Vipin B. Singh

Spectroscopic studies and normal coordinate analysis of bilirubin

The infrared spectrum of bilirubin has been recorded in the spectral region 200-4000 cm(-1). The Raman spectrum has also been recorded using the second harmonic (530 nm) radiation of a 200 mW Nd-YAG laser. In order to confirm the vibrational assignment of the bands obtained from experimental observation, a normal coordinate analysis has been carried out using the semi-empirical AM1 method through MOPAC 5.1 computer program. Electronic absorption spectrum of bilirubin dissolved in CHCl3 has been recorded in the spectral region 300-600 nm. A broad spectrum is observed with peak maxima at 454.2 nm. The photoacoustic spectrum of this molecule (in the powder form) has also been recorded for the first time which shows certain discrete features.

Ab initio and DFT studies of the structure and vibrational spectra of anhydrous caffeine

Vibrational spectra and molecular structure of anhydrous caffeine have been systematically investigated by second order Moller-Plesset (MP2) perturbation theory and density functional theory (DFT) calculations. Vibrational assignments have been made and many previous ambiguous assignments in IR and Raman spectra are amended. The calculated DFT frequencies and intensities at B3LYP/6-311++G(2d,2p) level, were found to be in better agreement with the experimental values. It was found that DFT with B3LYP functional predicts harmonic vibrational wave numbers more close to experimentally observed value when it was performed on MP2 optimized geometry rather than DFT geometry. The calculated TD-DFT vertical excitation electronic energies of the valence excited states of anhydrous caffeine are found to be in consonance to the experimental absorption peaks.

Vibrational spectra of 2 (3H) benzofuranone studied by Raman, IR spectroscopy and AM1 semiempirical molecular orbital calculations

Raman spectra of 2 (3H) benzofuranone have been recorded in the region 400-3200 cm(-1) and the IR spectra have been recorded in the region 200-4000 cm(-1). Vibrational frequencies for the fundamental modes of this bicyclic heteroatomic molecule have also been calculated using Austin method 1 (AM1) semiempirical molecular orbital method. Vibrational assignments have been made for the fundamental modes and the observed combination and overtone bands are also assigned. A splitting in the carbonyl group (C=O stretching) frequency observed at 1640-1660 cm(-1) in both Raman and IR spectra, is explained as Fermi-resonance. Net atomic charges for each atom of this molecule along with its heat of formation were also calculated. It is evident from the calculations that the 2 (3H) benzofuranone is more stable than the 3 (2H) benzofuranone in contrast to earlier estimates.

Spectroscopic signatures and structural motifs in isolated and hydrated serotonin: a computational study

The conformational landscapes of neutral serotonin and its hydrated complex were characterized by MP2, CC2 and DFT methods. The ground state geometry optimization of the twenty three lowest energy structures of serotonin have been performed employing higher basis sets. The MP2, CC2 and DFT (M06-2X, ωB97X-D and B3LYP-D3) calculations predict that the Gph-out/anti conformation of serotonin is the most stable which is in agreement with the experimental rotational spectroscopy (Cabezas et al., Phys. Chem. Chem. Phys., 2012, 14, 13618) and is in contrast to the resonance-enhanced two photon ionization (R2PI) and UV–UV hole burning (UVHB) spectroscopy results. Computed wave-numbers and intensities of the observed conformers are found in consonance with the experiment (LeGreve et al., J. Am. Chem. Soc., 2007, 129, 4028). The predicted intensity of the OH bending fundamental provides a useful diagnostic for the 5-OH anti and syn conformations. The computed hydrogen bond geometries of the experimentally observed Sero1–(H2O)1 and Sero1–(H2O)2 clusters are found in remarkable agreement with the experiment. The Sero1–(H2O)2 involving the Gph(out) conformation is used to form a strong water dimer bridge to the 5-OH with a binding energy of 104 kJ mol−1. The low-lying excited states of each experimentally observed conformer of serotonin have been determined by means of coupled cluster singles and approximate doubles (CC2) and TDDFT methods and a satisfactory interpretation of the electronic absorption spectra is obtained. One striking feature is the coexistence of the blue and red shift of the vertical excitation energies of the 1Lb (ππ*) and the 1La (ππ*) state upon forming a complex with water. The effect of hydration on the lowest 1Lb (ππ*) excited state due to the bulk water environment was mimicked by a combination of a polarizable continuum solvent model (PCM) and conductor like screening model (COSMO), for the different serotonin conformations which shows a red shift. The lowest excited state (1Lb) of the most stable Sero1–(H2O)1 structure shows a significant shift of 1.15 A for a water molecule towards the 5-OH group due to the S0–S1 electronic excitation.