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 Mo/ller–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 ΔHf,0○(NCO): (a) HNCO(X̃ 1A′)→H(2S)+NCO(2Π), (b) HNCO(X̃ 1A′)→H++NCO−, (c) N(4S)+CO→NCO(2Π), (d) HCN+O(3P)→H(2S)+NCO(2Π), and (e) NH(3Σ−)+CO→H(2S)+NCO(2Π). 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 ΔHf,0○(NCO)=31.4(5) kcal mol−1. Appurtenant refinements were obtained for the heat of formation of isocyanic acid, ΔHf,0○(HNCO)=−27.5(5) kcal mol−1, and hydrogen cyanide, ΔHf,0○(HCN)=31.9(5) kcal mol−1. The final proposals for ΔHf,0○(NCO) and ΔHf,0○(HNCO) resolve outstanding discrepancies with experiment and provide updates for thermochemical cycles of relevance to combustion chemistry.