A nuclear magnetic resonance study of molecular disorder in the solid state of some medium ring hydrocarbons

Abstract

Since the discovery of nuclear magnetic resonance in bulk matter in 1945, n.m.r studies of molecular motion in solids have provided a powerful insight into the details of molecular reorientation processes that occur in solids. By studying the variation of the n.m.r. parameters as a function of temperature it is usually possible to obtain information regarding such phenomena as solid-solid phase transitions, including the anomalous behaviour of heat capacity associated with order-disorder and co-operative processes, rotational disorder and self-diffusion. In suitable cases it is possible to estimate the origin and magnitude of the potential barriers of the crystalline field which hinder the various reorientation processes. The cycloalkane ring series (CH₂)[sub]n from cyclopropane n =3 to cyclohexane n=6 have all previously been studied in the solid state by n.m.r. The results indicate that considerable molecular mobility occurs in the solid state o these substances and that in general the solid-solid phase transitions detected by hear capacity measurements are associated with the onset of molecular reorientation and in some cases self-diffusion. The current interest in the molecular configuration of the medium ring hydrocarbons, and the extensive study by Finke et al. (1956) of the low temperature thermal properties of cycloheptane n=7, and cyclooctane n=8, showing three and two solid-solid phase transitions respectively, prompted an n.m.r. study of cycloheptane and cyclooctane. The results of this investigation together with those of a related seven membered ring hydrocarbon 1, 3, 5 cycloheptatriene C₇H₈ are presented in this thesis, and show that extensive molecular motion exists well below the melting points of all three substances, and information is given regarding the form of the motion in the various crystalline phases.