Study of Molecular Orbital Degeneracy in C7H7

Abstract

The paramagnetic resonance of the cycloheptatrienyl radical (C₇H₇) has been observed in single crystals of thiourea and naphthalene and in polycrystalline thiourea, naphthalene, cycloheptatriene, and argon at temperatures ranging from 4.2° to 300°K. The spectra are of two types: (a) high‐temperature spectra—eight equally spaced lines with splittings in the range 3.5–4.1 G; (b) low‐temperature spectra—highly anisotropic spectra which are markedly different for each crystalline environment. The transitions from (a) to (b) occur within 5°K, and the transition temperatures are 40°, 20°, and around 13°K in thiourea, naphthalene, and cycloheptatriene, respectively. The high‐temperature spectra are readily accounted for in terms of seven equivalent protons and a uniform electron spin distribution about the ring. The high‐temperature spectra thus provide evidence that there is no significant static (Jahn—Teller) distortion of C₇H₇. The uniform spin distribution around the molecular ring is brought about by the combined effects of rapid molecular reorientation about the seven‐fold axis, plus the crystalline electric‐field splitting of the orbital degeneracy. That is, in a sense, the crystal field splitting ``locks'' the spin distribution to the lattice, and molecular reorientation moves the nuclei relative to a fixed spin distribution. The low‐temperature spectra correspond to the electronic ground state of (nonreorienting) C₇H₇, as determined by the crystalline electric field. The observed low‐temperature spectra of C₇H₇ in naphthalene are compared with spectra calculated using the Hückel, valence‐bond, approximate unrestricted Hartree—Fock LCAO—MO, and Pariser—Parr approximations to the molecular electronic structure of C₇H₇. The observed spectra appear to be in semiquantitative agreement with calculations based on the Hartree—Fock and Pariser—Parr approximations. However, detailed comparisons between these theoretical calculations and experiment are uncertain since both vibronic interactions as well as torsional molecular oscillations have an effect on the low‐temperature spin distribution.