Structure and bond shift kinetics of cyclooctatetraene studied by NMR in nematic solvents

Abstract

A nuclear magnetic resonance study of cyclooctatetraene (COT) in a number of different nematic solvents covering a wide temperature range (− 35 to 170°C) is reported. Proton spectra for both normal COT and partially deuterated COT under conditions of deuterium decoupling were studied. From an analysis of the spectra in terms of direct dipolar interactions, it is concluded that the COT molecule has a symmetrical tub (D2d) conformation, and coordinates for the protons, within a scale factor, are derived. The spectrum undergoes profound changes with temperature, which are interpreted in terms of a bond shift process. The activation energy is found to be ΔE = 10.9 kcal mole−1 and the rate at 0 °C: 1/τ = 2.5 × 102 sec−1. In an appendix, symmetry considerations are used to identify those spectral peaks which are not affected by the bond shift process. In another appendix, the exchange broadening in a system consisting of two equivalent protons with direct dipolar interaction is treated quantitatively.