Samuel Leutwyler | University of Bern, Switzerland (original) (raw)

Papers by Samuel Leutwyler

ABSTRACT Using explicitly-correlated coupled-cluster theory with single and double excitations, t... more ABSTRACT Using explicitly-correlated coupled-cluster theory with single and double excitations, the intermolecular distances and interaction energies of the T-shaped imidazole...benzene and pyrrole...benzene complexes have been computed in a large augmented correlation-consistent quadruple-zeta basis set, adding also corrections for connected triple excitations and remaining basis-set-superposition errors. The results of these computations are used to assess other methods such as Moller-Plesset perturbation theory (MP2), spin-component-scaled MP2 theory, dispersion-weighted MP2 theory, interference-corrected explicitly-correlated MP2 theory, dispersion-corrected double-hybrid density-functional theory (DFT), DFT-based symmetry-adapted perturbation theory, the random-phase approximation, explicitly-correlated ring-coupled-cluster-doubles theory, and double-hybrid DFT with a correlation energy computed in the random-phase approximation.

Resonant two-photon ionization combined with time-of-flight mass spectrometry was applied for the... more Resonant two-photon ionization combined with time-of-flight mass spectrometry was applied for the interrogation of the So-S I electronic-vibrational excitations of van der Waals complexes of fluorene (FL) with rare-gas atoms and N2 in supersonic jets. Energy-resolved and mass-resolved spectra of FL· Ne. FL· Ar. (n = 1-3), FL· Kr, FL· Xe, and FL· N, were recorded over the energy range 0-800 cm-I above the electronic origin of S I' The red microscopic spectral shifts of the electronic origins of FL. R (R = Ar, Kr, and Xe) complexes are dominated by dispersive interactions, being proportional to the polarizability of R. The vibrational level structure of FL. R. (R = Ar, Kr, and Xe) complexes exhibits intramolecular vibrational excitations of FL, as well as intermolecular vibrations, which involve the relative motion of FL and R in the complex. The spectra of FL· Ne and FL· N, reveal a rich vibrational structure in the vicinity of the electronic origin, indicating a substantial change of the nuclear configuration upon electronic excitation. Upper and lower bounds on the dissociation energies of FL· R (R = Ne, Kr, and Xe) and FL· Ar, were inferred from the vibrational level structure in the mass-resolved spectra, where the disappearance of the signal of the parent van der Waals ion and the appearance of the ion signal of the fragments mark the onset of the vibrational predissociation process.

In this paper we report the results of an experimental study of collision-free molecular multipho... more In this paper we report the results of an experimental study of collision-free molecular multiphoton dissociation (PJPD) and molecular multiphoton ionization (&@I) of nickelocene @KC loH,o), induced by the light of a tunable dye laser in the wavelength region 3750-5200 A. The spectral dependence of the ion signal reveals-a multitude of narrow (fwhm from CO.5 cm-' to 1.5 cm-' _) Intense peaks superimposed on a very weak background (relative amplitude ratio for peakslback-ground ~10~). The sharp resonances in the icn sig~at are attributed, on the basis of spectroscopic anzdysis, to two-photon resonant three-photon ionization of Ni(I) and to one-photon resonant three-photon ionization of Ni(I), the Ni(I) being produced by MPD of nicketocene. The ion signal in the spectral rarge 3750-3950 A reveals enhanced continuous background due to MPI of nickelocene. This ion signal spectra, together with studies of the intensity dependence of the,overall (nickelocene hlPD)-(Ni(1) MPPI) processes, as well as the (nickelocene molecular MPI) reaction, reveal four reactive processes. (a) Two-photon molecular MPI for hw > 3.10 eV photons. (b) Three-photon molecular hIpI forfiw = 3.10-2.10 eV. (c) Twephoton MPD at hw > 2.86 eV. (d) Three-photon MPD for Aw = 2.8-1.9 eV. The overall Cissociation enerm of nickelocene (Nicpa) to give Ni f 2cp was determined to be 5.76 t 0.60 eV and the nvo-photon ionization potential of this molecule is 6.29 + 0.015 eV. Our results provide dynamic evidence concerning a simultaneous " 'explosive " photodissxiation mechanism of Nicpz by process (c) and for the dominating role of the dissociative channel, characterized by a branching ratio of 250 in favor of predissociation over autoionization, for process (c) at 6.3-6.6 eV. The MPD processes (c) and (d) are expected to exhibit intramolecular erosion of phase coherence effects. Processes (c) and (d) are of high efficiency ~0.01 ions/molecule at saturation exhibited at laser power of =108 tV cm " .

