Comparative Study of Nonlocal Density Functional Theory and ab initio Methods: The Potential Energy Surface of sym-Triazine Reactions.

Abstract : Critical points on the potential energy surface (PES) for sym-triazine (C3N3H3) have been calculated using nonlocal density functional (NDFT) methods. The two decomposition mechanisms for sym-triazine investigated are a concerted triple dissociation of the sym-triazine ring to form the HCN products, where three-fold symmetry is maintained along the reaction path, and a step-wise decomposition mechanism involving formation of an intermediate dimer species. The NDFT structures, harmonic vibrational frequencies, and corresponding eigenvectors are compared with previously reported MP2 calculations. QCISD(T) energy refinements of the MP2 critical points are used for comparison with NDFT predictions. The NDFTs used are BP86; BLYP; B3LYP; and BPW91. Basis sets used are 631G**, 6-311++G**, and cc-pVTZ. The reaction endothermicity predicted by B3LYP and BPW91 are in closer agreement with experiment than the QCISD(T) and MP2 predictions using the largest basis set B3LYP predictions are within 1.1 kcal/mol of experiment BPW91, BP86, and BLYP frequencies agree most closely with experimental values for sym-triazine and HCN. DFT eigenvectors corresponding to vibrational modes for critical points on the PES compare well with MP2 predictions for most modes, indicating similarity in force fields and, therefore, atomic motion for the vibrations. Geometries predicted by all methods are in excellent agreement with experimental values for sym-triazine and HCN. All methods predict the concerted triple dissociation mechanism to be the low-energy decomposition pathway for sym-triazine.