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.
Published Version
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