Initiating Molecular Growth in the Gas Phase, and its implication in Interstellar Chemistry and Hydrocarbon Combustion

by Dr. Partha P. Bera (NASA Ames Research Centre, USA)

Tuesday, August 30, 2011 from 14:30 to 15:30 (Asia/Kolkata)
at Colaba Campus ( AG-80 )

Description

Large carbonaceous molecules are abundant in the interstellar medium and are believed to be responsible for some of the unassigned strong visible and infrared emission features. How these polyatomic carbonaceous molecules are formed in the interstellar medium is still shrouded in mystery. Molecular ions are in relative abundance in the strong radiation fields of the ISM, hence, barrierless ion-molecule interactions play a major role in guiding molecules towards each other and initiating reactions. Can these cold condensation pathways be a viable means of forming nitrogen containing carbonaceous chains, stacks and even cyclic compounds? In our study, complexation mechanism of selected observed carbon, hydrogen and nitrogen containing molecules, both neutral and ions, were investigated. The nature of bonding and spectra of the resulting cationic molecular complexes were examined with an emphasis on the formation of key biogenic molecules such as pyrimidine. Very accurate ab initio Moller-Plesset perturbation theory and coupled cluster (CC) methods were used along with large correlation consistent basis sets. Both linear and cyclic isomers of these molecular complexes are expected to form rather easily due to electrostatic interactions. Ensuing complexes serve as starting points for further growth into polycyclic aromatic hydrocarbons, the precursors of soot and grains, also important in combustion chemistry. Our mechanistic study of oxidation of pyrimidine leading to the formation of nucleobase uracil revealed key information about this transformation and its identification in the ISM. Time dependent density functional theory (TDDFT) and equations of motion coupled cluster (EOM-CC) theory calculations indicated that several complexes have huge oscillator strengths for charge-transfer type electronic excitations in the near infrared and visible regions of the electromagnetic spectrum.