Amit Samanta - Academia.edu (original) (raw)

Papers by Amit Samanta

Non-bonded interactions between a weakly acidic C-H group and an oxygen atom of a functional grou... more Non-bonded interactions between a weakly acidic C-H group and an oxygen atom of a functional group belonging to the same or of a different molecule are categorized as weak hydrogen bonds. Recently, our group has studied a number of dimeric molecular complexes by infrared spectroscopy, where CH--O hydrogen bonding has been invoked to be the dominant binding force for stability of those complexes in ground electronic state. An intriguing spectroscopic attribute of such hydrogen bonding, identified though in a handful of favourable cases, is spectral blue shifting of the donor C-H stretching fundamental. Citing examples of the complexes between various cyclic ketones and haloforms studied by our group, the correlation that has been understood partly between the spectral blue shifts and geometries of the complexes will be discussed. An important issue here is whether such hydrogen bonds are stabilized by cooperative interaction, an underlying feature for excess stability of interconnected classical hydrogen bonds, and the spectral manifestation of this effect for the complexes we have studied will be presented. IR spectroscopy measurements have been performed in the gas phase, in carbontetrachoride solution at different temperatures and by isolating the complexes in cold inert gas matrixes.

Journal of Molecular Structure, 2010

Fourier transform infrared (FTIR) spectroscopy of 1,2-cyclohexanedione (CHD) has been studied wit... more Fourier transform infrared (FTIR) spectroscopy of 1,2-cyclohexanedione (CHD) has been studied with the neat sample, low-pressure vapour, CCl 4 solution and in cold N 2 matrix. The matrix-isolation spectrum has been assigned exclusively in terms of transitions of the enol tautomer, that is stabilized by an intramolecular OÁ Á ÁHAO hydrogen bond (HB). The vibrational fundamentals of the diketo tautomer appear weakly in the spectra of CCl 4 solution and vapour. On the other hand, the spectrum of the neat sample shows significant population of the diketo tautomer. Thus the intermolecular interactions, which are dominant in the neat sample, stabilize the diketo form. The predictions of electronic structure calculations by B3LYP/6-311++G(d,p) and MP2/cc-pVTZ methods are found to be consistent with the measured tautomeric distribution in the cold inert gas matrix. The larger dipole moment of the diketo tautomer (5.61 D) compared to enol form (3.60 D) is proposed to be responsible for stability of the former in the neat sample. The vibrational fundamentals predicted by anharmonic calculation at B3LYP/6-311++G(d,p) level display excellent agreement with measured frequencies. The proposed assignments are further corroborated by noting the deuterium isotopic shifts of different bands and their predicted shifts by the same theoretical methods.

Vibrational Spectroscopy, 2011

Matrix isolation-FTIR spectra of 2-pyrrolidinone monomer and its two hydrogen bonded dimers are r... more Matrix isolation-FTIR spectra of 2-pyrrolidinone monomer and its two hydrogen bonded dimers are recorded in N 2 matrix at 8 K temperature and in CCl 4 solution at variable temperature within the range 30-45 • C. The transient singly hydrogen bonded dimer (SHBD) is identified in the cold N 2 by comparing the matrix spectrum with that in CCl 4 solution. The spectral shifts of both amide-I and amide-A bands indicate cooperative strengthening of hydrogen bonds in doubly hydrogen bonded dimer (DHBD). Density functional theory at B3PW91/6-311++G** level of calculation is found to be in good accord to the observed spectral features.

The vibronically resolved electronic spectra for S(1)&amp... more The vibronically resolved electronic spectra for S(1)<-->S(0) transitions of a mixed dimer between 2-pyridone (2PY) and formamide have been measured in a supersonic free jet expansion using laser-induced fluorescence spectroscopy. Quantum chemistry method at different levels of theory has been used to optimize the geometries of the dimer for the S(0) and S(1) electronic states and also to calculate the normal vibrational modes. Assignments for the vibronic bands observed in the dispersed fluorescence spectrum of the 0(0) (0) band have been suggested with the aid of the ground state frequencies calculated by density functional theoretical method. Spectral analysis reveals that electronic excitation causes extensive mixing of the low-frequency intermolecular vibrational modes of the dimer with some of the intramolecular modes of the 2PY moiety. This spectral behavior is consistent with the complete active space self-consistent field theoretical prediction that with respect to a number of geometrical parameters the dimer geometry in S(1) is significantly distorted from the geometry of the S(0) state.

