US EPA

8711972 

Journal Article 

NMR conformation studies of rotational energetics about the C-C bond in trisubstituted ethanes of the types CH2X-CHX2 and CH2X-CHY2 

Chen, JS; Shirts, RB; Lin, WC 

1986 

1 

Journal of Physical Chemistry
ISSN: 0022-3654 

90 

21 

4970-4975 

English 

10.1021/j100412a020 

We have derived, based on rotational averaging and the Karplus equation, an equation governing the temperature dependence of the H-H vicinal coupling constant, J, for substituted ethanes of the form CH2XCHX2 or CH2XCHY2. This model contrasts with the conventional rotational isomeric state model in that all rotational conformers are included instead of only the discrete stable forms. The model depends on an energy parameter, V1, which is a measure of the maximum interaction energy between two C-X bonds or between C-X and C-Y bonds as a function of rotational configuration about the C-C axis. From the temperature variation of J in an inert solvent (e.g., cyclohexane), the characteristic constant A in the Karplus equation is determined. The value of A thus obtained is assumed to be constant for that particular compound regardless of temperature and solvent conditions and is employed to obtain the energy difference between gauche and trans conformers, ΔE, in various solvents at various temperatures. The value of ΔE in the gas phase can also be estimated by using solvent theory. Temperature and/or solvent dependence of J for CH2ClCHCl2 and CH2BrCHBr2 are taken as examples to illustrate the utility and accuracy of this new approach. 1986 American Chemical Society.