Transition state structure and energetics of the N2O+X (X = Cl,Br) reactions

The structural and vibrational properties of the transition state of the N2O+x(X=Cl,Br) reactions have been characterized by ab initio methods using density functional theory. We have employed Becke's hybrid functional (B3LYP). and transition state optimizations were performed with 6-31G(d), 6-311G(2d,2p). 6-311+G(3d,2p), and 6-311+G(3df,2p) basis sets. For the chlorine atom reaction the coupled-cluster method (CCSD(T)) with 6-31 G(d) basis set was also used. All calculations resulted in transition state structures with a planar cis arrangement of atoms for both reactions. The geometrical parameters of transition states at B3LYP are very similar. and the reaction coordinates involve mainly the breaking of the N-O bond. At CCSD(T)6-31G(d) level a contribution of the C-Cl forming bond is also observed in the reaction coordinate. In addition, several highly accurate ab initio composite methods of Gaussian-n (G1, G2, G3), their variations (G2(MP2). G3//B3LYP). and complete basis set (CBS-Q, CBS-Q//B3LYP) series of models were applied to compute reaction energetics. All model chemistries predict exothermic reactions. The G3 and G2 methods result in the smallest deviations from experiment. 1.8 and 0 kcal mol(-1), for the enthalpies of reaction for N2O reaction with chlorine and bromine, respectively, The G3//B3LYP and G1 methods perforin best among the composite methods in predicting energies of the transition state, with a deviation of 1,9 and 3.0 kcal mol(-1), respectively. in the activation energies for the above processes. However, the B3LYP/6-311+G(3df,2p) method gives smaller deviations of 0.4 and -1.0 kcal mol(-1), respectively. The performance of the methodologies applied in predicting transition state energies was analyzed.