Matrix isolation infrared spectra of 1,2-cyclohexanedione (1,2-CHD) and 3-methyl-1,2-cyclohexaned... more Matrix isolation infrared spectra of 1,2-cyclohexanedione (1,2-CHD) and 3-methyl-1,2-cyclohexanedione are measured in a nitrogen matrix at 8K temperature. The spectra reveal that in the matrix environment both the molecules exist exclusively in monohydroxy tautomeric forms with an intramolecular O–H⋯O=C hydrogen bonding. In the case of 3-MeCD, the fundamental of OH stretching νO−−H band appears more red-shifted with larger bandwidth indicating that the intramolecular O–H⋯O hydrogen bond of this molecule is somewhat stronger compared to that of 1,2-CD. Electronic structure calculations at B3LYP/6-311++G∗∗ and MP2/cc-pVTZ levels predict that the monohydroxy tautomer of 1,2-CD is nearly 4.5 kcal/mol more stable than the corresponding diketo tautomer, but in the case of 3-MeCD, the stability difference between the diketo and preferred enol tautomer is more than 7.5 kcal/mol. Analysis of the geometric parameters reveals that the excess stabilization of the latter originates as a result of formation of an intramolecular O⋯H–O⋯H–C type interconnected hydrogen bonding network involving a methyl C-H bond, which interact in a cooperative fashion. The predicted infrared spectrum shows that the formation of such hydrogen bonding network causes large blue-shifting of the H-bonded methyl νC−−H transition, and this spectral prediction matches well with the features displayed in the measured spectrum.For intermolecular case, 1:1 complex between 1,2-cyclohexanedione and chloroform have been studied. Here two types of complex is possible, interconnected and bifurcated. In the interconnected complex a cooperative stabilizing effect and in the bifurcated complex an anti-cooperative destabilizing effect of the C–H⋯O hydrogen bond on the intramolecular O–H⋯O bond is observable. In the room temperature solution phase of FTIR spectra, the anti-cooperative complex is observable.

Journal of Physical Chemistry A, 2007

We report here the laser induced fluorescence excitation (FE) and dispersed fluorescence (DF) spe... more We report here the laser induced fluorescence excitation (FE) and dispersed fluorescence (DF) spectra of a 1:1 mixed dimer between 7-azaindole (7AI) and 2-pyridone (2PY) measured in a supersonic free jet expansion of helium. Density functional theoretical calculation at the B3LYP/6-311++G** level has been performed for predictions of the dimer geometry and normal mode vibrational frequencies in the ground electronic state. A planar doubly hydrogen-bonded structure has been predicted to be the most preferred geometry of the dimer. In the FE spectrum, sharp vibronic bands are observed only for excitation of the 2PY moiety. A large number of low-frequency vibronic bands show up in both the FE and DF spectra, and those bands have been assigned to in-plane hydrogen bond vibrations of the dimer. Spectral analyses reveal Duschinsky-type mixing among those modes in the excited state. No distinct vibronic band structure in the FE spectrum was observed corresponding to excitations of the 7AI moiety, and the observation has been explained in terms of nonradiative electronic relaxation routes involving the 2PY moiety.

Photodissociation (PD) is a natural means for removal of volatile organic materials from atmosphe... more Photodissociation (PD) is a natural means for removal of volatile organic materials from atmosphere. We have studied UV-PD reactions (excitation wavelengths = 254 and 266 nm) of various cyclic and acyclic ketones in the gas phase, and studied also how the reaction channels are altered when the molecules are embedded in cold inert gas matrixes. The gas-phase PD products are characterized by quadrupole mass spectrometry followed by low-energy electron-impact ionization and also by gas-phase FTIR spectroscopy of irradiated gases. PD yields are estimated by measuring the depletion of parent molecular ion signal following introduction of UV laser pulses into molecular beam before ionization and comparing the results with respect to a standard. The mechanisms of PD reactions are suggested by identifying the transient intermediates by mass spectrometry, and energy partitioning among the PD products are estimated by analyzing the shapes of the time-of-flight signals.

