ON THE DIMERIZATION PROCESS OF NITROSO-COMPOUNDS - A THEORETICAL-STUDY OF THE REACTION 2 HNO -] (HNO)(2)

Many organic C-nitroso compounds R-NO form stable dimers with a covalent NN bond. To gain insight into the dimerization reaction 2 R-NO reversible arrow (R-NO)(2) a theoretical study of the dimerization to a trans-form was performed using HNO as a model compound. Complete geometry optimizations were carried out at the HF, MP2 and QCISD levels using a 6-31G* basis. In the stationary points energies were calculated at the MP4(SDTQ) and QCISD(T) levels. For the equilibrium structure of the monomer and dimers stable RHF solutions were found, whereas for the TS UHF and UMPn calculations were applied. Extensive spin contamination was found in the UHF wavefunction, and projections up to s+4 were invoked. Relative energies were corrected for differences in ZPE. Calculations were made (a) for the least-motion path (C-2h symmetry) and (b) for a path with complete relaxation of all internal coordinates. Along the latter path a TS having virtually C-i symmetry was found. Along path (a) an activation energy of around 150 kcal/mol was predicted, in conformity with a symmetry forbidden reaction. On the relaxed path (b) the barrier to dimerization was estimated to be 10.7kcal/mol at the MP4(SDTQ)//MP2 level, and 10.9kcal/mol at the QCISD(T)//QCISD level. Unsealed ZPE corrections, calculated at the SCF level, changed these values to 12.7 and 12.9 kcal/mol, respectively. The reaction energy for the dimerization process is predicted to be -17.2kcal/mol at the MP4(SDTQ)MP2 level, corrected for ZPE. Calculations at the G1 level gave a corresponding value of - 16.4 kcal/mol. The equilibrium constant for the association to the trans dimer is estimated to be K-p = 259 atm, indicating that the dimer should be an observable species in the gas phase.