Gurvinder Gill - J. Org. Chem, Vol. 70, No. 26, 2005

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      J. Org. Chem, Vol. 70, No. 26, 2005


      Low-temperature     13C   NMR spectra of   cis-1,4-di-tert-butylcyclohexane  (1)   showed signals for the   twist-boat (1a)   and chair (1b)   conformations.   13C   NMR signals were assigned to specific   carbons

      based on   the different populations, different symmetries (time-averaged     C2v    for   1a   and time-averaged   C   for   1b),   and calculated chemical shifts (GIAO,   HF/6-311+G*).   In addition to slow ring inversion   and interconversion of the chair and twist-boat conformations,   slow rotation of the       tert-butyl   groups   was found. Most of the expected   13C   peaks were observed. Free-energy barriers of 6.83 and   6.35   kcal/mol were found for interconversion of   1a   (major) and   1b   (minor) at -148.1   °C. Conformational   space was searched with Allinger’s MM3 and MM4 programs, and free   energies were obtained for several low-energy conformations     1a-c.   Calculations were repeated with ab initio methods up   to   the HF/6-311 +G*   level. Molecular symmetries, relative free energies, relative   enthalpies and   entropies, frequencies, and NMR chemical shifts were obtained. A   boat conformation (1d;    C2v    symmetry) was generated and optimized as a transition state by ab   initio, MM3, and MM4   calculations.





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