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⁺/NC_2n+1H complexes (n = 0, 1, 2); these complexes, and CaNC₇H⁺, are also assessed through counterpoise-corrected MP2(thaw)/6-311 + 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⁺/NC_2n+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.