Correlation of Activation Energies and Bond Energies in CF3 Reactions

Abstract

Rate constants and activation energies have been measured for deuterium abstraction from CD4, (CH3)3CD, and Cl3CD, to give, respectively: 10—12.6 exp(—12.1/RT), 10—11.3 exp(—7.4/RT), 10—12.0×exp(—9.0/RT) cc/molecules·sec with activation energies in kilocalories. By means of the London—Polanyi—Eyring—Sato LPES method of evaluating potential energy surfaces (with the one adjustable parameter fitted to the activation energy of the reaction CF3+CH4) and by means of activated complex theory, the predicted kinetic quantities (Arrhenius A, kH/kD, and other activation energies) were calculated for the two series involving both H and D: H3C–H, C2H5–H, (CH3)2CH–H, (CH3)3–H; H3C–H, H2ClC–H, HCl2C–H, Cl3C–H. The results are compared with observed values from the literature and from this study. The observed trend of E with bond energy for the series was compared with that predicted by the LPES method and that predicted by the one-parameter Polanyi—Semenov rule. For the general family of reactions of CF3 on substituted hydrocarbons RH, the following evaluation of the predictive value of the LPES method is made: Arrhenius E, not predicted but used to select the one adjustable parameter; Arrhenius A, predicted within a factor of 10 when full N-atom model is used and predicted very poorly when simple three-atom model is used; kH/kD, excellent prediction over wide range of temperature if large tunneling correction is made; E trend with —ΔH, good prediction, but no better than the vastly simpler Polanyi—Semenov rule.