HARTRE-FOCK AND POST-HARTREE-FOCK STUDIES ON CIS-TRANS EQUILIBRIUM OFFLUOROFORMYL HYPOFLUORITE FC(O)OF - RELIABILITY OF PREDICTED THERMODYNAMIC PROPERTIES AND MOLECULAR-STRUCTURES

A Hartree-Fock and post-Hartree-Fock study on the cis-trans equilibriu m of fluoroformyl hypofluorite, FC(O)OF, is presented. Pople's split-v alence double- and triple-zeta quality basis sets, augmented with sets of polarization and diffuse functions (6-31G(d), 6-311G(d), 6-311G(2d f) and 6-311+G(2df)), correlation consistent polarized valence triple- zeta Dunning's (aug-cc-pVTZ), and medium-size polarized Sadlej's (MSPB S) basis sets were used at the HF and at the MP2, MP4(SDQ), MP4(SDTQ), CCSD, CCSD(T), and DFT correlated levels. For all applied methods, tr ans-FC(O)OF is predicted to be more stable than the cis isomeric form, in agreement with experimental data. Delta E(cis double right arrow t rans) values range from -1 . 70 (CCSD/6-311G(d)) to -0 . 16 kcal mol(- 1) (DFT/6-311G(d)) and the cis-trans barrier height ranges from 9 . 60 (MP2/6-311G(2df)) to 8 . 56 kcal mol(-1) (MP4(SDQ)/6-311+G(2df)). Rel iable geometries and IR harmonic vibrational frequencies were obtained using the DFT model with the Becke-Lee-Yang-Parr exchange-correlation potential.