Hyperconjugation. II. The Electronic Structures and Conjugation Energies of Alkylbenzenes

The theory of hyperconjugation has been rescrutinized for a series of alkylbenzenes using the empirical LCAO MO method including overlap integrals for neighboring atoms. A detailed comparison of the theoretically computed and empirically determined conjugation energies of benzene gives a value of −70. 0 kcal. for β, the reasonance integral for benzene C-C. Using this value hyperconjugation energies for toluene, ethylbenzene and tert-butylbenzene are 1.45, 1.24 and 0.79 kcals. respectively. The third-order conjugation energies are found to be considerably less than any previous evaluations. Comparison of the predicted and observed dipole moments, absorption frequencies and oscillator strengths show fairly satisfactory agreements between them.