Cooperativity is an important attribute of interconnected hydrogen bonds, and the classic example... more Cooperativity is an important attribute of interconnected hydrogen bonds, and the classic examples are the size and shape dependent stability of small water clusters. The effect involving C-H-O hydrogen bonds has been theoretically predicted and inferred also by analyzing crystallographic data. However, direct demonstration by infrared spectroscopy, in terms of spectral shifts, is scarce. We report here such effect probing the spectral shifts of various stretching fundamentals associated with an intramolecular O-H-O=C linkage of the tautomer of 1,2-cyclohexanedione. Two types of interconnected C-H-O bonds, intra- and intermolecular, with the above linkage are generated. In the first case, we have used 3-methyl 1,2-cyclohexane dione and spectral measurements reveal that the cooperative stabilization occurs with displaying a blue-shifting of the C-H stretching fundamental of the hydrogen-bonded methyl C-H group. For the intermolecular case, a 1:1 complex between 1,2-cyclohexanedione and chloroform has been used, and the complexation results further red-shifting of the O-H stretching fundamental and a concomitant blue-shifting of the C=O stretching fundamental. The details of the spectral effects are corroborated by predictions of electronic structure calculations.

Journal of Molecular Structure, 2011

Mid-infrared spectra of 3-methyl-1,2-cyclopentanedione (3-MeCPD) have been recorded by isolating ... more Mid-infrared spectra of 3-methyl-1,2-cyclopentanedione (3-MeCPD) have been recorded by isolating the molecule in a cold argon matrix (8 K) and also in CCl 4 solution at room temperature. The spectral features reveal that in both media, the molecule exists exclusively in an enol tautomeric form, which is stabilized by an intramolecular OAHÁ Á ÁO hydrogen bond. NBO analysis shows that the preferred conformer is further stabilized because of hyperconjugation interaction between the methyl and vinyl group of the enol tautomer. In CCl 4 solution, the molecule undergoes extensive self association and generates a doubly hydrogen bonded centrosymmetric dimer. The dimerization constant (K d ) is estimated to have a value of $9 L mol À1 at room temperature (25°C) and the thermodynamic parameters, DH°, DS°and DG°, of dimerization are estimated by measuring K d at several temperatures within the range 22-60°C. The same dimer is also produced when the matrix is annealed at a higher temperature. In addition, a non-centrosymmetric singly hydrogen bonded dimer is also identified in the argon matrix. A comparison between the spectral features of the two dimers indicates that the dimerization effect on doubly H-bonded case is influenced by cooperative interaction between the two H-bonds.

Chemical Physics Letters, 2005

Three conformational isomers of o-fluorobenzoic acid dimer have been identified in a supersonic j... more Three conformational isomers of o-fluorobenzoic acid dimer have been identified in a supersonic jet expansion by use of laser-induced fluorescence excitation and dispersed fluorescence spectroscopy. Using a mixed dimer of o-fluorobenzoic acid with benzoic acid, we have provided further evidence that the three isomers in the previous case originate due to two distinct internal rotational isomeric forms of each of the dimer moiety. Relative stability and geometries of all the dimers are computed by use of DFT theoretical method. The resolved fluorescence spectra of the three isomeric homodimers have been tentatively assigned by correlating the observed frequencies with the normal mode frequencies of the isomers predicted by DFT calculation.

Journal of Physical Chemistry A, 2010

Matrix-isolation infrared spectra of 1,2-cyclohexanedione (CD) and 3-methyl-1,2-cyclohexanedione ... more Matrix-isolation infrared spectra of 1,2-cyclohexanedione (CD) and 3-methyl-1,2-cyclohexanedione (3-MeCD) were measured in a nitrogen matrix at 8 K. The spectral features reveal that, in the matrix environment, both molecules exist exclusively in the monohydroxy tautomeric form, which is stabilized by an intramolecular OsH · · · OdC hydrogen bond (HB). The ν OsH band of the enol tautomer of 3-MeCD appears at a relatively lower frequency and displays a somewhat broader bandwidth compared to that of CD, and these spectral differences between the two molecules are interpreted as being due to the formation of an interconnected CsH · · · O HB, where the enolic oxygen is the HB acceptor and one of the CsH covalent bonds of the methyl group is the HB donor. Electronic structure calculations at the B3LYP/6-311++G**, MP2/6-311++G**, and MP2/cc-pVTZ levels predict that this tautomer (enol-2) is ∼3.5 kcal/mol more stable than a second enolic form (enol-1) where such interconnected H-bonding is absent. Theoretical analysis with a series of molecules having similar functional groups reveals that part of the excess stability (∼1 kcal/mol) of enol-2 originates from a cooperative interaction between the interconnected CsH · · · O and OsH · · · O HBs. In the IR spectrum, a weak band at 3007 cm -1 is assigned to ν CsH of the methyl CsH bond involved in the H-bonded network. The spectra predicted by both harmonic and anharmonic calculations reveal that this transition is largely blue-shifted compared to the fundamentals of the other two methyl CsH stretching frequencies that are not involved in H-bonding. The conclusions are corroborated further by natural bond orbital (NBO) analysis.