Electrical transport and optical data on amorphous-metal — rare-gas mixtures of Cs-Xe at 6 K are ... more Electrical transport and optical data on amorphous-metal — rare-gas mixtures of Cs-Xe at 6 K are presented. The Cs-Xe mixtures fit a model whereby most of the Cs is distributed randomly on the microscopic scale while a small amount exists in the form of granular clusters. Although preferential clustering occurs, it is not prominent enough to apply the model of classical percolation theory to the metal-nonmetal transition which is exhibited. The transition is located at 0.55 + 0.01 Cs atomic fraction based upon the closure of the optical gap and the threshold of extended-state conduction. The composition range over which the states at the Fermi energy are localized is very small or zero.

Pulsed dye-laser nradlatmn of supcrsorucally cooled hIn2(CO)lo and Fe,(C0)12 m the 3800-5000 A wa... more Pulsed dye-laser nradlatmn of supcrsorucally cooled hIn2(CO)lo and Fe,(C0)12 m the 3800-5000 A wavelength range mduces colluon-free multlphoton dlssoclatlonllo~atlon processes. The IOIUC fragments generated are the metal species M', MC and M; excluswely.

Tunable laser two-photon 1on1za11on oTkugc van dcr Wdsls molcculcs, combmcd wrth tune-of-fl@t mas... more Tunable laser two-photon 1on1za11on oTkugc van dcr Wdsls molcculcs, combmcd wrth tune-of-fl@t mass spectroscopy. WE applied to the idcntfxatlon ol the clcctiomc orlgn and of some low wbnllonal cwtatlons of the Su-S, clcctromc transihon ol fluorcwhr,, fluorcnc AI, and lluorcnc Krr produrcd In supcrsomc c\p.mslons

Chem Phys, 2006

The S1↔S0 vibronic spectra of supersonic jet-cooled 2-pyridone [pyridin-2-one (2PY)] and its N-H ... more The S1↔S0 vibronic spectra of supersonic jet-cooled 2-pyridone [pyridin-2-one (2PY)] and its N-H deuterated isotopomer (d-2PY) have been recorded by two-color resonant two-photon ionization, laser-induced fluorescence and emission, and fluorescence depletion spectroscopies. By combining these methods, the B origin of 2PY at 000+98cm-1 and the bands at +218 and +252cm-1 are identified as overtones of the S1 state out-of-plane vibrations ν1' and ν2', as are the analogous bands of d-2PY. Anharmonic double-minimum potentials are derived for the respective out-of-plane coordinates that predict further ν1' and ν2' overtones and combinations, reproducing ˜80% of the vibronic bands up to 600cm-1 above the 000 band. The fluorescence spectra excited at the electronic origins and the ν1' and ν2' out-of-plane overtone levels confirm these assignments. The S1 nonplanar minima and S1←S0 out-of-plane progressions are in agreement with the determination of nonplanar vibrationally averaged geometries for the 000 and 000+98cm-1 upper states by Held et al. [J. Chem. Phys. 95, 8732 (1991)]. The fluorescence lifetimes of the S1 state vibrations show strong mode dependence: Those of the out-of-plane levels decrease rapidly above 200cm-1 excess vibrational energy, while the in-plane vibrations ν5', ν8', and ν9' have longer lifetimes, although they are above or interspersed with the "dark" out-of-plane states. This is interpreted in terms of an S1' state reaction with a low barrier towards a conical intersection with a prefulvenic geometry. Out-of-plane vibrational states can directly surmount this barrier, whereas in-plane vibrations are much less efficient in this respect. Analysis of the fluorescence spectra allows to identify nine in-plane S0' state fundamentals, overtones of the S0 state ν1″ and ν2″ out-of-plane vibrations, and >30 other overtones and combination bands. The B3LYP /6-311++G(d,p) calculated anharmonic wave numbers are in very good agreement with the observed fundamentals, overtones, and combinations, with a deviation Δrms=1.3%.

