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Biography

Professor Gupta did his PhD in 1966 from Moscow University (USSR) at the age of 22 yrs. He has over 45 yrs. of research and teaching experience in the Departments of Physics and Chemistry at the Universite de Provence, Marseille (France), University of Jammu (India), University of Calabar, Calabar (Nigeria) , Helsinki University, Finland , and University of Lucknow (India) He has successfully executed several research projects sponsored by Department of Science & Technology (DST), Govt. of India, New Delhi, University Grants Commission (UGC), New Delhi and Council of Scientific & Industrial Research (CSIR), New Delhi, AICTE, New Delhi and Indian Space Research Organization (ISRO), Bangalore. He has published over 100 research papers in reputed journals and has two books to his credit. Major Fields of his Research Interest are: Molecular Spectroscopy and Molecular Structure , Quantum Chemistry and Astrochemistry , and Laser Spectroscopy.

Abstract

The detection of nucleic acid bases in carbonaceous meteorites suggests that their formation and survival is possible outside of the Earth. We have attempted to use quantum chemical techniques to explore if nucleobases like adenine, cytosine and thymine and some small nitrile compounds such as diaminomaleonitrile (DAMN), diaminofumaronitrile (DAFN) and 4-amino-1H-imidazole-5-carbonitrile (AICN) can possibly form in the interstellar space (in conditions of low temperatures ~10 K and density ≥104 molecules/cm3 ) by radical–radical and radical-molecule interaction schemes, both in the gas phase and in the icy-grains, through exothermic and barrier-less or low barrier pathways. Various reaction channels have been explored in all cases using ab initio and DFT methods . In the case of adenine, a six-step reaction pathway was discovered that lead to its formation with the participation of molecules/radicals like HCN, HCCN, NH2CN and CN, which are abundant in the interstellar space. The reaction process was found to be both barrierless and exothermic. Rate Coefficients of these reactions are found to be less than 10-9 cm3 sec-1. In view of the significance of the HCN tetramers diaminomaleonitrile (DAMN) and diaminofumaronitrile (DAFN) in reactions leading to the formation of adenine, the possibility of their formation from CN and H radicals has been explored using quantum chemical methods. The reaction leading to the formation of DAFN is found to be exothermic with a very large net energy gain but involves a potential barrier of about 4.75 kcal/mol. The reaction scheme also provides a simpler route to the formation of 4-amino-1H-imidazole-5-carbonitrile (AICN) and adenine. Two exothermic reaction pathways starting from propynylidyne (CCCH) and cyanoacetylene (HCCCN), respectively, have been identified for the synthesis of cytosine in the interstellar space. While the first reaction path is found to be totally exothermic, it involves a barrier of 12.5 kcal/mol in the gas phase while the second path is both exothermic and barrier-less. The later has, therefore, a greater probability of occurrence in the cold interstellar clouds (10–50 K). Studies were conducted on the formation of another important nucleic base thymine in the interstellar space. To devise a viable interstellar synthesis of thymine is challenging because of the presence of the methyl substituent. As our earlier studies showed, the cyanate ion (OCN-), which is also a stable radiation product in any Titan region having both nitriles and H2O-ice, plays an important role in the formation of cytosine. Based on extensive ab initio and DFT calculations using extended basis sets we have arrive at a simple reaction path based on the reaction between propyne and the cyanate ion which is both barrierless and exothermic and has high probability of occurrence both in the interstellar space and in the nitrile rich environments such as on the saturnian satellite Titan. In order to study the formation of the nucleobases in ice-grains, the influence of the bulk solvent on energetics of chemical reactions has been analyzed by the IPCM model. The solvent has been found to have only a marginal affect on the energies of the reactants, products and transition states. Global and local descriptors have been helpful in predicting the chemical reactivity and regio-selectivity of the radical additions. The quantum theory of atoms in molecules (QTAIM) has additionally been used to locate the changes in structure along a reaction path. Spectroscopic parameters such as rotational constants, rotation–vibration coupling constants, and centrifugal distortion constants have been calculated for some of the stable intermediate molecules to help in their identification and characterization.