Journal of Molecular Structure, 2010

Singly and doubly hydrogen-bonded dimers of d-valerolactam, a biologically important cyclic amide... more Singly and doubly hydrogen-bonded dimers of d-valerolactam, a biologically important cyclic amide, are studied by matrix isolation FTIR spectroscopy and quantum chemistry methods. The spectral features of the doubly hydrogen-bonded dimer (DHBD) are nearly similar to those displayed by the dimer produced in CCl 4 solution at room temperature. On the other hand, the matrix environment allows identification of singly hydrogen-bonded dimer (SHBD) which is a transient species in the solution and gas phases. Identification of the two species are made by noting the spectral shifts of amide-I and amide-A bands corresponding to the two dimers and observed changes are consistent with predictions of electronic structure theory calculations. A cooperative strengthening of hydrogen bond is manifested in the large increment of C@O and NAH bond lengths in DHBD, as compared to those in SHBD.

Journal of Physical Chemistry A, 2009

Blue-shifting C-H · · · O hydrogen bonded complexes between chloroform and three small cyclic ket... more Blue-shifting C-H · · · O hydrogen bonded complexes between chloroform and three small cyclic ketones (cyclohexanone, cyclopentanone, and cyclobutanone) have been identified by use of FTIR spectroscopy in CCl 4 solution at room temperature. The shifts of the C-H stretching fundamental of chloroform (ν C-H ) in the said three complexes are +1, +2, and +5 cm -1 , respectively, and the complexation results in enhancement of the ν C-H transition intensity in all three cases. The 1:1 stoichiometry of the complexes is suggested by identifying distinct isosbestic points between the carbonyl stretching (ν CdO ) fundamentals of the monomers and corresponding complexes for spectra measured with different chloroform to ketone concentrations. The ν CdO bands in the three complexes are red-shifted by 8, 19, and 6 cm -1 , and apparently have no correlation with the respective blue shifts of the ν C-H bands. Spectral analysis reveals that the complex with cyclohexanone is most stable, and the stability decreases with the ring size of the cyclic ketones. A qualitative explanation of the relative stabilities of the complexes is presented by correlating the hydrogen bond acceptor abilities of the carbonyl groups with the ring size of the cyclic ketones. Quantum mechanical calculations at the DFT/ B3LYP/6-311++G(d,p) and MP2/6-31+G(d) levels were performed for predictions of the shapes of the complexes, electronic structure parameters of C-H (donor) and CdO (acceptor) groups, intermolecular interaction energies, spectral shifts, and evolution of those properties when the hydrogen bond distance between the donor-acceptor moieties is scanned. The results show that the binding energies of the complexes are correlated with the dipole moments, proton affinity, and n(O) f σ*(C-H) hyperconjugative charge transfer abilities of the three ketones. NBO analysis reveals that the blue shifting of the ν C-H transition in a complex is the net effect of hyperconjugation and repolarization/rehybridization of the bond under the influence of the electric field of carbonyl oxygen.

Chemical Physics Letters, 2005

... Below, we present additional evidences in support of this assignment by measuring the FE and ... more ... Below, we present additional evidences in support of this assignment by measuring the FE and DF spectra of the mixed dimer between o-FBA and BA. ... The remaining bands are due to absorptions of the mixed dimers between BA and o-FBA (see Fig. 2). ...

Cyclic β-diketo compounds are known to show markedly different physical and chemical properties c... more Cyclic β-diketo compounds are known to show markedly different physical and chemical properties compared to their linear counterparts. 1,3-cyclohexanedione, the simplest molecule among the cyclic variants was found to exist exclusively in ketoenolic form in crystal whereas appreciable amount of diketo tautomer was identified in chloroform solution. We have studied this system by means of infrared spectroscopy to elucidate its tautomeric behavior under different environmental as well as thermal conditions ranging from solid argon matrix at 8 K to carbon tetrachloride and chloroform solution at room temperature and low pressure vapor at 330 K. Besides, we have monitored its homodimeric complexes and the effect of weak C-H---O hydrogen bonding on the keto-enol tautomeric equilibria. The potential energy surface of the ground electronic state has been computed by means of electronic structure calculation to corroborate the experimental findings.

