The heat of formation of NCO

摘要

The heat of formation of NCO has been determined rigorously by statehyphen;ofhyphen;thehyphen;artabinitioelectronic structure methods, including Moslash;llerndash;Plesset perturbation theory from second through fifth order (MP2ndash;MP5) and coupledhyphen;cluster and Brueckner methods incorporating various degrees of excitation lsqb;CCSD, CCSD(T), BD, BD(T), and BD(TQ)rsqb;. Five independent reactions were investigated to establish a consistent value for Dgr;Hf,0cir;(NCO): (a) HNCO(Xtilde;thinsp;1Arsquo;)rarr;H(2S)+NCO(2Pgr;), (b) HNCO(Xtilde;thinsp;1Arsquo;)rarr;H++NCOminus;, (c) N(4S)+COrarr;NCO(2Pgr;), (d) HCN+O(3P)rarr;H(2S)+NCO(2Pgr;), and (e) NH(3Sgr;minus;)+COrarr;H(2S)+NCO(2Pgr;). The onehyphen;particle basis sets employed in the study were comprised of as many as 377 contracted Gaussian functions and ranged in quality from lsqb;4s2p1drsqb; to lsqb;14s9p6d4frsqb; on the (C,N,O) atoms and from lsqb;2s1prsqb; to lsqb;8s6p4drsqb; on hydrogen. After the addition of bond additivity corrections evaluated from related reactions of precisely known thermochemistry, all five approaches were found to converge on the value Dgr;Hf,0cir;(NCO)=31.4(5) kcalthinsp;molminus;1. Appurtenant refinements were obtained for the heat of formation of isocyanic acid, Dgr;Hf,0cir;(HNCO)=minus;27.5(5) kcalthinsp;molminus;1, and hydrogen cyanide, Dgr;Hf,0cir;(HCN)=31.9(5) kcalthinsp;molminus;1. The final proposals for Dgr;Hf,0cir;(NCO) and Dgr;Hf,0cir;(HNCO) resolve outstanding discrepancies with experiment and provide updates for thermochemical cycles of relevance to combustion chemistry. thinsp;