B3LYP calculations using basis sets up to 6-311G(3d2f,3p2d) have been employed to predict accurate thermochemical properties related to the bond dissociation reaction trans-HONO(X¹A') -> HO(X²P) + NO(X² P). A systematic study of the influence of the basis set was performed and results were compared with experimental data and with other calculated results, obtained using standard Gaussian methods (G2 and G2MP2), complete basis set extrapolation methods (CBS) and ab initio calculations (CCSD(T) and QCISD). The results suggest that, for this kind of unimolecular process: B3LYP calculations produce bond dissociation enthalpies that are more accurate than standard ab initio methods and) the best agreement with the experimental enthalpy has been found with B3LYP/6-311G(3d2f,3p2d) calculations: 49.2 kcal mol-1 and 49.0 kcal mol-1, respectively. Also, entropy and Gibbs free energy have been calculated and equilibrium constants have been determined as Keq(T) = 1.16 × 10(28) × exp(-48.34/RT), for the temperature range 200 - 500 K.
nitrous acid; thermochemical properties; density functional theory; equilibrium constants
