Circumstellar Calcium Chemistry

Abstract

High-level ab initio calculations are used to explore several aspects of calcium chemistry of direct relevance to formation and reaction of calcium-containing molecules in circumstellar envelopes. Counterpoise-corrected G2 and G2(MP2) calculations have been used to determine the bond dissociation energies (BDEs) of Ca+/NC2n+1H complexes (n = 0, 1, 2); these complexes, and CaNC7H+, are also assessed through counterpoise-corrected MP2(thaw)/6-3.11 + G(3df,2p) calculations. The relative energies of isomers of the feasible Ca(CN), Ca(C3N), and Ca(C5N) products (which may arise from dissociative recombination of the Ca+/NC2n+1H complexes) are obtained from G2 and G2(MP2) calculations; these calculations also permit evaluation of the thermochemistry of the dissociative recombination reactions in question. Thermochemical data are presented for a possible loss mechanism for calcium-containing neutrals by reaction with molecular ions in circumstellar envelopes. Finally, we provide an empirical assessment of the prospects for detecting the Ca(CN), Ca(C3N), and Ca(C5N) radicals within circumstellar environments. Show more