Initiating molecular growth in the interstellar medium via dimeric complexes of observed ions and molecules^⋆

¹ MS: 245-6, Space Science and Astrobiology Division, NASA Ames Research Center, Mountain View, 94035 California, USA
e-mail: partha.p.bera@nasa.gov
² Department of Chemistry, University of California and Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, 94720 California, USA
³ MS: 245-1, Space Science and Astrobiology Division, NASA Ames Research Center, Mountain View, 94035 California, USA

Received: 19 April 2011
Accepted: 18 July 2011

Abstract

A feasible initiation step for particle growth in the interstellar medium (ISM) is simulated by means of ab initio quantum chemistry methods. The systems studied are dimer ions formed by pairing nitrogen containing small molecules known to exist in the ISM with ions of unsaturated hydrocarbons or vice versa. Complexation energies, structures of ensuing complexes and electronic excitation spectra of the encounter complexes are estimated using various quantum chemistry methods. Møller-Plesset perturbation theory (MP2), Z-averaged perturbation theory (ZAPT2), coupled cluster singles and doubles with perturbative triples corrections (CCSD(T)), and density functional theory (DFT) methods (B3LYP) were employed along with the correlation consistent cc-pVTZ and aug-cc-pVTZ basis sets. Two types of complexes are predicted. One type of complex has electrostatic binding with moderate (7−20 kcal mol^-1) binding energies, that are nonetheless significantly stronger than typical van der Waals interactions between molecules of this size. The other type of complex develops strong covalent bonds between the fragments. Cyclic isomers of the nitrogen containing complexes are produced very easily by ion-molecule reactions. Some of these complexes show intense ultraviolet-visible spectra for electronic transitions with large oscillator strengths at the B3LYP, ωB97, and equations of motion coupled cluster (EOM-CCSD) levels. The open shell nitrogen containing carbonaceous complexes especially exhibit a large oscillator strength electronic transition in the visible region of the electromagnetic spectrum.