Non-bonded interactions between a weakly acidic C-H group and an oxygen atom of a functional grou... more Non-bonded interactions between a weakly acidic C-H group and an oxygen atom of a functional group belonging to the same or of a different molecule are categorized as weak hydrogen bonds. Recently, our group has studied a number of dimeric molecular complexes by infrared spectroscopy, where CH--O hydrogen bonding has been invoked to be the dominant binding force for stability of those complexes in ground electronic state. An intriguing spectroscopic attribute of such hydrogen bonding, identified though in a handful of favourable cases, is spectral blue shifting of the donor C-H stretching fundamental. Citing examples of the complexes between various cyclic ketones and haloforms studied by our group, the correlation that has been understood partly between the spectral blue shifts and geometries of the complexes will be discussed. An important issue here is whether such hydrogen bonds are stabilized by cooperative interaction, an underlying feature for excess stability of interconnected classical hydrogen bonds, and the spectral manifestation of this effect for the complexes we have studied will be presented. IR spectroscopy measurements have been performed in the gas phase, in carbontetrachoride solution at different temperatures and by isolating the complexes in cold inert gas matrixes.

Journal of Molecular Structure, 2010

Fourier transform infrared (FTIR) spectroscopy of 1,2-cyclohexanedione (CHD) has been studied wit... more Fourier transform infrared (FTIR) spectroscopy of 1,2-cyclohexanedione (CHD) has been studied with the neat sample, low-pressure vapour, CCl 4 solution and in cold N 2 matrix. The matrix-isolation spectrum has been assigned exclusively in terms of transitions of the enol tautomer, that is stabilized by an intramolecular OÁ Á ÁHAO hydrogen bond (HB). The vibrational fundamentals of the diketo tautomer appear weakly in the spectra of CCl 4 solution and vapour. On the other hand, the spectrum of the neat sample shows significant population of the diketo tautomer. Thus the intermolecular interactions, which are dominant in the neat sample, stabilize the diketo form. The predictions of electronic structure calculations by B3LYP/6-311++G(d,p) and MP2/cc-pVTZ methods are found to be consistent with the measured tautomeric distribution in the cold inert gas matrix. The larger dipole moment of the diketo tautomer (5.61 D) compared to enol form (3.60 D) is proposed to be responsible for stability of the former in the neat sample. The vibrational fundamentals predicted by anharmonic calculation at B3LYP/6-311++G(d,p) level display excellent agreement with measured frequencies. The proposed assignments are further corroborated by noting the deuterium isotopic shifts of different bands and their predicted shifts by the same theoretical methods.

Vibrational Spectroscopy, 2011

Matrix isolation-FTIR spectra of 2-pyrrolidinone monomer and its two hydrogen bonded dimers are r... more Matrix isolation-FTIR spectra of 2-pyrrolidinone monomer and its two hydrogen bonded dimers are recorded in N 2 matrix at 8 K temperature and in CCl 4 solution at variable temperature within the range 30-45 • C. The transient singly hydrogen bonded dimer (SHBD) is identified in the cold N 2 by comparing the matrix spectrum with that in CCl 4 solution. The spectral shifts of both amide-I and amide-A bands indicate cooperative strengthening of hydrogen bonds in doubly hydrogen bonded dimer (DHBD). Density functional theory at B3PW91/6-311++G** level of calculation is found to be in good accord to the observed spectral features.

The vibronically resolved electronic spectra for S(1)&amp... more The vibronically resolved electronic spectra for S(1)<-->S(0) transitions of a mixed dimer between 2-pyridone (2PY) and formamide have been measured in a supersonic free jet expansion using laser-induced fluorescence spectroscopy. Quantum chemistry method at different levels of theory has been used to optimize the geometries of the dimer for the S(0) and S(1) electronic states and also to calculate the normal vibrational modes. Assignments for the vibronic bands observed in the dispersed fluorescence spectrum of the 0(0) (0) band have been suggested with the aid of the ground state frequencies calculated by density functional theoretical method. Spectral analysis reveals that electronic excitation causes extensive mixing of the low-frequency intermolecular vibrational modes of the dimer with some of the intramolecular modes of the 2PY moiety. This spectral behavior is consistent with the complete active space self-consistent field theoretical prediction that with respect to a number of geometrical parameters the dimer geometry in S(1) is significantly distorted from the geometry of the S(0) state.

