The use of vicinal HH coupling constants in rotational isomerism studies, I

Abstract

By using simple model potential energy functions for internal rotation of ethane derivatives, associated with the Karplus relationship for vicinal HH coupling as a function of the dihedral angle, an investigation was made to find out how serious an approximation it is to reduce the whole conformational problem to a case of rapid equilibrium between the three staggered conformers, especially as far as the calculated energy differences for these are concerned. A classical view of internal rotation was adopted and only systems for which two staggered conformers are isoenergetic were studied: CH 2XCH 2X, CH 2XCH 2Y, CH 2XCHX 2, CH 2YCHX 2 CHX 2CHX 2, CHX 2CHY 2, CHXYCHXY ( meso form). It is found that the errors are temperature dependent and can be quite large, the calculated Δ E values being always smaller than the real ones. Simple rationalizations are offered for these and other findings.