Pseudorotation-driven dynamical structure of the tropyl radical

Abstract

Despite intensive studies of the neutral tropyl radical, none of its structure, energetics, and vibrational modes are still clear. This system has puzzled scientists for over a decade since one vibrational mode frequency sharply varies from imaginary number 3000i cm(-1) to the real number 6000 cm(-1), depending on the calculation methods employed. We find that the origin of this peculiar mode is due to the pseudorotation (omega(irot)) involved in the interconversion of two nearly isoenergetic Jahn-Teller configurations (elongated structure B-2(1) and compressed structure (2)A(2) with C-2v symmetry). Here, we first report that this interconversion is not via D-7h or C-2v symmetry configuration but via C-s symmetry (i.e., by changing the C-2v axis). This interconversion barrier is found negligibly small. Thus, the two conformers are considered to be not two different structures but a dynamically identical structure with partial quantum statistical distributions on the potential energy surface. Owing to the nearly barrierless pseudorotation, the overall structure in a short time scale (less than femtosecond) would be C-s-like between (2)A(2) and B-2(1) configurations with small fluctuation of bond distances. However, the dynamical transitions between the B-2(1) and (2)A(2) configurations via 14 different pseudorotation pathways would make the tropyl radical have the effective D-7h structure in either a nonshort time scale (greater than femtosecond) or at nonlow temperatures, which explains the high temperature electron spin resonance experiments. (c) 2006 American Institute of Physics.