Journal of Molecular Structure, 1992

Ab initio electronic structure calculations for phenol and the hydrogen-bonded complexes phenol*H... more Ab initio electronic structure calculations for phenol and the hydrogen-bonded complexes phenol*H,O and d-phenol*D,O were performed at the HartreeFock 4-31G and 6-31G** levels. Both phenol and phenol.H,O were fully structure optimized. Based on the minimumenergy structures so obtained, full normal coordinate analyses were carried out. The resulting harmonic frequencies were scaled and compared to available experimental data. The agreement is satisfactory and allows for an assignment of a majority of the bands observed in the experimental spectra. Comparison with previous calculations on (H,O), reveals a considerable increase in the strength of the hydrogen bond on going from

Blum Carmen Jaggi Michael Liu Shi Xia Leutwyler Samuel Decurtins Silvio Theoretical Study on Annulated Donor Π Acceptor Systems Incorporating Tetrathiafulvalene and Perylenediimide Units Chimia 64 P 507, 2010

The Journal of Physical Chemistry B, May 1, 2007

Benchmark ab initio calculations at the correlated level are reported for ten isomers of the urac... more Benchmark ab initio calculations at the correlated level are reported for ten isomers of the uracil dimer (U.U): six are doubly N-H...O hydrogen bonded, three have a C-H...O and an N-H...O hydrogen bond, and one is doubly C-H...O hydrogen bonded. Their structures were optimized at the correlated level by using second-order Møller-Plesset perturbation theory (MP2), resolution of identity MP2 (RIMP2), and the binding energies D(e) calculated with the coupled-cluster method with singles, doubles, and iterative triples, CCSD(T). The MP2 and RIMP2 binding energies D(e) are extrapolated to the complete basis set (CBS) limit, using the aug-cc-pVXZ (X = D, T, Q) basis sets, giving binding energies accurate to +/-0.07 kcal/mol. With one exception, the correlation energy contributions at the CCSD(T) level increase the binding energies, although the changes are small, +0.03 to -0.27 kcal/mol (or 0.1% to 2.2%). The most stable isomer is the doubly N1-H...O hydrogen-bonded HB4 form, with D(e)[CCSD(T)]= -19.04 kcal/mol. The biologically relevant HB2 dimer has D(e)[CCSD(T)] = -12.64 kcal/mol, and the HB5 dimer that constitutes the main structural motif of the uracil crystal has -13.20 kcal/mol. The "Calcutta" dimer, which occurs in an RNA hexamer, is among the weakest isomers, with D(e)[CCSD(T)] = -9.81 kcal/mol. We compare to the binding energies calculated with the B3LYP, PBE, and PW91 density functionals; the PW91/6-311++G(d,p) binding energies agree with the CBS benchmark values, to within <2%. A useful single-molecule descriptor for the strengths of the individual hydrogen bonds can be derived from the gas-phase acidity DeltaE(0)(A-H) of the N-H or C-H donor groups and the gas-phase proton affinity PA(0)(B) of the C=O groups of the uracil monomer. The calculated hydrogen bond energies D(e)(infinity) correlate well with the difference between gas-phase acidity and basicity, DeltaE(0)(A-H) - PA(0)(B).

Chemistry a European Journal, 2009

Chemical Physics Letters, 1989

Lobsiger Simon Frey Hans Martin Leutwyler Samuel Jet Spectroscopy of the Deoxythymine Tautomers 5 Methyl 2 Hydroxypyrimidine and 5 Methyl 2 Pyrimidinone Chimia 64 P 581, 2010

Journal of Chemical Physics, 1995

ABSTRACT A four-dimensional intermolecular torsional potential energy surface was calculated for ... more ABSTRACT A four-dimensional intermolecular torsional potential energy surface was calculated for the cyclic homodromic water tetramer at the level of second-order Møller-Plesset perturbation theory, using a large basis set (82 basis functions per monomer). The four torsional angles ωi (i = 1–4), which describe the positions of the “free” O–H bonds relative to the planar hydrogen-bonded O⋯O⋯O⋯O framework were varied, while all other internal coordinates were held fixed. Ab initio calculations were performed at 307 points in the asymmetric unit of the torsional space. The points were fitted using the same seven-parameter analytical potential function as previously employed for the torsional surface of the water trimer [Bürgi et al., J. Chem. Phys. 103, 1077 (1995)]. Fits were performed for four interaction energy ranges spanning 1100, 1500, 3500 and 11 500 cm−1 relative to the global minimum, respectively. A number of important stationary points were studied in greater detail and geometry optimized using larger basis sets at the MP2 level. Torsional interconversion pathways are mapped in terms of isopotential surface representations and contour plots. This accurate and rapidly computable potential can be employed to calculate the coupled anharmonic torsional level energies, wave functions and associated molecular properties, presented in the following paper [D. Sabo et al., J. Chem. Phys. 109, 5404 (1998)]. © 1998 American Institute of Physics.

