Is overlap repulsion between C–H bonds responsible for differences in enthalpies of formation among isomers in alkanes, alkenes and simple derivatives? An empirical inquiry
It is suggested that overlap repulsion between vicinal C–H bonds should be explicitly invoked in empirical calculations of the enthalpies of formation of simple organic gaseous substances. The extent of such repulsion is deemed to be proportional to the sum (over all bonds Ci–Cj) of the products aiaj where ai and aj are the numbers of hydrogen atoms bonded to carbon atoms i and j. With this proportionality constant c, and the bond energy terms E(C–C) and E(C–H), the enthalpies of formation of alkanes CnH2n+2 (n=2–7) can be reproduced within the limits of experimental uncertainty; steric corrections are necessary only where there are at least two contiguous, sterically active C–C bonds. Equally good results are obtained for alkyl radicals and alkenes, although the large experimental uncertainties for many haloalkanes preclude the satisfactory determination of all required parameters. These results may open the way to more elaborate quantum mechanical calculations of overlap repulsion which could prove to be more economical than standard abinitio or density functional methods.