Gurvinder Gill - Conformational Study of cis-1,4-Di-tert-butylcyclohexane by Dynamic NMR Spectroscopy and Computational Methods. Observation of Chair and Twist-Boat Conformations

Version 1

      Publication Details (including relevant citation   information):

      J. Org. Chem, Vol. 70, No. 26, 2005 10731

      Abstract:

      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

      Cs 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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