Chem Phys, 1988

Efficient excited-state proton transfer in neutral acid-base clusters α-naphtholṡBn has been dete... more Efficient excited-state proton transfer in neutral acid-base clusters α-naphtholṡBn has been detected and studied by a combination of laser spectroscopic techniques (resonant two-photon ionization, fluorescence excitation, and emission spectroscopy). S1 state proton transfer was observed for B=NH3 and n≥4, as evidenced by several criteria: (a) large red shift and substantial broadening of the R2PI spectra of the n≥4 clusters relative to those of the bare α-naphthol and smaller clusters; (b) very large Stokes shift (˜8000 cm-1) of the emission spectra of the n≥4 clusters; (c) complete broadening of the fluorescence emission band for the n≥4 clusters; and (d) a striking similarity of the emission band position and width of the latter spectra to the emission spectrum of the α-naphtholate anion in basic aqueous solution. No proton-transfer reaction was observed for small solvent clusters with B=NH3 and n≤3, nor for any of the pair complexes studied, which involve a single base partner [B=triethylamine, 3-dimethylamino-1-aminopropane, 1,4-bis(dimethylamino)butane] which we have studied so far. This behavior illustrates the difficulty of achieving charge separation in neutral gas-phase complexes or clusters. A critical gas-phase proton affinity PAcrit =248±3 kcal/mol was determined for proton transfer to take place in the α-naphtholṡBn (or base B) system.

Journal of Physical Chemistry, 1985

The Journal of Chemical Physics, Sep 21, 1999

Calculations of the torsional state dependence of A=B and C rotational constants of (D2O)3 are re... more Calculations of the torsional state dependence of A=B and C rotational constants of (D2O)3 are reported, for torsional energies up to 100 cm-1 above the ground state, extending our work on the rotational constants of all isotopomers of water trimer using ab initio four-dimensional torsional-stretching intermolecular potential-energy surface [D. Sabo et al., J. Phys. Chem. 110, 5745 (1999)]. Direct composition was made to the rotational constants measured by Viant et al. [J. Chem. Phys. 110, 4369 (1999)] for the same nine torsional levels of (D2O)3. In order to consistently reproduce the pronounced variations of the A=B and C rotational constants from one torsional eigenstate to the other, theory must take into account both the changes of rms torsional angles of the "free" O-D bonds and the changes in the oxygen-oxygen separation which accompany torsional excitation. The changes of C with torsional state, ΔC(n), are distinctly non-monotonic; they depend mainly on the rms (root-mean-square) torsional angle and to a lesser degree on the interoxygen distance. The changes of A=B with torsional state, ΔA(n)=ΔB(n), depend on both rms torsional angles and interoxygen distance, but with opposite signs, giving rise to an apparent monotonic decrease with n which is smaller than the changes due to either mechanism alone. The ΔA(n)=ΔB(n) and ΔC(n) changes from the (3+1)D torsional-stretching calculations are in good agreement with experiment up to n=5.

The Journal of Chemical Physics, Sep 8, 1995

Mass-selective ground-state vibrational spectra of jet-cooled carbazoleṡR (R=Ne, Ar, Kr, and Xe) ... more Mass-selective ground-state vibrational spectra of jet-cooled carbazoleṡR (R=Ne, Ar, Kr, and Xe) van der Waals complexes were obtained by populating ground-state intra- and intermolecular levels via stimulated emission pumping, followed by time delayed resonant two-photon ionization of the vibrationally hot complex. By tuning the dump laser frequency, S0 state vibrational modes were accessed from ≊200 cm-1 up to the dissociation energy D0. Upon dumping to ground-state levels above D0, efficient vibrational predissociation of the complexes occurred, allowing us to determine the S0 state van der Waals binding energies very accurately. The D0(S0) values are <214.5±0.5 cm-1 (R=Ne), 530.4±1.5 cm-1 (R=Ar), 687.9±4.0 cm-1 (R=Kr), and 890.8±1.6 cm-1 (R=Xe). In the S1 state, the corresponding binding energies are larger by 9% to 12%, being <222.9±1.0 cm-1, 576.3±1.6 cm-1, 756.4±4.5 cm-1, and 995.8±2.5 cm-1, respectively.

