Concerted hydrogen atom exchange between three HF molecules

We have investigated the termolecular reaction involving concerted hydrogen exchange between three HF molecules, with particular emphasis on the effects of correlation at the various stationary points along the reaction. Using an extended basis, we have located the geometries of the stable hydrogen-bonded trimer, which is of C(sub 3h) symmetry, and the transition state for hydrogen exchange, which is of D(sub 3h) symmetry. The energies of the exchange reation were then evaluated at the correlated level, using a large atomic natural orbital basis and correlating all valence electrons. Several correlation treatments were used, namely, configration interaction with single and double excitations, coupled-pair functional, and coupled-cluster methods. We are thus able to measure the effect of accounting for size-extensivity. Zero-point corrections to the correlated level energetics were determined using analytic second derivative techniques at the SCF level. Our best calculations, which include the effects of connected triple excitations in the coupled-cluster procedure, indicate that the trimer is bound by 9 +/- 1 kcal/mol relative to three separate monomers, in excellent agreement with previous estimates. The barrier to concerted hydrogen exchange is 15 kcal/mol above the trimer, or only 4.7 kcal/mol above three separated monomers. Thus the barrier to hydrogen exchange between HF molecules via this termolecular process is very low

THEORETICAL STUDY OF THE CONFORMATIONAL PROPERTIES AND TORSIONAL POTENTIAL FUNCTIONS OF METHACRYLOYL FLUORIDE

Author Institution: Analatom Incorporated; NASA Ames Research Center; Polyatomics Research InstitueThe suitability of ab initio self Consistent Field (SCF) molecular orbital and gradient methods for determining the conformational energies and torsional potentials of organic molecules such as methacryloyl fluoride $(CH_{2}=C(CH_{3})COF)$ is demonstrated. Often the barrier heights to internal rotations are not measured directly by experiment but determined extrapolation. However, theory allows for a direct determination. The two experimental $studies^{1,2}$ for methacryloyl fluoride, regarding the rotational barriers for the COF and $CH_{3}$ groups are evaluated in light of the theoretical study. In addition a normal mode analysis is made which complements the Infrared and Raman experiment of Durig and $coworkers^{1}$. The calculations suggest that to obtain reasonable barrier heights for larger molecules it would be sufficient to use the 4-31 G basis set, to optimize, using the SCF gradient method, the geometries of the conformations corresponding to local minima and between those, perform SCF calculations following a path of geometrical positions obtained by $interpolation.^{1}$J.R. Durig, P.A. Brletic and J.S. Church, J. Chem. Phys. 76, 1723 (1982) $^{2}$L.A. Glebova, A.V. Abramenkov, L. N. Margolin, A. A. Zenkin, Yu A. Pentin and V. I. Tyulin, Zh. Strukt. Khim. 20, 1030(1979