Nathanael Kidwell | College of William and Mary (original) (raw)

Papers by Nathanael Kidwell

In the context of combustion, both the formation and destruction of fused-ring aromatics are impo... more In the context of combustion, both the formation and destruction of fused-ring aromatics are important to understand in some detail. Determining the principal pathways from benzene to naphthalene and on to larger polyaromatic hydrocarbons is imperative, but with increased chemical complexity is likely to involve free radicals as intermediates which are themselves structurally complex, necessitating more detailed spectroscopic characterization for their identification. Our group has recently studied the vibronic spectroscopy of a series of C10H9 and C10H11 hydronaphthyl radicals, in which their thermochemical properties were evaluated with isomer specificity. Here, we extend this characterization to include infrared spectra in the alkyl CH stretch region, and explore the spectroscopic consequences of electronic excitation on these CH stretch absorptions. Utilizing resonant ion-dip infrared spectroscopy (RIDIRS), we report the infrared spectra of 2,3,4-Trihydronaphthyl (C10H11) and In...

In recent years, the Cassini satellite has been providing details about the composition of Titan&... more In recent years, the Cassini satellite has been providing details about the composition of Titan's atmosphere. Recent data has shown the existence of polycyclic aromatic hydrocarbons (PAHs) at higher altitudes than previously expected including masses tentatively ascribed to naphthalene and anthracene. The formation of indene (C9H9) and naphthalene (C10H8), the simplest PAHs, and their derivatives are of great interest as similar mechanisms may lead to the formation of larger fused-ring systems. In recent years it has been proposed that resonance-stabilized radicals (RSRs) may play an important role as intermediates along these pathways. RSRs gain extra stability by delocalizing the unpaired electron through a neighboring conjugated pi-system. Because of this extra stability, RSRs are able to build up in concentration, allowing for the creation of larger, more complex systems through their recombination with other RSRs. Mass-selective UV-visible spectra of two RSRs, phenylallyl ...

Chemical Science, 2011

Mass-selective two-color resonant two-photon ionization (2C-R2PI) spectra of two resonance stabil... more Mass-selective two-color resonant two-photon ionization (2C-R2PI) spectra of two resonance stabilized radicals (RSRs), 1-phenylallyl and benzylallenyl radicals, have been recorded under jetcooled conditions. These two radicals, while sharing the same radical conjugation, have unique properties. The D 0 -D 1 origin of the 1-phenylallyl radical is at 19208 cm À1 , with extensive vibronic structure extending over 2000 cm À1 above the D 1 origin. Much of this structure is assigned based on comparison with DFT and TDDFT calculations. Two-color photoionization efficiency scans reveal a sharp ionization threshold, providing a precise adiabatic ionization potential for the radical of 6.905 (2) eV. By comparison, the benzylallenyl radical has an electronic origin at 19703 cm À1 and Franck-Condon activity similar to phenylallyl. The photoionization efficiency curve shows a gradual onset with apparent threshold at $7.50(2) eV. Visible-visible holeburning was used to show that each radical exists in one isomeric form in the expansion. The CH stretch IR spectrum of each radical was taken using D 0 -resonant ion dip infrared spectroscopy (D 0 -RIDIRS) in a novel four-laser experiment. Comparison of the IR spectrum with the predictions of DFT B3LYP calculations leads to firm assignment of each radical as the trans isomer. TDDFT calculations on the excited states of benzylallenyl suggest the possibility that the excited state levels originally excited convert to an allplanar form prior to ionization. The potential role that these radicals could play in Titan's atmosphere as intermediates in formation pathways for polycyclic aromatic hydrocarbons (PAHs) is briefly discussed.

Journal of the American Chemical Society, 2012

Conformer-specific, vibrationally resolved electronic spectroscopy of benzylallene (4-phenyl-1,2-... more Conformer-specific, vibrationally resolved electronic spectroscopy of benzylallene (4-phenyl-1,2-butadiene) is presented along with a detailed analysis of the products formed via its ultraviolet photoexcitation. Benzylallene is the minor product of the recombination of benzyl and propargyl radicals. The mass-selective resonant two-photon ionization spectrum of benzylallene was recorded under jet-cooled conditions, with its S 0 −S 1 origin at 37 483 cm −1 .U V −UV holeburning spectroscopy was used to show that only one conformer was present in the expansion. Rotational band contour analysis provided rotational constants and transition dipole moment direction consistent with a conformation in which the allene side chain is in the anti position, pointing away from the phenyl ring. The photochemistry of benzylallene was studied in a pump−probe geometry in which photoexcitation occurred by counter-propagating the expansion with a photoexcitation laser. The laser was timed to interact with the gas pulse in a short tube that extended the collisional region of the expansion. The products were cooled during expansion of the gas mixture into vacuum, before being interrogated using mass-selective resonant two-photon ionization. The UV−vis spectra of the photochemical products were compared to literature spectra for identification. Several wavelengths were chosen for photoexcitation, ranging from the S 0 −S 1 origin transition (266.79 nm) to 193 nm. Comparison of the product spectral intensities as a function of photoexcitation wavelength provides information on the wavelength dependence of the product yields. Photoexcitation at 266.79 nm yielded five products (benzyl radical, benzylallenyl radical, 1-phenyl-1,3-butadiene, 1,2-dihydronaphthalene, and naphthalene), with naphthalene and benzylallenyl radicals dominant. At 193 nm, the benzylallenyl radical signal was greatly reduced in intensity, while three additional C 10 H 8 isomeric products were observed. An extensive set of calculations of key stationary points on the ground state C 10 H 10 and C 10 H 9 potential energy surfaces were carried out at the DFT B3LYP/6-311G(d,p) level of theory. Mechanisms for formation of the observed products are proposed based on these potential energy surfaces, constrained by the results of cursory studies of the photochemistry of 1-phenyl-1,3-butadiene and 4-phenyl-1-butyne. A role for tunneling on the excited state surface in the formation of naphthalene is suggested by studies of partially deuterated benzylallene, which blocked naphthalene formation.

