Carbon-carbon pi antibonding effects on the thermochemistry of alkanes, elucidated by angular overlap and MO calculations

It is postulated that the enthalpies of formation of alkanes can be rationa lised quantitatively in terms of an additive bond energy scheme with a cons tant C-H term (taking the methane value of 415.8 kJ mol(-1)) while the C-C bond energy is dependent upon the Tc-antibonding effects of the highest occ upied molecular orbitals (HOMOs). This antibonding effect can be treated by angular overlap methods with two adjustable parameters, e'(pi)(HH) (which represents the magnitude of the effect where the 2p(pi) orbitals concerned are engaged in sigma -overlap only with hydrogen 1s orbitals) and e'(pi)(HC ) (where one 2p(pi) orbital is sigma -bonded to hydrogen atoms and the othe r to carbon atoms); e'(pi)(CC) is deemed to be zero, so that the C-C bond e nergy in the absence of any pi antibonding effect takes the diamond value ( 357.4 kJ mol(-1)). The values for e'(pi)(CH) and e'(pi)(HH) are obtained fr om the experimental enthalpies of formation of C-2-C-6 alkanes having no 1, 4 steric interactions. It is then possible to calculate bond energy terms f or each of the types of bonds C-X-C-Y [X, Y = P(primary), S(secondary), T(t ertiary), Q(quaternary)]. These yield calculated enthalpies of formation fo r alkanes CnH2n (+ 2) (n = 2-8) in excellent agreement with experiment in t he absence of significant steric interactions. Corrections for 1,4 steric r epulsion are needed for C-S-C-Q, C-T-C-T and C-T-C-Q bonds but not for C-S- C-T. The HOMOs in CH4-nMe. (n = 2, 3) obtained from semi-empirical MO calcu lations (PM3) have the same relative orbital energies (b(2) > a(1) > b(1) f or n = 2, a(1) > e for n = 3) as found in the angular overlap calculations, and there is a clear correlation between the antibonding effects as measur ed by MO eigenvalues and the empirical thermochemical parameters. There is also an unexpectedly good agreement between the atomic fractional charges i n alkanes calculated by Mulliken population analysis of the PM3 MOs and tho se obtained by electronegativity equilibration.