Matrix isolation infrared spectra of 1,2-cyclohexanedione (1,2-CHD) and 3-methyl-1,2-cyclohexaned... more Matrix isolation infrared spectra of 1,2-cyclohexanedione (1,2-CHD) and 3-methyl-1,2-cyclohexanedione are measured in a nitrogen matrix at 8K temperature. The spectra reveal that in the matrix environment both the molecules exist exclusively in monohydroxy tautomeric forms with an intramolecular O–H⋯O=C hydrogen bonding. In the case of 3-MeCD, the fundamental of OH stretching νO−−H band appears more red-shifted with larger bandwidth indicating that the intramolecular O–H⋯O hydrogen bond of this molecule is somewhat stronger compared to that of 1,2-CD. Electronic structure calculations at B3LYP/6-311++G∗∗ and MP2/cc-pVTZ levels predict that the monohydroxy tautomer of 1,2-CD is nearly 4.5 kcal/mol more stable than the corresponding diketo tautomer, but in the case of 3-MeCD, the stability difference between the diketo and preferred enol tautomer is more than 7.5 kcal/mol. Analysis of the geometric parameters reveals that the excess stabilization of the latter originates as a result of formation of an intramolecular O⋯H–O⋯H–C type interconnected hydrogen bonding network involving a methyl C-H bond, which interact in a cooperative fashion. The predicted infrared spectrum shows that the formation of such hydrogen bonding network causes large blue-shifting of the H-bonded methyl νC−−H transition, and this spectral prediction matches well with the features displayed in the measured spectrum.For intermolecular case, 1:1 complex between 1,2-cyclohexanedione and chloroform have been studied. Here two types of complex is possible, interconnected and bifurcated. In the interconnected complex a cooperative stabilizing effect and in the bifurcated complex an anti-cooperative destabilizing effect of the C–H⋯O hydrogen bond on the intramolecular O–H⋯O bond is observable. In the room temperature solution phase of FTIR spectra, the anti-cooperative complex is observable.

Journal of Physical Chemistry A, 2007

We report here the laser induced fluorescence excitation (FE) and dispersed fluorescence (DF) spe... more We report here the laser induced fluorescence excitation (FE) and dispersed fluorescence (DF) spectra of a 1:1 mixed dimer between 7-azaindole (7AI) and 2-pyridone (2PY) measured in a supersonic free jet expansion of helium. Density functional theoretical calculation at the B3LYP/6-311++G** level has been performed for predictions of the dimer geometry and normal mode vibrational frequencies in the ground electronic state. A planar doubly hydrogen-bonded structure has been predicted to be the most preferred geometry of the dimer. In the FE spectrum, sharp vibronic bands are observed only for excitation of the 2PY moiety. A large number of low-frequency vibronic bands show up in both the FE and DF spectra, and those bands have been assigned to in-plane hydrogen bond vibrations of the dimer. Spectral analyses reveal Duschinsky-type mixing among those modes in the excited state. No distinct vibronic band structure in the FE spectrum was observed corresponding to excitations of the 7AI moiety, and the observation has been explained in terms of nonradiative electronic relaxation routes involving the 2PY moiety.

Photodissociation (PD) is a natural means for removal of volatile organic materials from atmosphe... more Photodissociation (PD) is a natural means for removal of volatile organic materials from atmosphere. We have studied UV-PD reactions (excitation wavelengths = 254 and 266 nm) of various cyclic and acyclic ketones in the gas phase, and studied also how the reaction channels are altered when the molecules are embedded in cold inert gas matrixes. The gas-phase PD products are characterized by quadrupole mass spectrometry followed by low-energy electron-impact ionization and also by gas-phase FTIR spectroscopy of irradiated gases. PD yields are estimated by measuring the depletion of parent molecular ion signal following introduction of UV laser pulses into molecular beam before ionization and comparing the results with respect to a standard. The mechanisms of PD reactions are suggested by identifying the transient intermediates by mass spectrometry, and energy partitioning among the PD products are estimated by analyzing the shapes of the time-of-flight signals.