The journal of physical chemistry. A, Jan 30, 2015

UV excitation of jet-cooled CH3CHOO on the B(1)A'-X(1)A' transition results in dissociati... more UV excitation of jet-cooled CH3CHOO on the B(1)A'-X(1)A' transition results in dissociation to two spin-allowed product channels: CH3CHO X(1)A' + O (1)D and CH3CHO a(3)A″ + O (3)P. The O (1)D and O (3)P products are detected using 2 + 1 REMPI at 205 and 226 nm, respectively, for action spectroscopy and velocity map imaging studies. The O (1)D action spectrum closely follows the previously reported UV absorption spectrum for jet-cooled CH3CHOO [ Beames et al. J. Chem. Phys. 2013 , 138 , 244307 ]. Velocity map images of the O (1)D products following excitation of CH3CHOO at 305, 320, and 350 nm exhibit anisotropic angular distributions indicative of rapid (ps) dissociation, along with broad and unstructured total kinetic energy (TKER) distributions that reflect the internal energy distribution of the CH3CHO X(1)A' coproducts. The O (3)P action spectrum turns on near the peak of the UV absorption spectrum (ca. 324 nm) and extends to higher energy with steadily increasin...

The Journal of Physical Chemistry B, 2014

The infrared (IR) spectroscopy of the alkyl CH stretch region (2750-3000 cm(-1)) of a series of b... more The infrared (IR) spectroscopy of the alkyl CH stretch region (2750-3000 cm(-1)) of a series of bicyclic hydrocarbons and free radicals has been studied under supersonic expansion cooling in the gas phase, and compared with a theoretical model that describes the local mode stretch-bend Fermi resonance interactions. The double resonance method of fluorescence-dip infrared (FDIR) spectroscopy was used on the stable molecules 1,2-dihydronaphthalene, 1,4-dihydronaphthalene, tetralin, indene, and indane using the S0-S1 origin transition as a monitor of transitions. Resonant ion-dip infrared (RIDIR) spectra were recorded for the trihydronaphthyl (THN) and inden-2-yl methyl (I2M) radicals. The previously developed model Hamiltonian (J. Chem. Phys. 2013, 138, 064308) incorporates cubic stretch-bend coupling with parameters obtained from density functional theory methods. Full dimensional calculations are compared to reduced dimensional Hamiltonian results in which anharmonic CH stretches and CH2 scissor modes are Fermi coupled. Excellent agreement between theoretical results is found. Scale factors of select terms in the reduced dimensional Hamiltonian, obtained by fitting the theoretical Hamiltonian predictions to the experimental spectra, are found to be similar to previous work. The resulting Hamiltonian predicts successfully all the major spectral features considered in this study. A simplified model is introduced in which the CH2 groups are decoupled. This model enables the assignment of many of the spectral features. The model results are extended to describe the CH stretch spectrum of the chair and twist-boat conformers of cyclohexane. The chair conformer is used to illustrate the shortcomings of the CH2 decoupling model.

The Journal of Chemical Physics, 2014

The alkyl and aromatic CH stretch infrared spectra of inden-2-ylmethyl (I2M, C10H9) and trihydron... more The alkyl and aromatic CH stretch infrared spectra of inden-2-ylmethyl (I2M, C10H9) and trihydronaphthyl (THN, C10H11) radicals have been recorded under jet-cooled conditions in the ground (D0) and first electronically excited (D1) states using resonant ion-dip infrared (RIDIR) spectroscopy. Previously, the vibronic spectroscopy of a series of C10H9 and C10H11 hydronaphthyl radicals were investigated and their thermochemical properties were evaluated with isomer specificity [J. A. Sebree et al., J. Phys. Chem. A 11, 6255-6262 (2010)]. We show here that one of the m/z 129 spectral carriers characterized in that work was misidentified as 2-hydronaphthyl (2-HN) radical, appearing in a discharge of 1,2-dihydronaphthalene in close proximity to 1-hydronaphthyl radical. The D0-RIDIR spectrum in the alkyl CH stretch region positively identifies the m/z 129 isomer as I2M, whose two-color resonant two-photon ionization (2C-R2PI) spectrum was recently reported by Schmidt and co-workers [T. P. Troy et al., Chem. Sci. 2, 1755-1765 (2011)]. Here, we further characterize the I2M and THN radicals by recording their gas phase IR spectra in the alkyl and aromatic CH stretch regions, and explore the spectroscopic consequences of electronic excitation on the CH stretch absorptions. A local-mode CH stretch Hamiltonian incorporating cubic stretch-bend coupling between anharmonic CH stretches and CH2 scissor modes is utilized to describe their Fermi resonance interactions. Excellent agreement between the experimental and theoretical results facilitates the interpretation of the D0- and D1-state RIDIR spectra of I2M, revealing that upon excitation the alkyl CH stretches decrease in frequency by 70 cm(-1), while the allyl-like CH stretches experience a modest blueshift. In comparison, the photophysics of THN are strikingly different in that the IR transitions that possess vibrational motion along the CβH and CδH bonds are absent in the D1-RIDIR spectrum yet are predicted to be present from the theoretical model. Several hypotheses are considered to account for the perturbations to these vibrations.

The Journal of Physical Chemistry Letters, 2014

The pure rotational spectrum of perfluoroiodoethane between 8.0 and 11.9 GHz has been measured on... more The pure rotational spectrum of perfluoroiodoethane between 8.0 and 11.9 GHz has been measured on a search accelerated, correct intensity Fourier transform microwave (SACI-FTMW) spectrometer. The spectra is dense with 247 measured transitions in the given region. Only the anti conformer was observed for which rotational constants are reported. Nuclear electric quadrupole coupling constants due to the iodine-127 were determined and are reported. Also, two dipole forbidden/quadrupole allowed DJ ¼ 2 transitions were observed in the spectra. The observation of these transitions has been rationalized on the basis of near degeneracies between energy levels connected by v ab .

