METHYL REORIENTATION IN SOLID 3-ETHYLCHRYSENE AND 3-ISOPROPYLCHRYSENE

We have measured the proton spin-lattice relaxation rate as a function of temperature in polycrystalline 3-ethylchrysene at nuclear magnetic resonance Larmor frequencies of 53.0 and 22.5 MHz and in polycrystall ine 3-isopropylchrysene at 53.0, 22.5 and 8.50 MHz. The syntheses of t hese new compounds are presented. The relatively large chrysene backbo ne creates an ideal and unique environment for the alkyl groups such t hat methyl group rotation is the only motion on the nuclear magnetic r esonance Larmor frequency timescale over a large temperature range. Th e relaxation rate data are interpreted in terms of the simplest possib le dynamical model; that of random hopping for the methyl group(s), al l of which are equivalent in the solid state. The barriers of 11-12 kJ mol(-1) are typical for methyl groups in 'isolated' ethyl and isoprop yl groups. (C) 1998 Elsevier Science B.V. All rights reserved.