Cooperativity is an important attribute of interconnected hydrogen bonds, and the classic example... more Cooperativity is an important attribute of interconnected hydrogen bonds, and the classic examples are the size and shape dependent stability of small water clusters. The effect involving C-H-O hydrogen bonds has been theoretically predicted and inferred also by analyzing crystallographic data. However, direct demonstration by infrared spectroscopy, in terms of spectral shifts, is scarce. We report here such effect probing the spectral shifts of various stretching fundamentals associated with an intramolecular O-H-O=C linkage of the tautomer of 1,2-cyclohexanedione. Two types of interconnected C-H-O bonds, intra- and intermolecular, with the above linkage are generated. In the first case, we have used 3-methyl 1,2-cyclohexane dione and spectral measurements reveal that the cooperative stabilization occurs with displaying a blue-shifting of the C-H stretching fundamental of the hydrogen-bonded methyl C-H group. For the intermolecular case, a 1:1 complex between 1,2-cyclohexanedione and chloroform has been used, and the complexation results further red-shifting of the O-H stretching fundamental and a concomitant blue-shifting of the C=O stretching fundamental. The details of the spectral effects are corroborated by predictions of electronic structure calculations.

Journal of Molecular Structure, 2011

Mid-infrared spectra of 3-methyl-1,2-cyclopentanedione (3-MeCPD) have been recorded by isolating ... more Mid-infrared spectra of 3-methyl-1,2-cyclopentanedione (3-MeCPD) have been recorded by isolating the molecule in a cold argon matrix (8 K) and also in CCl 4 solution at room temperature. The spectral features reveal that in both media, the molecule exists exclusively in an enol tautomeric form, which is stabilized by an intramolecular OAHÁ Á ÁO hydrogen bond. NBO analysis shows that the preferred conformer is further stabilized because of hyperconjugation interaction between the methyl and vinyl group of the enol tautomer. In CCl 4 solution, the molecule undergoes extensive self association and generates a doubly hydrogen bonded centrosymmetric dimer. The dimerization constant (K d ) is estimated to have a value of $9 L mol À1 at room temperature (25°C) and the thermodynamic parameters, DH°, DS°and DG°, of dimerization are estimated by measuring K d at several temperatures within the range 22-60°C. The same dimer is also produced when the matrix is annealed at a higher temperature. In addition, a non-centrosymmetric singly hydrogen bonded dimer is also identified in the argon matrix. A comparison between the spectral features of the two dimers indicates that the dimerization effect on doubly H-bonded case is influenced by cooperative interaction between the two H-bonds.

Chemical Physics Letters, 2005

Three conformational isomers of o-fluorobenzoic acid dimer have been identified in a supersonic j... more Three conformational isomers of o-fluorobenzoic acid dimer have been identified in a supersonic jet expansion by use of laser-induced fluorescence excitation and dispersed fluorescence spectroscopy. Using a mixed dimer of o-fluorobenzoic acid with benzoic acid, we have provided further evidence that the three isomers in the previous case originate due to two distinct internal rotational isomeric forms of each of the dimer moiety. Relative stability and geometries of all the dimers are computed by use of DFT theoretical method. The resolved fluorescence spectra of the three isomeric homodimers have been tentatively assigned by correlating the observed frequencies with the normal mode frequencies of the isomers predicted by DFT calculation.

Journal of Physical Chemistry A, 2010

Matrix-isolation infrared spectra of 1,2-cyclohexanedione (CD) and 3-methyl-1,2-cyclohexanedione ... more Matrix-isolation infrared spectra of 1,2-cyclohexanedione (CD) and 3-methyl-1,2-cyclohexanedione (3-MeCD) were measured in a nitrogen matrix at 8 K. The spectral features reveal that, in the matrix environment, both molecules exist exclusively in the monohydroxy tautomeric form, which is stabilized by an intramolecular OsH · · · OdC hydrogen bond (HB). The ν OsH band of the enol tautomer of 3-MeCD appears at a relatively lower frequency and displays a somewhat broader bandwidth compared to that of CD, and these spectral differences between the two molecules are interpreted as being due to the formation of an interconnected CsH · · · O HB, where the enolic oxygen is the HB acceptor and one of the CsH covalent bonds of the methyl group is the HB donor. Electronic structure calculations at the B3LYP/6-311++G**, MP2/6-311++G**, and MP2/cc-pVTZ levels predict that this tautomer (enol-2) is ∼3.5 kcal/mol more stable than a second enolic form (enol-1) where such interconnected H-bonding is absent. Theoretical analysis with a series of molecules having similar functional groups reveals that part of the excess stability (∼1 kcal/mol) of enol-2 originates from a cooperative interaction between the interconnected CsH · · · O and OsH · · · O HBs. In the IR spectrum, a weak band at 3007 cm -1 is assigned to ν CsH of the methyl CsH bond involved in the H-bonded network. The spectra predicted by both harmonic and anharmonic calculations reveal that this transition is largely blue-shifted compared to the fundamentals of the other two methyl CsH stretching frequencies that are not involved in H-bonding. The conclusions are corroborated further by natural bond orbital (NBO) analysis.

