Conformational analysis of the cis- and trans-1,2-bis(trimethylsilyl)cyclohexanes (BTMSC): molecular mechanics calculations and nuclear magnetic resonance spectroscopy. A preferred diaxial conformation for the trans-isomer

Abstract

Molecular mechanics calculations (MM3-92) suggest that a slightly distorted diaxial chair conformation of trans-1,2-bis(trimethylsilyl)cyclohexane is at least 1.26 kcal mol^-1 (1 cal = 4.184 J) lower in steric energy than the diequatorial form, and this is supported by NOED spectra, magnitudes of vic-¹H–¹H spin coupling constants, and low temperature (190 K) ¹³C NMR spectroscopic measurements, which place the free energy difference between 1.5 and 1.7 kcal mol^-1.