Ab initio study of the reaction mechanism of singlet and triplet N_2O and their intersystem crossing

摘要

The lowest singlet and triplet potential energy surfaces of N_2O 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 ～64 kcal mol~(-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 product from N_2 + O(~1D) without barrier with exothermicity of 88.3 and 23.9 kcal mol~(-1), respectively, at QCISD(T)/6-311+G(3df)//MP2/6-311+G(2d). On the triplet surface, no stable bound N_2O structure exists although some plateau on the surface is found in the vicinity of the bent structure B, 73-77 kcal mol~(-1) above linear A at the QCISD(T) and CCSD(T) levels, Singlet-triplet intersections are located both at the bent geometry (B1) with ∠ NNO = 114° and at the linear structure C. The computed energy of C, 60.3 kcal mol~(-1) at the MRCI(10,9)/6-311+G(3df) level, closely agree with the experimental activation energy for N_2O decomposition. C is minimum on the seam of crossing and had higher spin-orbit coupling than those for bent intersection structures. Thus, the spin-forbidden fragmentation N_2O(~1Σ~+) → N_2 (~1Σ_g~+) + O(~3P) 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.