Journal of Molecular Structure, 2010

Singly and doubly hydrogen-bonded dimers of d-valerolactam, a biologically important cyclic amide... more Singly and doubly hydrogen-bonded dimers of d-valerolactam, a biologically important cyclic amide, are studied by matrix isolation FTIR spectroscopy and quantum chemistry methods. The spectral features of the doubly hydrogen-bonded dimer (DHBD) are nearly similar to those displayed by the dimer produced in CCl 4 solution at room temperature. On the other hand, the matrix environment allows identification of singly hydrogen-bonded dimer (SHBD) which is a transient species in the solution and gas phases. Identification of the two species are made by noting the spectral shifts of amide-I and amide-A bands corresponding to the two dimers and observed changes are consistent with predictions of electronic structure theory calculations. A cooperative strengthening of hydrogen bond is manifested in the large increment of C@O and NAH bond lengths in DHBD, as compared to those in SHBD.

Journal of Physical Chemistry A, 2009

Blue-shifting C-H · · · O hydrogen bonded complexes between chloroform and three small cyclic ket... more Blue-shifting C-H · · · O hydrogen bonded complexes between chloroform and three small cyclic ketones (cyclohexanone, cyclopentanone, and cyclobutanone) have been identified by use of FTIR spectroscopy in CCl 4 solution at room temperature. The shifts of the C-H stretching fundamental of chloroform (ν C-H ) in the said three complexes are +1, +2, and +5 cm -1 , respectively, and the complexation results in enhancement of the ν C-H transition intensity in all three cases. The 1:1 stoichiometry of the complexes is suggested by identifying distinct isosbestic points between the carbonyl stretching (ν CdO ) fundamentals of the monomers and corresponding complexes for spectra measured with different chloroform to ketone concentrations. The ν CdO bands in the three complexes are red-shifted by 8, 19, and 6 cm -1 , and apparently have no correlation with the respective blue shifts of the ν C-H bands. Spectral analysis reveals that the complex with cyclohexanone is most stable, and the stability decreases with the ring size of the cyclic ketones. A qualitative explanation of the relative stabilities of the complexes is presented by correlating the hydrogen bond acceptor abilities of the carbonyl groups with the ring size of the cyclic ketones. Quantum mechanical calculations at the DFT/ B3LYP/6-311++G(d,p) and MP2/6-31+G(d) levels were performed for predictions of the shapes of the complexes, electronic structure parameters of C-H (donor) and CdO (acceptor) groups, intermolecular interaction energies, spectral shifts, and evolution of those properties when the hydrogen bond distance between the donor-acceptor moieties is scanned. The results show that the binding energies of the complexes are correlated with the dipole moments, proton affinity, and n(O) f σ*(C-H) hyperconjugative charge transfer abilities of the three ketones. NBO analysis reveals that the blue shifting of the ν C-H transition in a complex is the net effect of hyperconjugation and repolarization/rehybridization of the bond under the influence of the electric field of carbonyl oxygen.

Chemical Physics Letters, 2005

... Below, we present additional evidences in support of this assignment by measuring the FE and ... more ... Below, we present additional evidences in support of this assignment by measuring the FE and DF spectra of the mixed dimer between o-FBA and BA. ... The remaining bands are due to absorptions of the mixed dimers between BA and o-FBA (see Fig. 2). ...

Cyclic β-diketo compounds are known to show markedly different physical and chemical properties c... more Cyclic β-diketo compounds are known to show markedly different physical and chemical properties compared to their linear counterparts. 1,3-cyclohexanedione, the simplest molecule among the cyclic variants was found to exist exclusively in ketoenolic form in crystal whereas appreciable amount of diketo tautomer was identified in chloroform solution. We have studied this system by means of infrared spectroscopy to elucidate its tautomeric behavior under different environmental as well as thermal conditions ranging from solid argon matrix at 8 K to carbon tetrachloride and chloroform solution at room temperature and low pressure vapor at 330 K. Besides, we have monitored its homodimeric complexes and the effect of weak C-H---O hydrogen bonding on the keto-enol tautomeric equilibria. The potential energy surface of the ground electronic state has been computed by means of electronic structure calculation to corroborate the experimental findings.