COMPARATIVE-STUDY OF NONLOCAL DENSITY-FUNCTIONAL THEORY AND AB-INITIOMETHODS - THE POTENTIAL-ENERGY SURFACE OF SYM-TRIAZINE REACTIONS

Stable points and transition states on the potential energy surface (P ES) for sym-triazine (C3N3H3) have been calculated by using nonlocal d ensity functional (NDFT) methods. Two decomposition mechanisms for sym -triazine are investigated. The first is a concerted triple dissociati on of the sym-triazine ring to form the HCN products. Three-fold symme try is maintained along the reaction path for this mechanism. The seco nd is a stepwise decomposition mechanism involving the formation of an intermediate dimer species. The NDFT results, including structures, r elative energies, harmonic vibrational frequencies, and corresponding eigenvectors, are compared with previously reported nb initio calculat ions. These results include critical points located and characterized through normal mode analyses at the MP2 level. QCISD(T) energy refinem ents of the MP2 critical points are used for the comparison of DFT pre dictions. Basis set size dependence is also examined. The nonlocal den sity functionals used are the exchange functional of Becke and the cor relation energy functional of Perdew (BP86), Becke's exchange and the correlation energy functional of Lee, Yang, and Parr (BLYP), Becke's t hree-parameter hybrid exchange functional with the LYP correltation en ergy functional (B3LYP), and the Becke exchange with Perdew and Wang's 1991 gradient-corrected correlation functional (BPW91). Basis sets us ed are 6-31G*, 6-311++G**, and cc-pVTZ. The reaction endothermicity p redicted by B3LYP and BPW91 are in closer agreement with experiment th an the QCISD(T) and MP2 predictions using the largest basis set. B3LYP predictions are within 1.1 kcal/mol of experiment. BPW91, BPSG, and B LYP frequencies agree most closely with experimental values for sym-tr iazine and HCN. DFT eigenvectors corresponding to vibrational modes fo r critical points on the PES compare well with MP2 predictions for mos t 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 that the concerted triple-dissociation mechanism is the low-energy decomposition pathway for sym-triazine. DFT predicti ons of energies along the reaction path for the concerted triple-disso ciation reaction are in qualitative agreement with MP2. All DFT method s predict structures of species along the reaction path that are in qu antitative agreement with MP2 predictions.