A DENSITY-FUNCTIONAL STUDY OF MONOFLUOROPROPENE AND DIFLUOROPROPENE

A number of physical and chemical properties of all possible mono-and selected difluoropropenes have been investigated using semiempirical, standard ab initio and density functional methods, with particular emp hasis on hybrid HF-DFT functionals that mix part of the exact Hartree- Fock exchange in with the density functional. We have included two hyb rid functionals in our study-Becke's original 3-parameter ACM function al (also known as B3PW91) and the popular B3LYP functional. Results fo r the two functionals are very similar and generally among the best re ported, although the ACM functional seems to be better for geometries than B3LYP. The semiempirical methods (especially MNDO) give a poor pi cture of the chemistry of the monofluoropropenes, with relative energi es and derived potential energy surfaces often qualitatively incorrect . We make a number of predictions as to the geometries and energetics of the difluoropropenes; we consider our 'best' theoretical bond lengt hs to have a maximum error of 0.02 Angstrom. (C) 1998 Elsevier Science S.A. All rights reserved.