Ab initio calculations of nitrogen oxide reactions:: Formation of N2O2, N2O3, N2O4, N2O5, and N4O2 from NO, NO2, NO3, and N2O

Ab initio computational methods were used to obtain Delta H-r(o), Delta(r)G(o), and Delta S-r(o) for the reactions 2 NO -><- N2O2 (I), NO+NO2 -><- N2O3 (II), 2 NO2 -><- N2O4 (III), NO2+NO3 -><- N2O5 (IV), and 2 N2O -><- N4O2 (V) at 298.15 K. Optimized geometries and frequencies were obtained at the CCSD(T) level for all molecules except for NO, NO2, and NO3, for which UCCSD(T) was used. In all cases the aug-cc-pVDZ (avdz) basis set was employed. The electronic energies of all species were obtained from complete basis set extrapolations (to aug-cc-pV5Z) using five different extrapolation methods. The [U]CCSD(T)/avdz geometries and frequencies of the NxOy compounds are compared with literature values, and problems associated with the values and assignments of low-frequency modes are discussed. The standard entropies are compared with values cited in the NIST/JANAF tables [NIST-JANAF Thermochemical Tables, J. Phys. Chem. Ref. Data Monograph No. 9, 4th ed. edited by M. W. Chase, Jr. (American Chemical Society and American Institute of Physics, Woodbury, NY, 1988)]. With the exception of I, in which the dimer is weakly bound, and V, for which thermodynamic data appears to be lacking, the calculated standard thermodynamic functions of reaction are in good agreement with literature values obtained both from statistical mechanical and various equilibrium methods. A multireference-configuration interaction calculation (MRCI+Q) for I provides a D-e value that is consistent with previous calculations. The combined uncertainties of the NIST/JANAF values for Delta H-r(o), Delta(r)G(o), and Delta S-r(o) of II, III, and IV are discussed. The potential surface for the dissociation of N2O4 was explored using multireference methods. No evidence of a barrier to dissociation was found.