Nikhil Guchhait

Bio: Nikhil Guchhait is an academic researcher from Indian National Association. The author has contributed to research in topic(s): Excited state. The author has an hindex of 1, co-authored 1 publication(s) receiving 10 citation(s).

Low-frequency vibrations in S1 states of tetrahydroquinoline and N-deuterotetrahydroquinoline in supersonic jet

Abstract: The laser-induced fluorescence excitation (FE) and multiphoton ionisation (MPI) spectra of tetrahydroquinoline (THQ) and N-deuterotetrahydroquinoline (D-THQ), cooled by seeding in a supersonic jet expansion of He, have been investigated in the wavenumber range 32400 to 33200 cm−1. The vibronic origin for the S 0 → S 1 (ππ ∗ ) is found at 32450 cm− in THQ and 32466 cm−1 in D-THQ. The spectra show a prominent progression of a low-frequency vibration of 100 cm−1 which is assigned to the ring puckering “twist” motion in the excited state. There is another frequency close to the puckering “twist” frequency, which has been assigned to the puckering “bend” mode. Comparison of frequencies and intensities with corresponding values calculated from model potential shows that the molecules are twisted in both ground and excited states. An isotope shift of ∼ 16 cm−1 in the 0-0 band of D-THQ is observed and explained on the basis of zero-point energies.

Spectroscopic investigation of tetrahydroisoquinoline in supersonic jet

Abstract: Fluorescence excitation and dispersed emission spectra of tetrahydroisoquinoline are presented here. Two bands at 36 781 and 36 884 cm−1 are confirmed from the spectral hole burning studies as two origins for two conformers. These bands correspond to the inequivalent twist conformers with the hydrogen atom attached to the nitrogen atom at axial and equatorial positions, respectively. The former is found to be the most stable one. SVL spectra are explained on the basis of two theoretically calculated low-frequency vibrations. These correspond to the butterfly and puckering motions of the benzene chromophore, respectively.

Conformational landscape, stability, potential energy curves and vibrations of 1,2,3,4 tetrahydroquinoline

Abstract: The twisted conformer with the equatorial hydrogen of the NH group is computed to be the most stable conformer of tetrahydroquinoline in S0 and S1 states at various levels of quantum chemical computations. The planar structure for equatorial orientation of H atom is found to be higher by 4000 ± 800 cm−1 in S0 and ∼3000 cm−1 in S1. For the axial orientation of H atom the barrier is 9500 ± 300 cm−1 in S0 and ∼8000 cm−1 in S1. The twisting angle for the optimized conformer is 29° ± 2° in S0. Observed IR spectra corroborate well with the theoretical calculations. The characteristic low-frequency vibrations appear at 97 cm−1 and 154 cm−1 in S0. The excited state structure differs from S0 and is termed “half-twist”. Potential energy curves corresponding to twisting and bending motions, as well as the one connecting the two motions are found out. Frontier molecular orbital calculations associated with the electron density plots clearly identify the changes on the density on excitation. Molecular electrostatic potential is calculated and the sites of electrophilic interactions are noted. Hardnesses' and electrophilicities are calculated for all the conformers to check their relative stability. The maximum hardness principle and minimum electrophilicity principles are not valid for most of the conformers. Transition state is found to obey the minimum electrophilicity principle.

Fluorescence excitation spectra, Raman spectra and structure of isochroman in its S1 (π, π∗) state

Abstract: The laser induced fluorescence excitation (FE) spectra of jet-cooled isochroman is used along with the theoretical calculations to assign vibronic levels in 0–500 cm−1 region in the S1 (π, π∗) state. The origin of S0 → S1 transition appears at 36,989 cm−1. Six low-frequency vibrations are identified in the FE spectrum. These vibrations are compared to the corresponding levels in S0. The ab initio DFT calculations show the molecule to be in the twisted structure in both S0 and S1 states with a high barrier to planarity. This barrier is theoretically calculated to be 3572 cm−1 in S0 with the twisting angle as 33° and the corresponding values in S1 are 3916 cm−1 and 31°. Raman spectra are well corroborated by the theoretical calculations. TDDFT calculations are done on the optimized geometries and compared with the experimental results.

Ab initio conformational analysis of 1,2,3,4-tetrahydroquinoline and the high-resolution rotational spectrum of its lowest energy conformer

Abstract: The saturated part of the 1,2,3,4-tetrahydroquinoline (THQ) molecule allows for the possibility of multiple conformers' existence. High-resolution microwave spectroscopy, supported by high-level quantum chemistry calculations, was used to determine the precise molecular structures of the conformers of THQ. Via the MP2 calculations, we were able to discriminate four stable conformations, i.e. two pairs of energetically equivalent enantiomorphic conformers. The results of the calculations also indicate that energetically non-equivalent conformers are separated by a low energy barrier (104 cm−1) that allows for conformational cooling to occur. The high resolution rotational spectrum with resolved hyperfine structure in the frequency range of 7–20 GHz was obtained using both the In-phase/quadrature-phase-Modulation Passage-Acquired-Coherence Technique (IMPACT) and the coaxially oriented beam resonator arrangement (COBRA) to perform Fourier transform microwave (FTMW) spectroscopy. The precise values of the rotational constants, 14N nuclear hyperfine coupling parameters and centrifugal distortion parameters were determined from the measured transition frequencies. Based on our experimental results, only the most stable enantiomeric pair of THQ contributes to the rotational spectrum under the conditions of our experiment as the less stable conformers seem to efficiently relax to the lower energy conformers. Thus the experimentally evaluated molecular constants unambiguously define the lowest energy conformer of 1,2,3,4-tetrahydroquinoline.

Low-frequency large-amplitude vibrations of julolidine in the S0 and S1 states

Abstract: The vibrational modes of julolidine in the ground- and the first-excited states have been investigated by laser-induced fluorescence (LIF), multiphoton ionization (MPI) and dispersed emission spectroscopy. In the observed LIF excitation spectrum the mode 140 cm -1 appears and is assigned to the N-inversion mode of the S 1 state which gets coupled with the ring puckering modes of the fused saturated six-membered ring. A progression of 148 cm -1 intervals appears in the dispersed emission spectra, which on the basis of semi-empirical calculations has been identified as the N-inversion mode of the ground state.