The C7H6 Potential Energy Surface Revisited: Relative Energies and IR Assignment

Abstract

The geometries and force fields of phenylcarbene (PC) and cycloheptatrienylidene (CHT) in their singlet and triplet electronic states as well as of cycloheptatetraene (CHTE) and bicyclo[4.1.0]heptatriene (BCT) and the transition states for the formation and decay of the latter were evaluated by various methods. Relative single point energies were calculated at the CCSD(T)/cc-pVDZ//BLYP/6-31G* level. Finally, the effects of extending the basis set to triple-ζ quality were estimated by (R)MP2 calculations and carried over proportionally to CCSD(T). These calculations show that CHTE which has a strongly distorted allenic structure is the most stable species on that part of the C(CH)6 surface which was examined in the present study, followed by planar 3PC. The strained BCT is found to be nearly degenerate in energy with 1PC, but the high activation energy for its formation from 1PC together with the low activation energy for ring-opening to CHTE suggests that this species cannot persist under the experimental conditions employed for production of CHTE. In analogy to the case of cyclopentadienylidene, CHT exists in the form of a closed shell singlet (1A1) and two related pairs of open shell singlet and triplet states (1,3A2 and 1,3B1) which correspond to the Jahn−Teller distorted structures of the cycloheptatrienyl radical. The relative energies and the nature of the different CHT stationary points depend on the method of calculation, but it appears that the decrease in electron repulsion lowers the 1A2 state slightly below the 1A1 state so that the open shell species serves as a planar transition state for enantiomerization of CHTE with an estimated activation energy of ∼20 kcal/mol. The two triplets are very close in energy with the higher lying being either a transition state or a shallow minimum. The 1B1 state is an excited state of the open-shell singlet. The calculated IR spectra of the three most stable isomers were compared to those published previously by Chapman et al. whereby the assignment of the photoproduct of UV photolysis of phenyldiazomethane to CHTE was confirmed. A full study of the force fields of PC and CHTE is under way.