Tapas Chakraborty

Conformations of indan and 2-indanol: A combined study by UV laser spectroscopy and quantum chemistry calculation

Three conformational isomers of 2-indanol are identified by use of resonance enhanced two-photon ionization (R2PI) and single vibronic level dispersed fluorescence spectroscopy in a supersonic jet expansion. By combining the experimental results with the predictions of the ab initio quantum chemistry calculations at the MP2/6-311++G(d,p) level of theory, the major species is identified as a conformational isomer in which the hydroxyl hydrogen is involved in an intramolecular hydrogen bonding with the π-electrons of the aromatic ring. The theoretical estimate of the hydrogen bond energy is ∼6.5 kJ/mol. A comparative investigation with indan reveals that this weak hydrogen bonding in the former significantly affects the puckering potential of the five-member side ring. The dispersed fluorescence data indicate for a much higher ring-puckering barrier in the ground state than what has been suggested recently by measuring rotational spectra of the unsubstituted indan.

Blue Shifting C-H ··· O Hydrogen Bonded Complexes between Chloroform and Small Cyclic Ketones: Ring-Size Effects on Stability and Spectral Shifts

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 (nu(C-H)) in the said three complexes are +1, +2, and +5 cm(-1), respectively, and the complexation results in enhancement of the nu(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 (nu(C=O)) fundamentals of the monomers and corresponding complexes for spectra measured with different chloroform to ketone concentrations. The nu(C=O) 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 nu(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 C=O (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) --> sigma*(C-H) hyperconjugative charge transfer abilities of the three ketones. NBO analysis reveals that the blue shifting of the nu(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.

Cooperative strengthening of an intramolecular O—H···O hydrogen bond by a weak C—H···O counterpart: matrix-isolation infrared spectroscopy and quantum chemical studies on 3-methyl-1,2-cyclohexanedione

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 O-H...O=C hydrogen bond (HB). The nu(O-H) 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 C-H...O HB, where the enolic oxygen is the HB acceptor and one of the C-H 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 approximately 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 (approximately 1 kcal/mol) of enol-2 originates from a cooperative interaction between the interconnected C-H...O and O-H...O HBs. In the IR spectrum, a weak band at 3007 cm(-1) is assigned to nu(C-H) of the methyl C-H 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 C-H stretching frequencies that are not involved in H-bonding. The conclusions are corroborated further by natural bond orbital (NBO) analysis.

Effect of inclusion of cyclodextrin on excited state proton transfer: Carbazole—γ-cyclodextrin

Abstract The excited state proton transfer reaction of carbazole was investigated in the presence of γ-cyclodextrin (γ-CD) using steady state and time-resolved fluorometry. The deprotonation rate was enhanced for the carbazole—γ-CD inclusion complex compared with free carbazole, but the rate of reverse protonation was unaffected.

Vibrational coupling in carboxylic acid dimers

The vibrational level splitting in the ground electronic state of carboxylic acid dimers mediated by the doubly hydrogen-bonded networks are investigated using pure and mixed dimers of benzoic acid with formic acid as molecular prototypes. Within the 0-2000-cm(-1) range, the frequencies for the fundamental and combination vibrations of the two dimers are experimentally measured by using dispersed fluorescence spectroscopy in a supersonic jet expansion. Density-functional-theory calculations predict that most of the dimer vibrations are essentially in-phase and out-of-phase combinations of the monomer modes, and many of such combinations show significantly large splitting in vibrational frequencies. The infrared spectrum of the jet-cooled benzoic acid dimer, reported recently by Bakker et al. [J. Chem. Phys. 119, 11180 (2003)], has been used along with the dispersed fluorescence spectra to analyze the coupled g-u vibrational levels. Assignments of the dispersed fluorescence spectra of the mixed dimer are suggested by comparing the vibronic features with those in the homodimer spectrum and the predictions of density-functional-theory calculation. The fluorescence spectra measured by excitations of the low-lying single vibronic levels of the mixed dimer reveal that the hydrogen-bond vibrations are extensively mixed with the ring modes in the S1 surface.

Fluorescence excitation spectrum of jet-cooled dihydroanthracene

Abstract The lowest excited singlet site of 9,10-dihydroanthracene has been studied by supersonic-jet laser spectroscopy. The spectrum consists of a number of prominent low-frequency vibronic bands near the 0—0 band followed by a structureless continuum. The low-frequency vibronic bands are nicely fitted with a quadratic Gaussian-type potential for the excited stated. The height of the barrier of the double-minimum potential is found to be 94 cm −1 .

Correlation of ν(OH) spectral shifts of phenol-benzene O-H···π hydrogen-bonded complexes with donor's acidity: a combined matrix isolation, infrared spectroscopy, and quantum chemistry study.

