Ab initio study of the reaction mechanism of singlet and triplet N2O and their intersystem crossing

The lowest singlet and triplet potential energy surfaces of N2O and their intersection are studied using various ab initio methods including MP2, QCISD(T), CCSD(T), CASSCF and multireference configuration interaction method (MRCI). On the singlet surface, two local minima are found, linear NNO (A) and cyclic structure D. The latter lies similar to 64 kcalmol(-1) higher in energy and is separated from the former by a barrier of about 15 kcal mol(-1) at the QCISD(T)/6-311+G(3df)//MP2/6-311+G(2d) and CCSD(T)/6-311+G(3df)//B3LYP/6-311G(d) levels. Both A and D can be produced from N-2 + O(D-1) without barrier with exothermicity of 88.3 and 23.9 kcalmol(-1), respectively, at QCISD(T)/6-311 +G(3df)//MP2/6-311+G(2d). On the triplet surface, no stable bound N2O structure exists although some plateau on the surface is found in the vicinity of the bent structure B, 73-77 kcalmol(-1) above linear A at the QCISD(T) and CCSD(T) levels. Singlet-triplet intersections are located both at the bent geometry (B1) with angle NNO = 114 degrees and at the linear structure C. The computed energy of C, 60.3 kcalmol(-1) at the MRCI(10,9)/6-311+G(3df) level, closely agree with the experimental activation energy for N2O decomposition. C is minimum on the seam of crossing and has higher spin-orbit coupling than those for bent intersection structures. Thus, the spin-forbidden fragmentation N2O((1)Sigma(+)) --> N-2((1)Sigma(g)(+)) + O(P-3) should occur via structure C as a transition state. The calculations demonstrated that the use of QCISD(T), CCSD(T), full-valence active space CASSCF, or MRCI theoretical levels is essential to compute accurate relative energies of B1 and C. (C) 2000 Elsevier Science B.V. All rights reserved.