The mechanism of the reaction FH+H2C=CH2→H3C-CFH2.: Investigation of hidden intermediates with the unified reaction valley approach

The unified reaction valley approach (URVA) is used to investigate the mechanism of the reaction H2C=CH2 + FH --> H3C-CH2F (reaction I) at different levels of theory (HF, MP2 and CCSD(T)) with different basis sets (6-31G(d,p), 6-311 + +G(3df,3dp) and cc-pVTZ). URVA is based on the reaction path Hamiltonian, the intrinsic reaction coordinate, and the characterization of normal modes, reaction path vector and curvature vector in terms of generalized adiabatic modes associated with internal parameters that are used to describe the reaction complex. In addition, URVA combines the investigation of the harmonic reaction valley with the analysis of attractive and repulsive forces exerted on the nuclei by analyzing the changes of the electron density distribution along the reaction path. It is shown that reaction I involves two different chemical processes: (a) the simultaneous FH bond cleavage and CH bond formation leading to an intermediate structure with ion-pair character and (b) the formation of a CF bond and, by this, the final product. The properties of the reaction complex suggest the possibility that a hidden intermediate formed in process (a), which upon a change in the reaction conditions (environment, substitution pattern) can convert into a real intermediate (in solution: solvated ion pairs). Using the results of the URVA analysis of reaction I predictions with regard to the occurrence of hidden intermediates in related addition/cycloaddition reactions are made.