ABSTRACT Using explicitly-correlated coupled-cluster theory with single and double excitations, t... more ABSTRACT Using explicitly-correlated coupled-cluster theory with single and double excitations, the intermolecular distances and interaction energies of the T-shaped imidazole...benzene and pyrrole...benzene complexes have been computed in a large augmented correlation-consistent quadruple-zeta basis set, adding also corrections for connected triple excitations and remaining basis-set-superposition errors. The results of these computations are used to assess other methods such as Moller-Plesset perturbation theory (MP2), spin-component-scaled MP2 theory, dispersion-weighted MP2 theory, interference-corrected explicitly-correlated MP2 theory, dispersion-corrected double-hybrid density-functional theory (DFT), DFT-based symmetry-adapted perturbation theory, the random-phase approximation, explicitly-correlated ring-coupled-cluster-doubles theory, and double-hybrid DFT with a correlation energy computed in the random-phase approximation.

Resonant two-photon ionization combined with time-of-flight mass spectrometry was applied for the... more Resonant two-photon ionization combined with time-of-flight mass spectrometry was applied for the interrogation of the So-S I electronic-vibrational excitations of van der Waals complexes of fluorene (FL) with rare-gas atoms and N2 in supersonic jets. Energy-resolved and mass-resolved spectra of FL· Ne. FL· Ar. (n = 1-3), FL· Kr, FL· Xe, and FL· N, were recorded over the energy range 0-800 cm-I above the electronic origin of S I' The red microscopic spectral shifts of the electronic origins of FL. R (R = Ar, Kr, and Xe) complexes are dominated by dispersive interactions, being proportional to the polarizability of R. The vibrational level structure of FL. R. (R = Ar, Kr, and Xe) complexes exhibits intramolecular vibrational excitations of FL, as well as intermolecular vibrations, which involve the relative motion of FL and R in the complex. The spectra of FL· Ne and FL· N, reveal a rich vibrational structure in the vicinity of the electronic origin, indicating a substantial change of the nuclear configuration upon electronic excitation. Upper and lower bounds on the dissociation energies of FL· R (R = Ne, Kr, and Xe) and FL· Ar, were inferred from the vibrational level structure in the mass-resolved spectra, where the disappearance of the signal of the parent van der Waals ion and the appearance of the ion signal of the fragments mark the onset of the vibrational predissociation process.

In this paper we report the results of an experimental study of collision-free molecular multipho... more In this paper we report the results of an experimental study of collision-free molecular multiphoton dissociation (PJPD) and molecular multiphoton ionization (&@I) of nickelocene @KC loH,o), induced by the light of a tunable dye laser in the wavelength region 3750-5200 A. The spectral dependence of the ion signal reveals-a multitude of narrow (fwhm from CO.5 cm-' to 1.5 cm-' _) Intense peaks superimposed on a very weak background (relative amplitude ratio for peakslback-ground ~10~). The sharp resonances in the icn sig~at are attributed, on the basis of spectroscopic anzdysis, to two-photon resonant three-photon ionization of Ni(I) and to one-photon resonant three-photon ionization of Ni(I), the Ni(I) being produced by MPD of nicketocene. The ion signal in the spectral rarge 3750-3950 A reveals enhanced continuous background due to MPI of nickelocene. This ion signal spectra, together with studies of the intensity dependence of the,overall (nickelocene hlPD)-(Ni(1) MPPI) processes, as well as the (nickelocene molecular MPI) reaction, reveal four reactive processes. (a) Two-photon molecular MPI for hw > 3.10 eV photons. (b) Three-photon molecular hIpI forfiw = 3.10-2.10 eV. (c) Twephoton MPD at hw > 2.86 eV. (d) Three-photon MPD for Aw = 2.8-1.9 eV. The overall Cissociation enerm of nickelocene (Nicpa) to give Ni f 2cp was determined to be 5.76 t 0.60 eV and the nvo-photon ionization potential of this molecule is 6.29 + 0.015 eV. Our results provide dynamic evidence concerning a simultaneous " 'explosive " photodissxiation mechanism of Nicpz by process (c) and for the dominating role of the dissociative channel, characterized by a branching ratio of 250 in favor of predissociation over autoionization, for process (c) at 6.3-6.6 eV. The MPD processes (c) and (d) are expected to exhibit intramolecular erosion of phase coherence effects. Processes (c) and (d) are of high efficiency ~0.01 ions/molecule at saturation exhibited at laser power of =108 tV cm " .

