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A method is proposed for calculating internal rotation barriers in ethanelike molecules based on two effects, exchange interactions of the electrons in bonds adjacent to the bond about which internal rotation occurs and nonbonded or van der Waals interactions. A modified Buckingham or ``6-exp'' potential is used for the nonbonded interactions and a semiempirical method is used to obtain the constants in the 6-exp potential for the various kinds of nonbonded interactions. The equation for U(ω) contains one adjustable parameter, U0, which theoretically should be constant for all molecules within certain classes. An average value for U0 is obtained for each class from selected microwave values of ΔU by letting ΔU have the microwave experimental value and solving for U0. It was found that U0 was essentially constant within each class, thus allowing these average values to be used to calculate ΔU for all molecules in each class. Satisfactory agreement between the calculated values and experimental values of ΔU was obtained in all cases tested except two. A discussion is included of the extension of this method to more complex molecules with more than one internal rotational degree of freedom, allowing for other energy contributions such as distortion of bond angles. Brief mention is also made of the possible extension of the method to the conformational analysis of macromolecules such as polypeptides and synthetic polymers.
Scott et al. (Mon,) studied this question.