The Journal of Chemical Physics, 2014

The alkyl and aromatic CH stretch infrared spectra of inden-2-ylmethyl (I2M, C10H9) and trihydron... more The alkyl and aromatic CH stretch infrared spectra of inden-2-ylmethyl (I2M, C10H9) and trihydronaphthyl (THN, C10H11) radicals have been recorded under jet-cooled conditions in the ground (D0) and first electronically excited (D1) states using resonant ion-dip infrared (RIDIR) spectroscopy. Previously, the vibronic spectroscopy of a series of C10H9 and C10H11 hydronaphthyl radicals were investigated and their thermochemical properties were evaluated with isomer specificity [J. A. Sebree et al., J. Phys. Chem. A 11, 6255-6262 (2010)]. We show here that one of the m/z 129 spectral carriers characterized in that work was misidentified as 2-hydronaphthyl (2-HN) radical, appearing in a discharge of 1,2-dihydronaphthalene in close proximity to 1-hydronaphthyl radical. The D0-RIDIR spectrum in the alkyl CH stretch region positively identifies the m/z 129 isomer as I2M, whose two-color resonant two-photon ionization (2C-R2PI) spectrum was recently reported by Schmidt and co-workers [T. P. Troy et al., Chem. Sci. 2, 1755-1765 (2011)]. Here, we further characterize the I2M and THN radicals by recording their gas phase IR spectra in the alkyl and aromatic CH stretch regions, and explore the spectroscopic consequences of electronic excitation on the CH stretch absorptions. A local-mode CH stretch Hamiltonian incorporating cubic stretch-bend coupling between anharmonic CH stretches and CH2 scissor modes is utilized to describe their Fermi resonance interactions. Excellent agreement between the experimental and theoretical results facilitates the interpretation of the D0- and D1-state RIDIR spectra of I2M, revealing that upon excitation the alkyl CH stretches decrease in frequency by 70 cm(-1), while the allyl-like CH stretches experience a modest blueshift. In comparison, the photophysics of THN are strikingly different in that the IR transitions that possess vibrational motion along the CβH and CδH bonds are absent in the D1-RIDIR spectrum yet are predicted to be present from the theoretical model. Several hypotheses are considered to account for the perturbations to these vibrations.

Chemical Science, 2011

Mass-selective two-color resonant two-photon ionization (2C-R2PI) spectra of two resonance stabil... more Mass-selective two-color resonant two-photon ionization (2C-R2PI) spectra of two resonance stabilized radicals (RSRs), 1-phenylallyl and benzylallenyl radicals, have been recorded under jetcooled conditions. These two radicals, while sharing the same radical conjugation, have unique properties. The D 0 -D 1 origin of the 1-phenylallyl radical is at 19208 cm À1 , with extensive vibronic structure extending over 2000 cm À1 above the D 1 origin. Much of this structure is assigned based on comparison with DFT and TDDFT calculations. Two-color photoionization efficiency scans reveal a sharp ionization threshold, providing a precise adiabatic ionization potential for the radical of 6.905 (2) eV. By comparison, the benzylallenyl radical has an electronic origin at 19703 cm À1 and Franck-Condon activity similar to phenylallyl. The photoionization efficiency curve shows a gradual onset with apparent threshold at $7.50(2) eV. Visible-visible holeburning was used to show that each radical exists in one isomeric form in the expansion. The CH stretch IR spectrum of each radical was taken using D 0 -resonant ion dip infrared spectroscopy (D 0 -RIDIRS) in a novel four-laser experiment. Comparison of the IR spectrum with the predictions of DFT B3LYP calculations leads to firm assignment of each radical as the trans isomer. TDDFT calculations on the excited states of benzylallenyl suggest the possibility that the excited state levels originally excited convert to an allplanar form prior to ionization. The potential role that these radicals could play in Titan's atmosphere as intermediates in formation pathways for polycyclic aromatic hydrocarbons (PAHs) is briefly discussed.

Chemical Science, 2011

Mass-selective two-color resonant two-photon ionization (2C-R2PI) spectra of two resonance stabil... more Mass-selective two-color resonant two-photon ionization (2C-R2PI) spectra of two resonance stabilized radicals (RSRs), 1-phenylallyl and benzylallenyl radicals, have been recorded under jetcooled conditions. These two radicals, while sharing the same radical conjugation, have unique properties. The D 0 -D 1 origin of the 1-phenylallyl radical is at 19208 cm À1 , with extensive vibronic structure extending over 2000 cm À1 above the D 1 origin. Much of this structure is assigned based on comparison with DFT and TDDFT calculations. Two-color photoionization efficiency scans reveal a sharp ionization threshold, providing a precise adiabatic ionization potential for the radical of 6.905 (2) eV. By comparison, the benzylallenyl radical has an electronic origin at 19703 cm À1 and Franck-Condon activity similar to phenylallyl. The photoionization efficiency curve shows a gradual onset with apparent threshold at $7.50(2) eV. Visible-visible holeburning was used to show that each radical exists in one isomeric form in the expansion. The CH stretch IR spectrum of each radical was taken using D 0 -resonant ion dip infrared spectroscopy (D 0 -RIDIRS) in a novel four-laser experiment. Comparison of the IR spectrum with the predictions of DFT B3LYP calculations leads to firm assignment of each radical as the trans isomer. TDDFT calculations on the excited states of benzylallenyl suggest the possibility that the excited state levels originally excited convert to an allplanar form prior to ionization. The potential role that these radicals could play in Titan's atmosphere as intermediates in formation pathways for polycyclic aromatic hydrocarbons (PAHs) is briefly discussed.

Journal of the American Chemical Society, Jan 18, 2012

Conformer-specific, vibrationally resolved electronic spectroscopy of benzylallene (4-phenyl-1,2-... more Conformer-specific, vibrationally resolved electronic spectroscopy of benzylallene (4-phenyl-1,2-butadiene) is presented along with a detailed analysis of the products formed via its ultraviolet photoexcitation. Benzylallene is the minor product of the recombination of benzyl and propargyl radicals. The mass-selective resonant two-photon ionization spectrum of benzylallene was recorded under jet-cooled conditions, with its S(0)-S(1) origin at 37,483 cm(-1). UV-UV holeburning spectroscopy was used to show that only one conformer was present in the expansion. Rotational band contour analysis provided rotational constants and transition dipole moment direction consistent with a conformation in which the allene side chain is in the anti position, pointing away from the phenyl ring. The photochemistry of benzylallene was studied in a pump-probe geometry in which photoexcitation occurred by counter-propagating the expansion with a photoexcitation laser. The laser was timed to interact wit...

The journal of physical chemistry. A, Jan 26, 2014

A theoretical model Hamiltonian [J. Chem. Phys. 2013, 138, 064308] for describing vibrational spe... more A theoretical model Hamiltonian [J. Chem. Phys. 2013, 138, 064308] for describing vibrational spectra associated with the CH stretch of CH2 groups is extended to molecules containing methyl and methoxy groups. Results are compared to the infrared (IR) spectroscopy of four molecules studied under supersonic expansion cooling in gas phase conditions. The molecules include 1,1-diphenylethane (DPE), 1,1-diphenylpropane (DPP), 2-methoxyphenol (guaiacol), and 1,3-dimethoxy-2-hydroxybenzene (syringol). Transforming the bending normal mode vibrations of CH3 groups to local scissor vibrations leads to model Hamiltonians which share many features present in our model Hamiltonian for the stretching vibrations of CH2 Fermi coupled to scissor modes. The central difference arises from the greater scissor-scissor coupling present in the CH3 case. Comparing anharmonic couplings between these modes and the stretch-bend Fermi coupling for a variety of systems, it is observed that the anharmonic coupl...