Electrical transport and optical data on amorphous-metal — rare-gas mixtures of Cs-Xe at 6 K are ... more Electrical transport and optical data on amorphous-metal — rare-gas mixtures of Cs-Xe at 6 K are presented. The Cs-Xe mixtures fit a model whereby most of the Cs is distributed randomly on the microscopic scale while a small amount exists in the form of granular clusters. Although preferential clustering occurs, it is not prominent enough to apply the model of classical percolation theory to the metal-nonmetal transition which is exhibited. The transition is located at 0.55 + 0.01 Cs atomic fraction based upon the closure of the optical gap and the threshold of extended-state conduction. The composition range over which the states at the Fermi energy are localized is very small or zero.

Pulsed dye-laser nradlatmn of supcrsorucally cooled hIn2(CO)lo and Fe,(C0)12 m the 3800-5000 A wa... more Pulsed dye-laser nradlatmn of supcrsorucally cooled hIn2(CO)lo and Fe,(C0)12 m the 3800-5000 A wavelength range mduces colluon-free multlphoton dlssoclatlonllo~atlon processes. The IOIUC fragments generated are the metal species M', MC and M; excluswely.

Tunable laser two-photon 1on1za11on oTkugc van dcr Wdsls molcculcs, combmcd wrth tune-of-fl@t mas... more Tunable laser two-photon 1on1za11on oTkugc van dcr Wdsls molcculcs, combmcd wrth tune-of-fl@t mass spectroscopy. WE applied to the idcntfxatlon ol the clcctiomc orlgn and of some low wbnllonal cwtatlons of the Su-S, clcctromc transihon ol fluorcwhr,, fluorcnc AI, and lluorcnc Krr produrcd In supcrsomc c\p.mslons

Chem Phys, 2006

The S1↔S0 vibronic spectra of supersonic jet-cooled 2-pyridone [pyridin-2-one (2PY)] and its N-H ... more The S1↔S0 vibronic spectra of supersonic jet-cooled 2-pyridone [pyridin-2-one (2PY)] and its N-H deuterated isotopomer (d-2PY) have been recorded by two-color resonant two-photon ionization, laser-induced fluorescence and emission, and fluorescence depletion spectroscopies. By combining these methods, the B origin of 2PY at 000+98cm-1 and the bands at +218 and +252cm-1 are identified as overtones of the S1 state out-of-plane vibrations ν1' and ν2', as are the analogous bands of d-2PY. Anharmonic double-minimum potentials are derived for the respective out-of-plane coordinates that predict further ν1' and ν2' overtones and combinations, reproducing ˜80% of the vibronic bands up to 600cm-1 above the 000 band. The fluorescence spectra excited at the electronic origins and the ν1' and ν2' out-of-plane overtone levels confirm these assignments. The S1 nonplanar minima and S1←S0 out-of-plane progressions are in agreement with the determination of nonplanar vibrationally averaged geometries for the 000 and 000+98cm-1 upper states by Held et al. [J. Chem. Phys. 95, 8732 (1991)]. The fluorescence lifetimes of the S1 state vibrations show strong mode dependence: Those of the out-of-plane levels decrease rapidly above 200cm-1 excess vibrational energy, while the in-plane vibrations ν5', ν8', and ν9' have longer lifetimes, although they are above or interspersed with the "dark" out-of-plane states. This is interpreted in terms of an S1' state reaction with a low barrier towards a conical intersection with a prefulvenic geometry. Out-of-plane vibrational states can directly surmount this barrier, whereas in-plane vibrations are much less efficient in this respect. Analysis of the fluorescence spectra allows to identify nine in-plane S0' state fundamentals, overtones of the S0 state ν1″ and ν2″ out-of-plane vibrations, and >30 other overtones and combination bands. The B3LYP /6-311++G(d,p) calculated anharmonic wave numbers are in very good agreement with the observed fundamentals, overtones, and combinations, with a deviation Δrms=1.3%.