O-H stretching infrared fundamentals (νOH) of phenol and a series of fluorophenol monomers and their 1:1 complexes with benzene have been measured under a matrix isolation condition (8 K). Spectral analysis reveals that ring fluorine substitutions have little effect on phenolic νO-H as long as the molecules in the matrix are fully dispersed as monomers. The substitution effects are pronouncedly manifested only when the phenols are complexed with benzene, and the measured shift in phenolic νOH from the monomer value varies from ∼78 cm(-1) in phenol to ∼98 cm(-1) in 3,4,5-trifluorophenol. The spectral shifts are found to display a linear correlation with the aqueous phase acid dissociation constants (pKa) of the phenols. The spectral changes predicted by electronic structure calculations at several levels of theory are found to be consistent with the observations. Such correlations are also found to exist with respect to different energetic, geometric, and other electronic structure parameters of the complexes. Atoms in Molecules (AIM) analysis shows a distinct bond critical point due to accumulation of electron density at the hydrogen-bonding site. The variation of electron densities both on the hydrogen bond as well the donor O-H group is in accordance with the experimentally observed νO-H of the various fluorophenol-benzene complexes. Partitioning of binding energies into components following the Morokuma-Kitaura scheme shows that the π-hydrogen-bonded complexes are stabilized predominantly by dispersion interactions, although electrostatics, polarization, and charge-transfer terms have appreciable contribution to overall binding energies. NBO analysis reveals that hyperconjugative charge-transfers from the filled π-orbitals of the hydrogen bond acceptor (benzene) to the antibonding σ*(O-H) orbital of the donors (phenols) display correlations which are fully consistent with the observed variations of spectral shifts. The analysis also shows that the O-H bond dipole moments of all the phenolic species are nearly the same, implying that local electrostatics has only a little effect at the site of hydrogen bonding.

CH···O interaction lowers hydrogen transfer barrier to keto–enol tautomerization of β-cyclohexanedione: combined infrared spectroscopic and electronic structure calculation study

Molecular association and keto-enol tautomerization of β-cyclohexanedione (β-CHD) have been investigated in argon matrix and also in a thin solid film prepared by depositing pure β-CHD vapor on a cold (8 K) KBr window. Infrared spectra reveal that, in low-pressure vapor and argon matrix, the molecules are exclusively in diketo tautomeric form. The CH···O hydrogen bonded dimers of the diketo tautomer are produced by annealing the matrix at 28 K. No indication is found for keto-enol tautomerization of β-CHD in dimeric complexes in argon matrix within the temperature range of 8-28 K. On the other hand, in thin film of pure diketo tautomer, the conversion initiates only when the film is heated at temperatures above 165 K. The observed threshold appears to be associated with excitation of the intermolecular modes, and the IR spectra recorded at high temperatures display narrowing of vibrational bandwidths, which has been associated with reorientations of the molecules in the film. The nonoccurrence of tautomerization of the matrix isolated dimer is consistent with the barrier predicted by electronic structure calculations at B3LYP/6-311++G** and MP2/6-311++G** levels of theory. The transition state calculation predicts that CH···O interaction has a dramatic effect on lowering of the tautomerization barrier, from more than 60 kcal/mol for the bare molecule to ~35-45 kcal/mol for dimers.

Fluorescence spectroscopy of jet-cooled chiral (±)-indan-1-ol and its cluster with (±)-methyl- and ethyl-lactate

The laser-induced fluorescence excitation, dispersed fluorescence, and IR-UV double resonance spectra of chiral (+/-)-indan-1-ol have been measured in a supersonic expansion. Three low energy conformers of the molecule have been identified, and the ground state vibrational modes of each conformer are tentatively assigned with the aid of quantum chemistry calculations. The frequencies of the nu(OH) and nu(CH) stretch modes of the two most abundant conformers have been measured by fluorescence dip IR spectroscopy and have been used for their assignment. The dispersed fluorescence spectra clearly indicate the coupling of low-frequency modes, as was seen in other substituted indanes such as 1-aminoindan and 1-amino-2-indanol. (R)- and (S)-indan-1-ol distinctly form different types of clusters with (R)- and (S)- methyl- and ethyl-lactate. Both hetero- and homochiral clusters are characterized by complex spectra which exhibit a progression built on low-frequency intermolecular modes.

Observation of exciplex emission from the mixed dimer of naphthalene and 2-methoxynaphthalene: A laser-induced fluorescence study in supersonic jet

Exciplex formation in the van der Waals mixed dimer of naphthalene and 2-methoxynaphthalene has been investigated by using laser-induced fluorescence spectroscopy in supersonic jet. In contrast to the barrierless excimer formation in the S1 state of naphthalene homodimer, the exciplex formation in the mixed dimer occurs with a vibrational energy barrier of 466 cm−1. The energy gap between the S1 states of two molecules in the mixed dimer is 1000 cm−1. Therefore, the contribution of the exciton resonance interaction to the stability of the exciplex state is considered to be insignificant. Charge transfer interaction and the mixing of the charge transfer state with the locally excited state due to orbital overlap between two molecules at suitable geometry are proposed to be the decisive factors in formation of the exciplex.