The heavier pnictogen and chalcogen analogues of isocyanic and cyanic acids and their dimers: A high level ab initio study

The CCSD(T)/cc-pVTZ//CCSD/cc-pVTZ method is used to determine the geometries and energetics of the isomers HX(sic)C(sic)Y vs HYC(sic)X (X(sic)N, P, As; Y(sic)O, S) and their dimers from chain dimerizations and head-to-head or head-to-tail [2 + 2] cyclodimerizations. The HOC(sic)X structures with C(sic)X triple bonds lie at energies at least 18.5 kcal/mol above their HX(sic)C(sic)O isomers. However, the energy differences between the HX(sic)C(sic)S and HSC(sic)X isomers are found to be particularly small, especially in the [H,P,C,S] and [H,As,C,S] systems. For (HNCY)(2), the lowest energy dimers are the chain isomers, which lie ~11 kcal/mol below the lowest energy cyclic dimers aNO containing a NCNC ring and cNS containing a NCSC ring. Formation of the remaining dimers through dimerization from two monomers is predicted to be endothermic and thus thermodynamically disfavored. However, the energies of the chain isomers in the other (HXCY)(2) (X(sic)P, As; Y(sic)O, S) series are higher than those of the corresponding isomeric lowest energy cyclodimers. For (HXCO)(2) (X(sic)P, As), the lowest energy structures are the head-to-head dimers hPO and hAsO containing a CCX(sic)X ring. For (HXCS)(2) (X(sic)P, As), the lowest energy structures are the head-to-head dimers gPS and gAsS with a CCXS ring.