Journal of Molecular Structure, 1992

Ab initio electronic structure calculations for phenol and the hydrogen-bonded complexes phenol*H... more Ab initio electronic structure calculations for phenol and the hydrogen-bonded complexes phenol*H,O and d-phenol*D,O were performed at the HartreeFock 4-31G and 6-31G** levels. Both phenol and phenol.H,O were fully structure optimized. Based on the minimumenergy structures so obtained, full normal coordinate analyses were carried out. The resulting harmonic frequencies were scaled and compared to available experimental data. The agreement is satisfactory and allows for an assignment of a majority of the bands observed in the experimental spectra. Comparison with previous calculations on (H,O), reveals a considerable increase in the strength of the hydrogen bond on going from

Blum Carmen Jaggi Michael Liu Shi Xia Leutwyler Samuel Decurtins Silvio Theoretical Study on Annulated Donor Π Acceptor Systems Incorporating Tetrathiafulvalene and Perylenediimide Units Chimia 64 P 507, 2010

The Journal of Physical Chemistry B, May 1, 2007

Benchmark ab initio calculations at the correlated level are reported for ten isomers of the urac... more Benchmark ab initio calculations at the correlated level are reported for ten isomers of the uracil dimer (U.U): six are doubly N-H...O hydrogen bonded, three have a C-H...O and an N-H...O hydrogen bond, and one is doubly C-H...O hydrogen bonded. Their structures were optimized at the correlated level by using second-order Møller-Plesset perturbation theory (MP2), resolution of identity MP2 (RIMP2), and the binding energies D(e) calculated with the coupled-cluster method with singles, doubles, and iterative triples, CCSD(T). The MP2 and RIMP2 binding energies D(e) are extrapolated to the complete basis set (CBS) limit, using the aug-cc-pVXZ (X = D, T, Q) basis sets, giving binding energies accurate to +/-0.07 kcal/mol. With one exception, the correlation energy contributions at the CCSD(T) level increase the binding energies, although the changes are small, +0.03 to -0.27 kcal/mol (or 0.1% to 2.2%). The most stable isomer is the doubly N1-H...O hydrogen-bonded HB4 form, with D(e)[CCSD(T)]= -19.04 kcal/mol. The biologically relevant HB2 dimer has D(e)[CCSD(T)] = -12.64 kcal/mol, and the HB5 dimer that constitutes the main structural motif of the uracil crystal has -13.20 kcal/mol. The &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;quot;Calcutta&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;quot; dimer, which occurs in an RNA hexamer, is among the weakest isomers, with D(e)[CCSD(T)] = -9.81 kcal/mol. We compare to the binding energies calculated with the B3LYP, PBE, and PW91 density functionals; the PW91/6-311++G(d,p) binding energies agree with the CBS benchmark values, to within &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;lt;2%. A useful single-molecule descriptor for the strengths of the individual hydrogen bonds can be derived from the gas-phase acidity DeltaE(0)(A-H) of the N-H or C-H donor groups and the gas-phase proton affinity PA(0)(B) of the C=O groups of the uracil monomer. The calculated hydrogen bond energies D(e)(infinity) correlate well with the difference between gas-phase acidity and basicity, DeltaE(0)(A-H) - PA(0)(B).

Chemistry a European Journal, 2009

Chemical Physics Letters, 1989

Lobsiger Simon Frey Hans Martin Leutwyler Samuel Jet Spectroscopy of the Deoxythymine Tautomers 5 Methyl 2 Hydroxypyrimidine and 5 Methyl 2 Pyrimidinone Chimia 64 P 581, 2010

Journal of Chemical Physics, 1995

ABSTRACT A four-dimensional intermolecular torsional potential energy surface was calculated for ... more ABSTRACT A four-dimensional intermolecular torsional potential energy surface was calculated for the cyclic homodromic water tetramer at the level of second-order Møller-Plesset perturbation theory, using a large basis set (82 basis functions per monomer). The four torsional angles ωi (i = 1–4), which describe the positions of the “free” O–H bonds relative to the planar hydrogen-bonded O⋯O⋯O⋯O framework were varied, while all other internal coordinates were held fixed. Ab initio calculations were performed at 307 points in the asymmetric unit of the torsional space. The points were fitted using the same seven-parameter analytical potential function as previously employed for the torsional surface of the water trimer [Bürgi et al., J. Chem. Phys. 103, 1077 (1995)]. Fits were performed for four interaction energy ranges spanning 1100, 1500, 3500 and 11 500 cm−1 relative to the global minimum, respectively. A number of important stationary points were studied in greater detail and geometry optimized using larger basis sets at the MP2 level. Torsional interconversion pathways are mapped in terms of isopotential surface representations and contour plots. This accurate and rapidly computable potential can be employed to calculate the coupled anharmonic torsional level energies, wave functions and associated molecular properties, presented in the following paper [D. Sabo et al., J. Chem. Phys. 109, 5404 (1998)]. © 1998 American Institute of Physics.

