Deuterium conformational equilibrium isotope effects in 1,3,5‐cycloheptatriene‐7‐d

Abstract

AbstractWe report a reinvestigation of the conformational equilibrium of 1,3,5‐cycloheptatriene‐7‐d (CHT‐7‐d) by solution 1H NMR at 500 MHz over a wide range of temperatures, and ab initio calculations up to the MP4 level. Lineshape analysis provided a barrier to ring inversion of approximately 6 kcal mol−1. Equilibrium constants were measured in CBrF3 and CClF2H by: integration when the ring‐inversion rate is low; lineshape analysis when the ­ring‐inversion rate is on the order of the chemical shift difference; and temperature‐dependent chemical shift differences between CHT and CHT‐7‐d when the ring‐inversion rate is fast. The three independent measure‐ments confirm an equilibrium biased toward an equatorial deuterium, with ΔG° = 52 ± 8 cal mol−1 (at −173 °C), ΔH° = 55 ± 8 cal mol−1 and ΔS° = 0.03 ± 0.05 cal mol−1 K−1 in CBrF3. Supporting ab initio calculations yield: ΔH° = 34 cal mol−1, ΔS° = 0.014 cal mol−1 K−1 (MP2/6‐31G*); ΔH° = 57 cal mol−1 and ΔS° = 0.03 cal mol−1 K−1 (RHF/6‐31G*); and a barrier to ring inversion in CHT of 4.26 kcal mol−1 (RHF/6‐31G*), 9.76 kcal mol−1 (MP2/6‐31G*), and 7.34 kcal mol−1 (MP4/6‐31G* single point). The differences in calculated barrier heights due to exclusion or inclusion of Møller–Plesset energies indicate that the ground state is stabilized relative to the transition state at ­the MP2 and MP4 levels. This implies that homoconjugation is important in CHT, a neutral molecule. Our experimental values differ from those originally deduced by Jensen and Smith (J. Am. Chem. Soc. 1964; 86: 956): ΔG° = 72 cal mol−1, ΔH° = 142 ± 30 cal mol−1 and ΔS° = 0.7 ± 0.3 cal mol−1 K−1. Copyright © 2001 John Wiley & Sons, Ltd.