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 covale nt NN bond. To gain insight into the dimerization reaction 2 R-NO reve rsible arrow (R-NO)(2) a theoretical study of the dimerization to a tr ans-form was performed using HNO as a model compound. Complete geometr y 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 th e monomer and dimers stable RHF solutions were found, whereas for the TS UHF and UMPn calculations were applied. Extensive spin contaminatio n was found in the UHF wavefunction, and projections up to s+4 were in voked. Relative energies were corrected for differences in ZPE. Calcul ations were made (a) for the least-motion path (C-2h symmetry) and (b) for a path with complete relaxation of all internal coordinates. Alon g 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. U nsealed ZPE corrections, calculated at the SCF level, changed these va lues 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(S DTQ)MP2 level, corrected for ZPE. Calculations at the G1 level gave a corresponding value of - 16.4 kcal/mol. The equilibrium constant for t he association to the trans dimer is estimated to be K-p = 259 atm, in dicating that the dimer should be an observable species in the gas pha se.