Theoretical investigation of the potential energy surface for the NH2+NO reaction via density functional theory and ab initio molecular electronic structure theory

Abstract

The potential energy surface of the NH2+NO reaction, which involves nine intermediates (1–9) as well as twenty-three possible transition states (a–w), has been fully characterized at the B3LYP/cc-pVQZ//B3LYP/6-311G(d,p)+ZPE[B3LYP/6-311G(d,p)] and modified Gaussian-2 (G2M) levels of theory. The reaction is shown to have three different groups of products (HN2+OH, N2O+H2, and N2+H2O denoted as A, B, and C, respectively) and a very complicated reaction mechanism. The first reaction path is initiated by the N–N bond association of the reactants to form an intermediate H2NNO, 1, which then undergoes a 1,3-H migration to yield an isomer pair HNNOH (2,3) (separated by a low energy torsional barrier) which can then proceed along three different paths. Because of the essential role it would play kinetically, the enthalpy of the NH2+NO→HN2+OH reaction has been further investigated using various levels of theory. The best theoretical results of this study predicted it to be 0.9 and 2.4 kcal mol−1 at the B3LYP and CCSD(T) levels, respectively, using a relatively large basis set (AUG-cc-pVQZ) based on the geometry optimized at the B3LYP/6-311G(d,p) level of theory. It has been found that TS g(4→B) is expected to be the rate-determining transition state responsible for the NH2+NO→N2O+H2 reaction. TS g lies above the reactants by only 2.6 kcal mol−1 according to the G2M prediction. On the other hand, TS h(3→7) is a new transition state discovered in this work which may allow some kinetic contribution from the NH2+NO→N2+H2O reaction under high temperature conditions due to its relatively low energy as well as its loose transition state property. A modified G2 additivity scheme based on the G2(DD) approach has been shown to be necessary for better predicting the energetics for TS h, which gives a value of 2.3 kcal mol−1 in energy with respect to the reactants. Generally, the cost-effective B3LYP method is found to give very good predictions for the optimized geometries and vibrational frequencies of various species in the system if compare them with those optimized at the QCISD/6-311G(d,p) and 12-in-11 CASSCF/cc-pVDZ levels of theory. Furthermore, it is noticeable in this study that most of the relative energies calculated via the B3LYP method are more close to the G2M results than those predicted at the PMP4 and CCSD(T) levels using the same 6-311G(d,p) basis set.