Semi-empirical SCF MO calculations of the electronic structures of CH3F and CH3F– and their significance for adiabatic electrode processes involving bond-breaking
The electronic structure and geometry of CH3F– ion is calculated by the CNDO molecular orbital method and the results calibrated by parallel calculations on CH3F. The C—F equilibrium distance increases by a very small amount (0.031 Å) on addition of an electron to CH3F. The half-occupied orbital in CH3F–(in Unrestricted Hartree-Fock approximation) is antibonding between the carbon and fluorine, but bonding between fluorine and the hydrogens. The HCH angle is increased from 109·3 to 114·3° as a result of the increased F—H bonding. The negative charge of the anion is calculated to be distributed between the hydrogens and the fluorine, the carbon atom bearing a small positive charge.
The implications of these calculations for the mechanism of the electrochemical reduction of alkyl halide molecules, in which it has been postulated that the first electron-transfer step leads to formation of the molecular negative ion, is discussed. An interpretation of the observed trend of exchange rate constants for methyl halides is proposed. A similar interpretation for a series of substituted halides leads to the prediction that in complete electro-reduction of a particular stereoisomer, configurational inversion will be more complete the higher the atomic number of the halogen.
For the CH3F/CH3F– exchange, the calculated energy of internal rearrangement in forming the transition state is small (< 1 kcal/mole). The possibility of complete calculation of the rate constants of a variety of electrode processes involving bond fission for which the primary step is of the type XY + e(M)→(XY)– is discussed. It is concluded that this is now possible, using relatively simple methods of calculation, but that in order to locate the oxidation-reduction potential of the primary step, more detailed methods will need to be used to obtain the ground-state energy of the (XY)– ions.