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 mech anism of the reaction H2C=CH2 + FH --> H3C-CH2F (reaction I) at different l evels 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 Hami ltonian, the intrinsic reaction coordinate, and the characterization of nor mal modes, reaction path vector and curvature vector in terms of generalize d adiabatic modes associated with internal parameters that are used to desc ribe 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 densi ty distribution along the reaction path. It is shown that reaction I involv es two different chemical processes: (a) the simultaneous FH bond cleavage and CH bond formation leading to an intermediate structure with ion-pair ch aracter 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 hidd en intermediate formed in process (a), which upon a change in the reaction conditions (environment, substitution pattern) can convert into a real inte rmediate (in solution: solvated ion pairs). Using the results of the URVA a nalysis of reaction I predictions with regard to the occurrence of hidden i ntermediates in related addition/cycloaddition reactions are made.