The Journal of Physical Chemistry B, 2014

The infrared (IR) spectroscopy of the alkyl CH stretch region (2750-3000 cm(-1)) of a series of b... more The infrared (IR) spectroscopy of the alkyl CH stretch region (2750-3000 cm(-1)) of a series of bicyclic hydrocarbons and free radicals has been studied under supersonic expansion cooling in the gas phase, and compared with a theoretical model that describes the local mode stretch-bend Fermi resonance interactions. The double resonance method of fluorescence-dip infrared (FDIR) spectroscopy was used on the stable molecules 1,2-dihydronaphthalene, 1,4-dihydronaphthalene, tetralin, indene, and indane using the S0-S1 origin transition as a monitor of transitions. Resonant ion-dip infrared (RIDIR) spectra were recorded for the trihydronaphthyl (THN) and inden-2-yl methyl (I2M) radicals. The previously developed model Hamiltonian (J. Chem. Phys. 2013, 138, 064308) incorporates cubic stretch-bend coupling with parameters obtained from density functional theory methods. Full dimensional calculations are compared to reduced dimensional Hamiltonian results in which anharmonic CH stretches and CH2 scissor modes are Fermi coupled. Excellent agreement between theoretical results is found. Scale factors of select terms in the reduced dimensional Hamiltonian, obtained by fitting the theoretical Hamiltonian predictions to the experimental spectra, are found to be similar to previous work. The resulting Hamiltonian predicts successfully all the major spectral features considered in this study. A simplified model is introduced in which the CH2 groups are decoupled. This model enables the assignment of many of the spectral features. The model results are extended to describe the CH stretch spectrum of the chair and twist-boat conformers of cyclohexane. The chair conformer is used to illustrate the shortcomings of the CH2 decoupling model.

The Journal of Physical Chemistry A, 2013

The state-dependent spectroscopy of α-methylbenzyl radical (α-MeBz) has been studied under jet-co... more The state-dependent spectroscopy of α-methylbenzyl radical (α-MeBz) has been studied under jet-cooled conditions. Two-color resonant two-photon ionization (2C-R2PI), laser-induced fluorescence, and dispersed fluorescence spectra were obtained for the D0-D1 electronic transition of this prototypical resonance-stabilized radical in which the methyl group is immediately adjacent to the primary radical site. Extensive Franck-Condon activity in hindered rotor levels was observed in the excitation spectrum, reflecting a reorientation of the methyl group upon electronic excitation. Dispersed fluorescence spectra from the set of internal rotor levels are combined with the excitation spectrum to obtain a global fit of the barrier heights and angular change of the methyl group in both D0 and D1 states. The best-fit methyl rotor potential in the ground electronic state (D0) is a flat-topped 3-fold potential (V3" = 151 cm(-1), V6" = 34 cm(-1)) while the D1 state has a lower barrier (V3' = 72 cm(-1), V6' = 15 cm(-1)) with Δφ = ± π/3, π, consistent with a reorientation of the methyl group upon electronic excitation. The ground state results are compared with calculations carried out at the DFT B3LYP level of theory using the 6-311+G(d,p) basis set, and a variety of excited state calculations are carried out to compare against experiment. The preferred geometry of the methyl rotor in the ground state is anti, which switches to syn in the D1 state and in the cation. The calculations uncover a subtle combination of effects that contribute to the shift in orientation and change in barrier in the excited state relative to ground state. Steric interaction favors the anti conformation, while hyperconjugation is greater in the syn orientation. The presence of a second excited state close by D1 is postulated to influence the methyl rotor properties. A resonant ion-dip infrared (RIDIR) spectrum in the alkyl and aromatic CH stretch regions was also recorded, probing in a complementary way the state-dependent conformation of α-MeBz. Using a scheme in which infrared depletion occurs between excitation and ionization steps of the 2C-R2PI process, analogous infrared spectra in D1 were also obtained, probing the response of the CH stretch fundamentals to electronic excitation. A reduced-dimension Wilson G-matrix model was implemented to simulate and interpret the observed infrared results. Finally, photoionization efficiency scans were carried out to determine the adiabatic ionization threshold of α-MeBz (IP = 6.835 ± 0.002 eV) and provide thresholds for ionization out of specific internal rotor levels, which report on the methyl rotor barrier in the cation state.

The Journal of Chemical Physics, 2012

Vibrationally and rotationally resolved electronic spectra of diphenylmethane-d 5 ) are reported ... more Vibrationally and rotationally resolved electronic spectra of diphenylmethane-d 5 ) are reported in the isolated-molecule environment of a supersonic expansion. While small, the asymmetry induced by deuteration of one of the aromatic rings is sufficient to cause several important effects that change the principle mechanism of vibronic coupling between the close-lying S 1 and S 2 states, and spectroscopic signatures such coupling produces. The splitting between S 1 and S 2 origins is 186 cm −1 , about 50% greater than its value in DPM-d 0 (123 cm −1 ), and an amount sufficient to bring the S 2 zero-point level into near-resonance with the v = 1 level in the S 1 state of a low-frequency phenyl flapping mode, ν R = 191 cm −1 . Dispersed fluorescence spectra bear clear evidence that v(R) = 1 Herzberg-Teller coupling dominates the near-resonant internal mixing between the S 1 and S 2 manifolds. The fluorescence into each pair of Franck-Condon active ring modes shows an asymmetry that suggests incorrectly that the S 1 and S 2 states may be electronically localized. From rotationally resolved studies, the S 0 and S 1 states have been well-fit to asymmetric rotor Hamiltonians while the S 2 state is perturbed and not fit. The transition dipole moment (TDM) orientation of the S 1 state is nearly perpendicular to the C 2 symmetry axes with 66(2)%:3(1)%:34(2)% a:b:c hybrid-type character while that of the S 2 origin contains 50(10)% a:c-type (S 1 ) and 50(10)% b-type (S 2 ) character. A model is put forward that explains qualitatively the TDM compositions and dispersed emission patterns without the need to invoke electronic localization. The experimental data discussed here serve as a foundation for a multi-mode vibronic coupling model capable of being applied to asymmetric bichromophores, as presented in the work of B. Nebgen and L. V. Slipchenko ["Vibronic coupling in asymmetric bichromophores: Theory and application to diphenylmethane-d 5 ," J. Chem. Phys. (submitted)].

