Amareshwar Kumar Rai

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.

An exploration of conformational search of leucine molecule and their vibrational spectra in gas phase using ab initio methods.

An extensive exploration of the conformational space has been carried out to characterize all possible gas phase structures of leucine. A total of 324 unique trial structures for canonical leucine were generated by considering all possible combinations of single bond rotamers. All trial structures were optimized at the B3LYP/6-311G* level of the DFT method. A total of 77 unique and stationary canonical conformers were found. Further, 15 most stable conformers were reoptimized at B3LYP/6-311++G** level and their respective relative energies, vertical ionization energies, hydrogen bonding patterns, rotational constants and dipole moments were calculated. A single point energy calculations for leucine conformers have also been done at both B3LYP/6-311++G(2df, p) and MP2/6-311++G(2df, p) levels. The good agreement between our estimates of rotational constants for two most stable conformers and available experimental measurements supports the reliability of the B3LYP/6-311++G** level of theory for describing the conformational behavior of leucine molecule. The proton affinity and gas phase basicity were also determined. Using the statistical approach, conformational distributions at various temperatures have also been performed and analyzed. Vibrational spectra were also calculated. It is also observed that zwitterions of leucine are not stable in gas phase.

Spectroscopic studies of some antidiabetic drugs.

The infrared absorption spectrum of antidiabetic drugs Pioglitazone and Pioglitazone.HCl has been recorded at high resolution in the spectral region 200-4000 cm(-1). Most of the frequencies in the latter compound are slightly lower in magnitude than corresponding value in the former. The absorption intensities are however slightly larger. In order to confirm the vibrational assignment of the bands obtained from experimental observation, a normal co-ordinate analysis has been carried out using semi empirical AM1 method through MOPAC 5.1 computer program. In addition to these compounds, the vibrational frequencies for some of the intermediate metabolite of Pioglitazone have also been calculated.

Spectral study of the D-X system of the diatomic mercury chloride molecule

The vibrational and rotational structure in the D-X system of the HgCl molecule has been studied by photographing it in the third order of a 10.6 m concave grating spectrograph. Rotational constants in the (0, 0) and (0, 1) bands have been determined from the partially resolved rotational structure. Variation of intensity in the bands of different sequences have been explained on the basis of Franck-Condon factors estimated from Morse curves for the two combining states. Vibrational constants for the isotopic molecule HgCl37 have also been determined.

Quantum chemical studies on the conformational structure of bacterial peptidoglycans and action of penicillin on cell wall

Abstract A partial conformational analysis of bacterial peptidoglycans has been carried out using semi-empirical quantum chemical calculation at the AM1 level. It is seen that the presence of the four peptide units attached to one of the pyranose rings considerably affects the minimum energy conformation of the N -acetylglucosamine (NAG)– N -acetyl muramic acid (NAM) units obtained in earlier work. However, in agreement with the result of earlier theoretical studies the AM1 method also disfavours a chitin like structure for the glycan moiety of the peptidoglycan. It is also found that in the minimum energy conformation there is a likelihood of formation of several hydrogen bonds between the sidegroups which might further stabilize the structure. The net atomic charges do not seem to be much affected as the conformation changes. The serine unit in the cell wall transmidase enzyme plays an important role in cross linking the peptide tails of successive NAG–NAM units in order to form the bacterial cell wall. A calculation of the total energy of serine+the peptide tail as a function of their separation yields an estimate of the strength of relevant interaction. It is found that this interaction is almost equal in strength to the interaction between serine and a molecule of penicillin. It is inferred that because of this equality the presence of penicillin negatively affects the formation of cell wall in bacteria.

Rotational structure in the A-X system of InBr

Rotational structure in the A-X system of the indium monobromide molecule has been studied with moderately high resolution. Five bands, namely (2, 0), (1, 0), (0, 0) (0, 1) and (0, 2), were rotationally analysed and rotational constants were determined in the A and the X states for both the isotopic molecules In 79Br and In 81Br. Predissociation has been observed at J=134 in the upsilon =2 level of the A 3 Pi 0 state. This gives an upper limit to the dissociation energy of the InBr molecule of 3.5 eV.

Effect of dc voltage on the laser optogalvanic spectrum of iodine molecule

Abstract The Doppler-limited laser optogalvanic spectrum of the B-X system of I 2 molecule has been recorded in the 570-630 nm region. It is found that the discrete spectrum of the B-X system is superposed over a weak continuum extending throughout this region. Upon increasing the applied voltage, the signal corresponding to some of the discrete bands in the lower wavelength region changes sign (from positive to negative). The continuum signal in the region 570–605 nm changes from positive to negative while the other part is much less affected by a change of voltage.

Ultraviolet bands in the emission spectrum of the diatomic mercury iodide molecule

The ultraviolet bands of diatomic mercury iodide extending from 3095.0 AA to 2110.0 AA have been investigated under moderately high resolution and dispersion. All the previously known systems, namely C-X, D-X and one component of the E-X, could be recorded with good intensity. The vibrational assignments in the C-X system could be confirmed using mercury isotope shifts. A large number of new bands involving higher vibrational quanta have been observed in the reinvestigated systems. The new measurements coupled with computer analysis of this have enabled the authors to determine precise vibrational constants for X, C, D, E1 and E2 states of this molecule. A new vibrational analysis is proposed for the E-X system and the nature of the E state is shown to be 2 Pi . Approximate rotational constants have also been determined for the C state.

Optogalvanic signal in the B‐X system of HgBr: Effect of discharge voltage and laser power

Laser Optogalvanic (LOG) signals have been detected from a D.C. discharge in flowing HgBr2 vapour. Different laser lines from a 4 watt Ar+ laser have been used for excitation. The signals are identified with transitions in the B‐X system of this molecule and variation in the signal strength with discharge voltage and input laser power has been investigated. The LOG signal is seen to vary with laser power and the variation is wavelength dependent. An explanation for this dependence is offered.

Two-photon optogalvanic detection of theF′(O u + ) ion pair state of I2

Two-photon transition inF′(Ou+) ion pair state of iodine from theB3ΠOu+ state is detected for the first time using laser optogalvanic spectroscopy. TheB3ΠOu+ is populated by a d.c. discharge running at constant voltage.