THEORETICAL INVESTIGATION OF THE POTENTIAL-ENERGY SURFACE FOR THE NH2-FUNCTIONAL THEORY AND AB-INITIO MOLECULAR ELECTRONIC-STRUCTURE THEORY(NO REACTION VIA DENSITY)

The potential energy surface of the NH2+NO reaction, which involves ni ne intermediates (1-9) as well as twenty-three possible transition sta tes (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) level s of theory. The reaction is shown to have three different groups of p roducts (HN2+OH, N2O+H-2, and N-2+H2O denoted as A, B, and C, respecti vely) and a very complicated reaction mechanism. The first reaction pa th is initiated by the N-N bond association of the reactants to form a n intermediate H2NNO, 1, which then undergoes a 1,3-H migration to yie ld an isomer pair HNNOH (2,3) (separated by a low energy torsional bar rier) which can then proceed along three different paths. Because of t he essential role it would play kinetically, the enthalpy of the NH2+N O-->HN2(+)OH reaction has been further investigated using various leve ls 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, respec tively, 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 be en found that TS g(4-->B) is expected to be the rate-determining trans ition state responsible for the NH2+NO-->N2O+H-2 reaction. TS g lies a bove the reactants by only 2.6 kcal mol(-1) according to the G2M predi ction. On the other hand, TS h(3-->7) is a new transition state discov ered in this work which may allow some kinetic contribution from the N H2+NO-->N-2+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, w hich gives a value of 2.3 kcal mol(-1) in energy with respect to the r eactants. Generally, the cost-effective B3LYP method is found to give very good predictions for the optimized geometries and vibrational fre quencies of various species in the system if compare them with those o ptimized at the QCISD/6-311G(d,p) and 12-in-11 CASSCF/cc-pVDZ levels o f theory. Furthermore, it is noticeable in this study that most of the relative energies calculated via the B3LYP method are more close to t he G2M results than those predicted at the PMP4 and CCSD(T) levels usi ng the same 6-311G(d,p) basis set. (C) 1997 American Institute of Phys ics.