In the context of combustion, both the formation and destruction of fused-ring aromatics are impo... more In the context of combustion, both the formation and destruction of fused-ring aromatics are important to understand in some detail. Determining the principal pathways from benzene to naphthalene and on to larger polyaromatic hydrocarbons is imperative, but with increased chemical complexity is likely to involve free radicals as intermediates which are themselves structurally complex, necessitating more detailed spectroscopic characterization for their identification. Our group has recently studied the vibronic spectroscopy of a series of C10H9 and C10H11 hydronaphthyl radicals, in which their thermochemical properties were evaluated with isomer specificity. Here, we extend this characterization to include infrared spectra in the alkyl CH stretch region, and explore the spectroscopic consequences of electronic excitation on these CH stretch absorptions. Utilizing resonant ion-dip infrared spectroscopy (RIDIRS), we report the infrared spectra of 2,3,4-Trihydronaphthyl (C10H11) and In...

In recent years, the Cassini satellite has been providing details about the composition of Titan&... more In recent years, the Cassini satellite has been providing details about the composition of Titan's atmosphere. Recent data has shown the existence of polycyclic aromatic hydrocarbons (PAHs) at higher altitudes than previously expected including masses tentatively ascribed to naphthalene and anthracene. The formation of indene (C9H9) and naphthalene (C10H8), the simplest PAHs, and their derivatives are of great interest as similar mechanisms may lead to the formation of larger fused-ring systems. In recent years it has been proposed that resonance-stabilized radicals (RSRs) may play an important role as intermediates along these pathways. RSRs gain extra stability by delocalizing the unpaired electron through a neighboring conjugated pi-system. Because of this extra stability, RSRs are able to build up in concentration, allowing for the creation of larger, more complex systems through their recombination with other RSRs. Mass-selective UV-visible spectra of two RSRs, phenylallyl ...

Chemical Science, 2011

Mass-selective two-color resonant two-photon ionization (2C-R2PI) spectra of two resonance stabil... more Mass-selective two-color resonant two-photon ionization (2C-R2PI) spectra of two resonance stabilized radicals (RSRs), 1-phenylallyl and benzylallenyl radicals, have been recorded under jetcooled conditions. These two radicals, while sharing the same radical conjugation, have unique properties. The D 0 -D 1 origin of the 1-phenylallyl radical is at 19208 cm À1 , with extensive vibronic structure extending over 2000 cm À1 above the D 1 origin. Much of this structure is assigned based on comparison with DFT and TDDFT calculations. Two-color photoionization efficiency scans reveal a sharp ionization threshold, providing a precise adiabatic ionization potential for the radical of 6.905 (2) eV. By comparison, the benzylallenyl radical has an electronic origin at 19703 cm À1 and Franck-Condon activity similar to phenylallyl. The photoionization efficiency curve shows a gradual onset with apparent threshold at $7.50(2) eV. Visible-visible holeburning was used to show that each radical exists in one isomeric form in the expansion. The CH stretch IR spectrum of each radical was taken using D 0 -resonant ion dip infrared spectroscopy (D 0 -RIDIRS) in a novel four-laser experiment. Comparison of the IR spectrum with the predictions of DFT B3LYP calculations leads to firm assignment of each radical as the trans isomer. TDDFT calculations on the excited states of benzylallenyl suggest the possibility that the excited state levels originally excited convert to an allplanar form prior to ionization. The potential role that these radicals could play in Titan's atmosphere as intermediates in formation pathways for polycyclic aromatic hydrocarbons (PAHs) is briefly discussed.

Journal of the American Chemical Society, 2012

Conformer-specific, vibrationally resolved electronic spectroscopy of benzylallene (4-phenyl-1,2-... more Conformer-specific, vibrationally resolved electronic spectroscopy of benzylallene (4-phenyl-1,2-butadiene) is presented along with a detailed analysis of the products formed via its ultraviolet photoexcitation. Benzylallene is the minor product of the recombination of benzyl and propargyl radicals. The mass-selective resonant two-photon ionization spectrum of benzylallene was recorded under jet-cooled conditions, with its S 0 −S 1 origin at 37 483 cm −1 .U V −UV holeburning spectroscopy was used to show that only one conformer was present in the expansion. Rotational band contour analysis provided rotational constants and transition dipole moment direction consistent with a conformation in which the allene side chain is in the anti position, pointing away from the phenyl ring. The photochemistry of benzylallene was studied in a pump−probe geometry in which photoexcitation occurred by counter-propagating the expansion with a photoexcitation laser. The laser was timed to interact with the gas pulse in a short tube that extended the collisional region of the expansion. The products were cooled during expansion of the gas mixture into vacuum, before being interrogated using mass-selective resonant two-photon ionization. The UV−vis spectra of the photochemical products were compared to literature spectra for identification. Several wavelengths were chosen for photoexcitation, ranging from the S 0 −S 1 origin transition (266.79 nm) to 193 nm. Comparison of the product spectral intensities as a function of photoexcitation wavelength provides information on the wavelength dependence of the product yields. Photoexcitation at 266.79 nm yielded five products (benzyl radical, benzylallenyl radical, 1-phenyl-1,3-butadiene, 1,2-dihydronaphthalene, and naphthalene), with naphthalene and benzylallenyl radicals dominant. At 193 nm, the benzylallenyl radical signal was greatly reduced in intensity, while three additional C 10 H 8 isomeric products were observed. An extensive set of calculations of key stationary points on the ground state C 10 H 10 and C 10 H 9 potential energy surfaces were carried out at the DFT B3LYP/6-311G(d,p) level of theory. Mechanisms for formation of the observed products are proposed based on these potential energy surfaces, constrained by the results of cursory studies of the photochemistry of 1-phenyl-1,3-butadiene and 4-phenyl-1-butyne. A role for tunneling on the excited state surface in the formation of naphthalene is suggested by studies of partially deuterated benzylallene, which blocked naphthalene formation.

