3678
Conformational Inversion Rates in the Dimethylcyclohexanes and in Some cis-Decalins Don K. Dalling, David M. Grant,*’ and LeRoy F. Johnson
Contributionf r o m the Department of Chemistry, University of Utah, Salt Lake City, Utah 84112, and f r o m the Instrument Division, Varian Associates, Palo Alto, California 94303. Received October 2, 1970 Abstract: Carbon-13 magnetic resonance (cmr) dataover the temperature range 211-318°K has been used to study the kinetic process associated with conformational inversion in some dimethylcyclohexaneand decalin ring systems. Rate constants and activation enthalpies and entropies are reported for 1,l-,cis-1,2-,trans-l,3-, and cis-l,4-dimethylcyclohexane and for the parent and 9-methyl-cis-decalin. The results may be rationalized in terms of previous data on similar systems and steric interactions. The advantages of cmr technique in kinetic studies of conformational
interconversions are delineated. uclear magnetic resonance has been used extensively for the determination of the thermodynamic parameters associated with conformational equilibration of hydrocarbons such as cyclohexane and related compounds. Most studies have utilized protons3-’6 or f l ~ o r i n e - 1 9 ~ ’magnetic -~~ resonance. In this work we would like t o suggest and to investigate the use of carbon-13 magnetic resonance (cmr) as an alternative to that of ’H or I9Fwhich, under favorable circumstances, offers some unique and desirable advantages. Proton resonance has proved very useful for the investigation of conformational equilibration, but it suffers from two deficiencies. First, proton resonances usually exhibit complex, homonuclear spin-spin splittings which create difficulty in determining rate constants from the line width and the coalescence temperature studies normally used. This problem has been circumvented in numerous cases by extensive deuteration and subsequent double resonance of the deuterium. 2 2 The second limitation arises from the fact that the chem-
N
(1) University of Utah. (2) For revie\\s see (a) J. E. Anderson, Quart. Reo., Chem. Soc., 19, 426 (1965); (b) F. G . Riddell, ibid., 21, 364 (1967); (c) L. W. Reeves, Adcan. P h j s . Org. Chem., 3, 187 (1965).
(3) F. R. Jensen, D. S. Noyce, C. H . Sederholm, and A. J. Berlin, J . Amer. Chem. SOC.,82, 1256 (1960); 84, 386 (1962). (4) R . I
