On the formation of metal cyanides and related compounds in the circumstellar envelope of IRC + 10216

Abstract

Various pathways to the metal cyanides Na(CN), Mg(CN) and Al(CN) - some of which have been detected in the circumstellar envelope IRC + 10216 - are critically assessed. Calculations of rate coefficients for radiative association processes of the types M+ + HCN, M+ + HNC and M + CN, which are supported by ab initio calculations of the relevant bond strengths, molecular geometries and vibrational frequencies, allow us to conclude that none of these previously proposed pathways to M(CN) can account for the observed abundances of T-Na(CN), MgCN and MgNC within IRC + 10216. We propose that the observed sodium and magnesium cyanides are best accounted for by a mechanism involving radiative association of Na + and Mg+ with cyanopolyynes such as HCsN and HC7N, and discuss prospects for the formation of Al(CN) by an analogous mechanism. We also estimate the abundance ratiosn(NaCN) :n(T-Na(CN)) andn(AlCN) :n(AlNC), assuming an ion/ molecule source for these species and based on calculated thermochemical and spectroscopic properties for these isomers; the results suggest that virtually all Na(CN) and Al(CN) should be in the form of T-Na(CN) and linear AINC respectively, with poor prospects for the detection of the higher energy isomers NaCN and AlCN. Larger homologues of the metal cyanides, such as MNC2n + 1 (M = N a, Mg, Al; n = 1, 2, 3, 4 ... ), may also arise in the dissociative recombination of MNC2n +1H+ ions. We assess the prospects for formation of the species MgC2 and MgCCH, and propose also that the metal amides MNH2 (M=Na, Mg, Al) constitute another class of compound whose formation in IRC + 10216 via an ion/ molecule mechanism, namely the reaction of M + with NH3 , appears highly likely.