THE C7H6 POTENTIAL-ENERGY SURFACE REVISITED - RELATIVE ENERGIES AND IR ASSIGNMENT

The geometries and force fields of phenylcarbene (PC) and cycloheptatr ienylidene (CHT) in their singlet and triplet electronic states as wel l as of cycloheptatetraene (CHTE) and bicyclo[4.1.0]heptatriene (BCT) and the transition states for the formation and decay of the latter we re evaluated by various methods. Relative single point energies were c alculated at the CCSD(T)/cc-pVDZ//BLYP/6-31G level. Finally, the effe cts of extending the basis set to triple-zeta quality were estimated b y (R)MF2 calculations and carried over proportionally to CCSD(T). Thes e calculations show that CHTE which has a strongly distorted allenic s tructure is the most stable species on that part of the C(CH)(6) surfa ce which was examined in the present study, followed by planar (PC)-P- 3. The strained BCT is found to be nearly degenerate in energy with (P C)-P-1, but the high activation energy for its formation from (PC)-P-1 together with the low activation energy for ring-opening to CHTE sugg ests that this species cannot persist under the experimental condition s employed for production of CHTE. In analogy to the case of cyclopent adienylidene, CHT exists in the form of a closed shell singlet ((1)A(1 )) and two related pairs of open shell singlet and triplet states ((1, 3)A(2) and B-1.3(1)) which correspond to the Jahn-Teller distorted str uctures of the cycloheptatrienyl radical, The relative energies and th e nature of the different CHT stationary points depend on the method o f calculation, but it appears that the decrease in electron repulsion lowers the (1)A(2) State slightly below the (1)A(1) state so that the open shell species serves as a planar transition state for enantiomeri zation of CHTE with an estimated activation energy of similar to 20 kc al/ mel. The two triplets are very close in energy with the higher lyi ng being either a transition state or a shallow minimum. The B-1(1) st ate is an excited state of the open-shell singlet. The calculated IR s pectra of the three most stable isomers were compared to those publish ed previously by Chapman et al. whereby the assignment of the photopro duct of UV photolysis of phenyldiazomethane to CHTE was confirmed. A f ull study of the force fields of PC and CHTE is under way.