CI CALCULATIONS ON DIDEHYDROBENZENES PREDICT HEATS OF FORMATION FOR THE META AND PARA ISOMERS THAT ARE SUBSTANTIALLY HIGHER THAN PREVIOUS EXPERIMENTAL VALUES

CI calculations with the 6-311G* basis set on o-didehydrobenzene (1) predict a singlet-triplet energy difference of 34.4 kcal/mol, which is about 3 kcal/mol smaller than the value measured by Leopold, Miller, and Lineberger. Calculations at this level also predict energy differe nces between the singlet ground state of 1 and those of the meta (2) a nd para (3) isomers of respectively 15.8 and 28.4 kcal/mol. These valu es are both larger by about 6 kcal/mol than the values for the differe nces between the heats of formation obtained from the experiments of W enthold, Paulino, and Squires. Computational evidence is presented tha t calculations which increased DELTAE(ST) in 1 by 3 kcal/mol would als o increase the energy differences computed between 1 and 2 and 1 and 3 by about the same amount. In addition, the calculated values of DELTA E(ST) in 2 and 3 are both at least 8 kcal/mol smaller than the values estimated from two different types of experimental data for DELTAH-deg rees f for the triplets and the experimental values of DELTAH-degrees f reported for the singlets. Calculations of the bond strengths in sin glet and triplet 1 support the experimental value of DELTAH-degrees f = 106 +/- 3 kcal/mol for singlet 1, but the calculations predict bond strengths in 2 and 3 that are about 8 kcal/mol smaller than the values obtained from their experimental heats of formation. Since the bicycl ic isomers (4 and 5) of 2 and 3 are both calculated to be higher in en ergy than their monocyclic counterparts, the formation of 4 and 5 in t he experiments of Wenthold, Paulino, and Squires cannot reconcile the heats of formation, measured by them, with the energies for the lowest singlet states of 2 and of 3, calculated by us. On the basis of our c omputational results, it is proposed that DELTAH-degrees f for 2 and 3 is higher than that of 1 by respectively greater-than-or-equal-to 18 and greater-than-or-equal-to 30 kcal/mol.