K. Takegoshi, F. Imashiro, T. Terao
Feb 1, 1984
Citations
0
Influential Citations
15
Citations
Journal
Journal of Chemical Physics
Abstract
We have investigated methyl rotation in 4‐methyl and 4,5‐dimethyl substituted phenanthrenes, fluorenes, and fluorenones by the temperature dependence of the 1H spin‐lattice relaxation times and high‐resolution solid‐state 13C NMR spectra. Distinctly unsymmetric curves of the relaxation times with temperature are ascribed to the existence of inequivalent methyl groups in the unit cell. The inequivalence is considered to be brought about by distortion about the central C4a–C4b bonds and anisotropic intermolecular forces in the crystalline state. The steric effect of the protruding 5‐hydrogen atom in a planar molecular structure gives rise to a remarkably high barrier (5.05±0.13 kcal mol−1) to rotation of the methyl group in 4‐methylphenanthrene (1).