Nuclear magnetic resonance study of hindered rotations of the π-bonded ring in crystalline benzenetricarbonylchromium

Abstract

N.m.r. broad line measurements on crystalline benzenetricarbonylchromium at various temperatures show two narrowings of the proton absorption line-width, centred at about –165 and –45°C. The corresponding activation energies are E₁= 3.4 ± 0.4 and E₂= 7.2 ± 0.6 kcal mol^–1. A more reliable value for the activation energy E₁, obtained by measurements of the spin-lattice relaxation time, results in E₁= 4.2 ± 0.1 kcal mol^–1. The low temperature narrowing is attributed to the rotation of the arene groups, in agreement with our second moment calculations of the absorption signal. Comparison of E₁ with the corresponding activation energies for C₆H₆ and C₆F₆ indicates that most of the rotational hindrance is due to the packing forces. Hypotheses are advanced for the interpretation of the second line-width narrowing.