The journal of physical chemistry. A, Jan 30, 2015

UV excitation of jet-cooled CH3CHOO on the B(1)A'-X(1)A' transition results in dissociati... more UV excitation of jet-cooled CH3CHOO on the B(1)A'-X(1)A' transition results in dissociation to two spin-allowed product channels: CH3CHO X(1)A' + O (1)D and CH3CHO a(3)A″ + O (3)P. The O (1)D and O (3)P products are detected using 2 + 1 REMPI at 205 and 226 nm, respectively, for action spectroscopy and velocity map imaging studies. The O (1)D action spectrum closely follows the previously reported UV absorption spectrum for jet-cooled CH3CHOO [ Beames et al. J. Chem. Phys. 2013 , 138 , 244307 ]. Velocity map images of the O (1)D products following excitation of CH3CHOO at 305, 320, and 350 nm exhibit anisotropic angular distributions indicative of rapid (ps) dissociation, along with broad and unstructured total kinetic energy (TKER) distributions that reflect the internal energy distribution of the CH3CHO X(1)A' coproducts. The O (3)P action spectrum turns on near the peak of the UV absorption spectrum (ca. 324 nm) and extends to higher energy with steadily increasin...

The Journal of Physical Chemistry B, 2014

The infrared (IR) spectroscopy of the alkyl CH stretch region (2750-3000 cm(-1)) of a series of b... more The infrared (IR) spectroscopy of the alkyl CH stretch region (2750-3000 cm(-1)) of a series of bicyclic hydrocarbons and free radicals has been studied under supersonic expansion cooling in the gas phase, and compared with a theoretical model that describes the local mode stretch-bend Fermi resonance interactions. The double resonance method of fluorescence-dip infrared (FDIR) spectroscopy was used on the stable molecules 1,2-dihydronaphthalene, 1,4-dihydronaphthalene, tetralin, indene, and indane using the S0-S1 origin transition as a monitor of transitions. Resonant ion-dip infrared (RIDIR) spectra were recorded for the trihydronaphthyl (THN) and inden-2-yl methyl (I2M) radicals. The previously developed model Hamiltonian (J. Chem. Phys. 2013, 138, 064308) incorporates cubic stretch-bend coupling with parameters obtained from density functional theory methods. Full dimensional calculations are compared to reduced dimensional Hamiltonian results in which anharmonic CH stretches and CH2 scissor modes are Fermi coupled. Excellent agreement between theoretical results is found. Scale factors of select terms in the reduced dimensional Hamiltonian, obtained by fitting the theoretical Hamiltonian predictions to the experimental spectra, are found to be similar to previous work. The resulting Hamiltonian predicts successfully all the major spectral features considered in this study. A simplified model is introduced in which the CH2 groups are decoupled. This model enables the assignment of many of the spectral features. The model results are extended to describe the CH stretch spectrum of the chair and twist-boat conformers of cyclohexane. The chair conformer is used to illustrate the shortcomings of the CH2 decoupling model.

The Journal of Chemical Physics, 2014

The alkyl and aromatic CH stretch infrared spectra of inden-2-ylmethyl (I2M, C10H9) and trihydron... more The alkyl and aromatic CH stretch infrared spectra of inden-2-ylmethyl (I2M, C10H9) and trihydronaphthyl (THN, C10H11) radicals have been recorded under jet-cooled conditions in the ground (D0) and first electronically excited (D1) states using resonant ion-dip infrared (RIDIR) spectroscopy. Previously, the vibronic spectroscopy of a series of C10H9 and C10H11 hydronaphthyl radicals were investigated and their thermochemical properties were evaluated with isomer specificity [J. A. Sebree et al., J. Phys. Chem. A 11, 6255-6262 (2010)]. We show here that one of the m/z 129 spectral carriers characterized in that work was misidentified as 2-hydronaphthyl (2-HN) radical, appearing in a discharge of 1,2-dihydronaphthalene in close proximity to 1-hydronaphthyl radical. The D0-RIDIR spectrum in the alkyl CH stretch region positively identifies the m/z 129 isomer as I2M, whose two-color resonant two-photon ionization (2C-R2PI) spectrum was recently reported by Schmidt and co-workers [T. P. Troy et al., Chem. Sci. 2, 1755-1765 (2011)]. Here, we further characterize the I2M and THN radicals by recording their gas phase IR spectra in the alkyl and aromatic CH stretch regions, and explore the spectroscopic consequences of electronic excitation on the CH stretch absorptions. A local-mode CH stretch Hamiltonian incorporating cubic stretch-bend coupling between anharmonic CH stretches and CH2 scissor modes is utilized to describe their Fermi resonance interactions. Excellent agreement between the experimental and theoretical results facilitates the interpretation of the D0- and D1-state RIDIR spectra of I2M, revealing that upon excitation the alkyl CH stretches decrease in frequency by 70 cm(-1), while the allyl-like CH stretches experience a modest blueshift. In comparison, the photophysics of THN are strikingly different in that the IR transitions that possess vibrational motion along the CβH and CδH bonds are absent in the D1-RIDIR spectrum yet are predicted to be present from the theoretical model. Several hypotheses are considered to account for the perturbations to these vibrations.

The Journal of Physical Chemistry Letters, 2014

The pure rotational spectrum of perfluoroiodoethane between 8.0 and 11.9 GHz has been measured on... more The pure rotational spectrum of perfluoroiodoethane between 8.0 and 11.9 GHz has been measured on a search accelerated, correct intensity Fourier transform microwave (SACI-FTMW) spectrometer. The spectra is dense with 247 measured transitions in the given region. Only the anti conformer was observed for which rotational constants are reported. Nuclear electric quadrupole coupling constants due to the iodine-127 were determined and are reported. Also, two dipole forbidden/quadrupole allowed DJ ¼ 2 transitions were observed in the spectra. The observation of these transitions has been rationalized on the basis of near degeneracies between energy levels connected by v ab .

