THE HEAT OF FORMATION OF NCO

The heat of formation of NCO has been determined rigorously by state-of-the-art ab initio electronic structure methods, including Moller-Plesset perturbation theory from second through fifth order (MP2-MP5) and coupled-cluster and Brueckner methods incorporating various degrees of excitation [CCSD, CCSD(T), BD, BD(T), and BD(TQ)]. Five independent reactions were investigated to establish a consistent value for DELTAH(f,0)o (NCO): (a) HNCO (X1A') --> H(S-2) + NCO(2PI), (b) HNCO(X1A') --> H+ +HNCO-, (c) N(4S) +CO --> NCO(2PI), (d) HCN+O(P3) -->H(S2) + NCO(2PI), and (e) NH(3SIGMA-) + CO --> H(S2) +NCO(2PI). The one-particle basis sets employed in the study were comprised of as many as 377 contracted Gaussian functions and ranged in quality from [4s2p1d] to [14s9p6d4f] on the (C,N,O) atoms and from [2s1p] to [8s6p4d] 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 DELTAH(f,0)o(NCO) = 31.4(5) kcal mol-1. Appurtenant refinements were obtained for the heat of formation of isocyanic acid, DELTAH(f.0)o(HNCO) = - 27.5 (5) kcal mol-1, and hydrogen cyanide, DELTAH(f,0)o (HCN) = 31.9 (5) kcal mol-1. The final proposals for DELTAH(f,0)0 (NCO) and DELTAH(f,0)o(HNCO) resolve outstanding discrepancies with experiment and provide updates for thermochemical cycles of relevance to combustion chemistry.