A density functional study of the global potential energy surfaces of the [H,C,N,O] system in singlet and triplet states

Abstract

Global potential energy surfaces (PESs) of the [H,C,N,O] system in singlet and triplet states have been investigated using the hybrid density functional B3LYP/6−311G(d,p) method. Isocyanic acid, HNCO 1, has been found to be the most stable isomer for both multiplicities. The adiabatic singlet–triplet splitting for 1 is 82.6 kcal/mol. In the singlet state, HNCO is energetically followed by cyanic acid, HOCN 2, 28.7 kcal/mol higher than 1, fulminic acid, HCNO 3 (67.9 kcal/mol), and isofulminic acid, HONC 4 (87.1 kcal/mol). In the triplet state, the branched NC(H)O isomer 37 is 0.3 kcal/mol higher than 31, followed by HOCN 32 (27.9 kcal/mol relative to triplet HNCO) and HCNO 33 (40.6 kcal/mol). The barriers for intramolecular rearrangements within singlet and triplet [H,C,N,O] system have been calculated to be high, and the isomerization processes in most cases are not expected to compete with fragmentations. Several minima on the singlet–triplet seam of crossing, relevant to the singlet [H,C,N,O] decomposition reactions, have also been found. The global features of the singlet and triplet PES have been applied to several important reactions, such as NH(3Σ−)+CO, thermal decomposition of HNCO, O(3P)+HCN, O(3P)+HNC, and CH(2Π)+NO(2Π). For these reactions, major product channels have been speculated and their activation energies have been reported. Adiabatic ionization potentials for singlet and triplet [H,C,N,O] have been found to be high, in the range of 180–270 kcal/mol.