The Journal of Chemical Physics, 2014

The alkyl and aromatic CH stretch infrared spectra of inden-2-ylmethyl (I2M, C10H9) and trihydron... more The alkyl and aromatic CH stretch infrared spectra of inden-2-ylmethyl (I2M, C10H9) and trihydronaphthyl (THN, C10H11) radicals have been recorded under jet-cooled conditions in the ground (D0) and first electronically excited (D1) states using resonant ion-dip infrared (RIDIR) spectroscopy. Previously, the vibronic spectroscopy of a series of C10H9 and C10H11 hydronaphthyl radicals were investigated and their thermochemical properties were evaluated with isomer specificity [J. A. Sebree et al., J. Phys. Chem. A 11, 6255-6262 (2010)]. We show here that one of the m/z 129 spectral carriers characterized in that work was misidentified as 2-hydronaphthyl (2-HN) radical, appearing in a discharge of 1,2-dihydronaphthalene in close proximity to 1-hydronaphthyl radical. The D0-RIDIR spectrum in the alkyl CH stretch region positively identifies the m/z 129 isomer as I2M, whose two-color resonant two-photon ionization (2C-R2PI) spectrum was recently reported by Schmidt and co-workers [T. P. Troy et al., Chem. Sci. 2, 1755-1765 (2011)]. Here, we further characterize the I2M and THN radicals by recording their gas phase IR spectra in the alkyl and aromatic CH stretch regions, and explore the spectroscopic consequences of electronic excitation on the CH stretch absorptions. A local-mode CH stretch Hamiltonian incorporating cubic stretch-bend coupling between anharmonic CH stretches and CH2 scissor modes is utilized to describe their Fermi resonance interactions. Excellent agreement between the experimental and theoretical results facilitates the interpretation of the D0- and D1-state RIDIR spectra of I2M, revealing that upon excitation the alkyl CH stretches decrease in frequency by 70 cm(-1), while the allyl-like CH stretches experience a modest blueshift. In comparison, the photophysics of THN are strikingly different in that the IR transitions that possess vibrational motion along the CβH and CδH bonds are absent in the D1-RIDIR spectrum yet are predicted to be present from the theoretical model. Several hypotheses are considered to account for the perturbations to these vibrations.

Chemical Science, 2011

Mass-selective two-color resonant two-photon ionization (2C-R2PI) spectra of two resonance stabil... more Mass-selective two-color resonant two-photon ionization (2C-R2PI) spectra of two resonance stabilized radicals (RSRs), 1-phenylallyl and benzylallenyl radicals, have been recorded under jetcooled conditions. These two radicals, while sharing the same radical conjugation, have unique properties. The D 0 -D 1 origin of the 1-phenylallyl radical is at 19208 cm À1 , with extensive vibronic structure extending over 2000 cm À1 above the D 1 origin. Much of this structure is assigned based on comparison with DFT and TDDFT calculations. Two-color photoionization efficiency scans reveal a sharp ionization threshold, providing a precise adiabatic ionization potential for the radical of 6.905 (2) eV. By comparison, the benzylallenyl radical has an electronic origin at 19703 cm À1 and Franck-Condon activity similar to phenylallyl. The photoionization efficiency curve shows a gradual onset with apparent threshold at $7.50(2) eV. Visible-visible holeburning was used to show that each radical exists in one isomeric form in the expansion. The CH stretch IR spectrum of each radical was taken using D 0 -resonant ion dip infrared spectroscopy (D 0 -RIDIRS) in a novel four-laser experiment. Comparison of the IR spectrum with the predictions of DFT B3LYP calculations leads to firm assignment of each radical as the trans isomer. TDDFT calculations on the excited states of benzylallenyl suggest the possibility that the excited state levels originally excited convert to an allplanar form prior to ionization. The potential role that these radicals could play in Titan's atmosphere as intermediates in formation pathways for polycyclic aromatic hydrocarbons (PAHs) is briefly discussed.

Chemical Science, 2011

Mass-selective two-color resonant two-photon ionization (2C-R2PI) spectra of two resonance stabil... more Mass-selective two-color resonant two-photon ionization (2C-R2PI) spectra of two resonance stabilized radicals (RSRs), 1-phenylallyl and benzylallenyl radicals, have been recorded under jetcooled conditions. These two radicals, while sharing the same radical conjugation, have unique properties. The D 0 -D 1 origin of the 1-phenylallyl radical is at 19208 cm À1 , with extensive vibronic structure extending over 2000 cm À1 above the D 1 origin. Much of this structure is assigned based on comparison with DFT and TDDFT calculations. Two-color photoionization efficiency scans reveal a sharp ionization threshold, providing a precise adiabatic ionization potential for the radical of 6.905 (2) eV. By comparison, the benzylallenyl radical has an electronic origin at 19703 cm À1 and Franck-Condon activity similar to phenylallyl. The photoionization efficiency curve shows a gradual onset with apparent threshold at $7.50(2) eV. Visible-visible holeburning was used to show that each radical exists in one isomeric form in the expansion. The CH stretch IR spectrum of each radical was taken using D 0 -resonant ion dip infrared spectroscopy (D 0 -RIDIRS) in a novel four-laser experiment. Comparison of the IR spectrum with the predictions of DFT B3LYP calculations leads to firm assignment of each radical as the trans isomer. TDDFT calculations on the excited states of benzylallenyl suggest the possibility that the excited state levels originally excited convert to an allplanar form prior to ionization. The potential role that these radicals could play in Titan's atmosphere as intermediates in formation pathways for polycyclic aromatic hydrocarbons (PAHs) is briefly discussed.

Journal of the American Chemical Society, Jan 18, 2012

Conformer-specific, vibrationally resolved electronic spectroscopy of benzylallene (4-phenyl-1,2-... more Conformer-specific, vibrationally resolved electronic spectroscopy of benzylallene (4-phenyl-1,2-butadiene) is presented along with a detailed analysis of the products formed via its ultraviolet photoexcitation. Benzylallene is the minor product of the recombination of benzyl and propargyl radicals. The mass-selective resonant two-photon ionization spectrum of benzylallene was recorded under jet-cooled conditions, with its S(0)-S(1) origin at 37,483 cm(-1). UV-UV holeburning spectroscopy was used to show that only one conformer was present in the expansion. Rotational band contour analysis provided rotational constants and transition dipole moment direction consistent with a conformation in which the allene side chain is in the anti position, pointing away from the phenyl ring. The photochemistry of benzylallene was studied in a pump-probe geometry in which photoexcitation occurred by counter-propagating the expansion with a photoexcitation laser. The laser was timed to interact wit...

The journal of physical chemistry. A, Jan 26, 2014

A theoretical model Hamiltonian [J. Chem. Phys. 2013, 138, 064308] for describing vibrational spe... more A theoretical model Hamiltonian [J. Chem. Phys. 2013, 138, 064308] for describing vibrational spectra associated with the CH stretch of CH2 groups is extended to molecules containing methyl and methoxy groups. Results are compared to the infrared (IR) spectroscopy of four molecules studied under supersonic expansion cooling in gas phase conditions. The molecules include 1,1-diphenylethane (DPE), 1,1-diphenylpropane (DPP), 2-methoxyphenol (guaiacol), and 1,3-dimethoxy-2-hydroxybenzene (syringol). Transforming the bending normal mode vibrations of CH3 groups to local scissor vibrations leads to model Hamiltonians which share many features present in our model Hamiltonian for the stretching vibrations of CH2 Fermi coupled to scissor modes. The central difference arises from the greater scissor-scissor coupling present in the CH3 case. Comparing anharmonic couplings between these modes and the stretch-bend Fermi coupling for a variety of systems, it is observed that the anharmonic coupl...

