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.
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