Chem Phys, 1988

Efficient excited-state proton transfer in neutral acid-base clusters α-naphtholṡBn has been dete... more Efficient excited-state proton transfer in neutral acid-base clusters α-naphtholṡBn has been detected and studied by a combination of laser spectroscopic techniques (resonant two-photon ionization, fluorescence excitation, and emission spectroscopy). S1 state proton transfer was observed for B=NH3 and n≥4, as evidenced by several criteria: (a) large red shift and substantial broadening of the R2PI spectra of the n≥4 clusters relative to those of the bare α-naphthol and smaller clusters; (b) very large Stokes shift (˜8000 cm-1) of the emission spectra of the n≥4 clusters; (c) complete broadening of the fluorescence emission band for the n≥4 clusters; and (d) a striking similarity of the emission band position and width of the latter spectra to the emission spectrum of the α-naphtholate anion in basic aqueous solution. No proton-transfer reaction was observed for small solvent clusters with B=NH3 and n≤3, nor for any of the pair complexes studied, which involve a single base partner [B=triethylamine, 3-dimethylamino-1-aminopropane, 1,4-bis(dimethylamino)butane] which we have studied so far. This behavior illustrates the difficulty of achieving charge separation in neutral gas-phase complexes or clusters. A critical gas-phase proton affinity PAcrit =248±3 kcal/mol was determined for proton transfer to take place in the α-naphtholṡBn (or base B) system.

Journal of Physical Chemistry, 1985

The Journal of Chemical Physics, Sep 21, 1999

Calculations of the torsional state dependence of A=B and C rotational constants of (D2O)3 are re... more Calculations of the torsional state dependence of A=B and C rotational constants of (D2O)3 are reported, for torsional energies up to 100 cm-1 above the ground state, extending our work on the rotational constants of all isotopomers of water trimer using ab initio four-dimensional torsional-stretching intermolecular potential-energy surface [D. Sabo et al., J. Phys. Chem. 110, 5745 (1999)]. Direct composition was made to the rotational constants measured by Viant et al. [J. Chem. Phys. 110, 4369 (1999)] for the same nine torsional levels of (D2O)3. In order to consistently reproduce the pronounced variations of the A=B and C rotational constants from one torsional eigenstate to the other, theory must take into account both the changes of rms torsional angles of the "free" O-D bonds and the changes in the oxygen-oxygen separation which accompany torsional excitation. The changes of C with torsional state, ΔC(n), are distinctly non-monotonic; they depend mainly on the rms (root-mean-square) torsional angle and to a lesser degree on the interoxygen distance. The changes of A=B with torsional state, ΔA(n)=ΔB(n), depend on both rms torsional angles and interoxygen distance, but with opposite signs, giving rise to an apparent monotonic decrease with n which is smaller than the changes due to either mechanism alone. The ΔA(n)=ΔB(n) and ΔC(n) changes from the (3+1)D torsional-stretching calculations are in good agreement with experiment up to n=5.

The Journal of Chemical Physics, Sep 8, 1995

Mass-selective ground-state vibrational spectra of jet-cooled carbazoleṡR (R=Ne, Ar, Kr, and Xe) ... more Mass-selective ground-state vibrational spectra of jet-cooled carbazoleṡR (R=Ne, Ar, Kr, and Xe) van der Waals complexes were obtained by populating ground-state intra- and intermolecular levels via stimulated emission pumping, followed by time delayed resonant two-photon ionization of the vibrationally hot complex. By tuning the dump laser frequency, S0 state vibrational modes were accessed from ≊200 cm-1 up to the dissociation energy D0. Upon dumping to ground-state levels above D0, efficient vibrational predissociation of the complexes occurred, allowing us to determine the S0 state van der Waals binding energies very accurately. The D0(S0) values are <214.5±0.5 cm-1 (R=Ne), 530.4±1.5 cm-1 (R=Ar), 687.9±4.0 cm-1 (R=Kr), and 890.8±1.6 cm-1 (R=Xe). In the S1 state, the corresponding binding energies are larger by 9% to 12%, being <222.9±1.0 cm-1, 576.3±1.6 cm-1, 756.4±4.5 cm-1, and 995.8±2.5 cm-1, respectively.