The Journal of Physical Chemistry B, 2014

The infrared (IR) spectroscopy of the alkyl CH stretch region (2750-3000 cm(-1)) of a series of b... more The infrared (IR) spectroscopy of the alkyl CH stretch region (2750-3000 cm(-1)) of a series of bicyclic hydrocarbons and free radicals has been studied under supersonic expansion cooling in the gas phase, and compared with a theoretical model that describes the local mode stretch-bend Fermi resonance interactions. The double resonance method of fluorescence-dip infrared (FDIR) spectroscopy was used on the stable molecules 1,2-dihydronaphthalene, 1,4-dihydronaphthalene, tetralin, indene, and indane using the S0-S1 origin transition as a monitor of transitions. Resonant ion-dip infrared (RIDIR) spectra were recorded for the trihydronaphthyl (THN) and inden-2-yl methyl (I2M) radicals. The previously developed model Hamiltonian (J. Chem. Phys. 2013, 138, 064308) incorporates cubic stretch-bend coupling with parameters obtained from density functional theory methods. Full dimensional calculations are compared to reduced dimensional Hamiltonian results in which anharmonic CH stretches and CH2 scissor modes are Fermi coupled. Excellent agreement between theoretical results is found. Scale factors of select terms in the reduced dimensional Hamiltonian, obtained by fitting the theoretical Hamiltonian predictions to the experimental spectra, are found to be similar to previous work. The resulting Hamiltonian predicts successfully all the major spectral features considered in this study. A simplified model is introduced in which the CH2 groups are decoupled. This model enables the assignment of many of the spectral features. The model results are extended to describe the CH stretch spectrum of the chair and twist-boat conformers of cyclohexane. The chair conformer is used to illustrate the shortcomings of the CH2 decoupling model.

The Journal of Physical Chemistry A, 2013

The state-dependent spectroscopy of α-methylbenzyl radical (α-MeBz) has been studied under jet-co... more The state-dependent spectroscopy of α-methylbenzyl radical (α-MeBz) has been studied under jet-cooled conditions. Two-color resonant two-photon ionization (2C-R2PI), laser-induced fluorescence, and dispersed fluorescence spectra were obtained for the D0-D1 electronic transition of this prototypical resonance-stabilized radical in which the methyl group is immediately adjacent to the primary radical site. Extensive Franck-Condon activity in hindered rotor levels was observed in the excitation spectrum, reflecting a reorientation of the methyl group upon electronic excitation. Dispersed fluorescence spectra from the set of internal rotor levels are combined with the excitation spectrum to obtain a global fit of the barrier heights and angular change of the methyl group in both D0 and D1 states. The best-fit methyl rotor potential in the ground electronic state (D0) is a flat-topped 3-fold potential (V3" = 151 cm(-1), V6" = 34 cm(-1)) while the D1 state has a lower barrier (V3' = 72 cm(-1), V6' = 15 cm(-1)) with Δφ = ± π/3, π, consistent with a reorientation of the methyl group upon electronic excitation. The ground state results are compared with calculations carried out at the DFT B3LYP level of theory using the 6-311+G(d,p) basis set, and a variety of excited state calculations are carried out to compare against experiment. The preferred geometry of the methyl rotor in the ground state is anti, which switches to syn in the D1 state and in the cation. The calculations uncover a subtle combination of effects that contribute to the shift in orientation and change in barrier in the excited state relative to ground state. Steric interaction favors the anti conformation, while hyperconjugation is greater in the syn orientation. The presence of a second excited state close by D1 is postulated to influence the methyl rotor properties. A resonant ion-dip infrared (RIDIR) spectrum in the alkyl and aromatic CH stretch regions was also recorded, probing in a complementary way the state-dependent conformation of α-MeBz. Using a scheme in which infrared depletion occurs between excitation and ionization steps of the 2C-R2PI process, analogous infrared spectra in D1 were also obtained, probing the response of the CH stretch fundamentals to electronic excitation. A reduced-dimension Wilson G-matrix model was implemented to simulate and interpret the observed infrared results. Finally, photoionization efficiency scans were carried out to determine the adiabatic ionization threshold of α-MeBz (IP = 6.835 ± 0.002 eV) and provide thresholds for ionization out of specific internal rotor levels, which report on the methyl rotor barrier in the cation state.

The Journal of Chemical Physics, 2012

Vibrationally and rotationally resolved electronic spectra of diphenylmethane-d 5 ) are reported ... more Vibrationally and rotationally resolved electronic spectra of diphenylmethane-d 5 ) are reported in the isolated-molecule environment of a supersonic expansion. While small, the asymmetry induced by deuteration of one of the aromatic rings is sufficient to cause several important effects that change the principle mechanism of vibronic coupling between the close-lying S 1 and S 2 states, and spectroscopic signatures such coupling produces. The splitting between S 1 and S 2 origins is 186 cm −1 , about 50% greater than its value in DPM-d 0 (123 cm −1 ), and an amount sufficient to bring the S 2 zero-point level into near-resonance with the v = 1 level in the S 1 state of a low-frequency phenyl flapping mode, ν R = 191 cm −1 . Dispersed fluorescence spectra bear clear evidence that v(R) = 1 Herzberg-Teller coupling dominates the near-resonant internal mixing between the S 1 and S 2 manifolds. The fluorescence into each pair of Franck-Condon active ring modes shows an asymmetry that suggests incorrectly that the S 1 and S 2 states may be electronically localized. From rotationally resolved studies, the S 0 and S 1 states have been well-fit to asymmetric rotor Hamiltonians while the S 2 state is perturbed and not fit. The transition dipole moment (TDM) orientation of the S 1 state is nearly perpendicular to the C 2 symmetry axes with 66(2)%:3(1)%:34(2)% a:b:c hybrid-type character while that of the S 2 origin contains 50(10)% a:c-type (S 1 ) and 50(10)% b-type (S 2 ) character. A model is put forward that explains qualitatively the TDM compositions and dispersed emission patterns without the need to invoke electronic localization. The experimental data discussed here serve as a foundation for a multi-mode vibronic coupling model capable of being applied to asymmetric bichromophores, as presented in the work of B. Nebgen and L. V. Slipchenko ["Vibronic coupling in asymmetric bichromophores: Theory and application to diphenylmethane-d 5 ," J. Chem. Phys. (submitted)].