Method for Calculating Internal Rotation Barriers

摘要

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 ``6hyphen;exp'' potential is used for the nonbonded interactions and a semiempirical method is used to obtain the constants in the 6hyphen;exp potential for the various kinds of nonbonded interactions. The equation forU(ohgr;) contains one adjustable parameter,U0, which theoretically should be constant for all molecules within certain classes. An average value forU0is obtained for each class from selected microwave values of Dgr;Uby letting Dgr;Uhave the microwave experimental value and solving forU0. It was found thatU0was essentially constant within each class, thus allowing these average values to be used to calculate Dgr;Ufor all molecules in each class. Satisfactory agreement between the calculated values and experimental values of Dgr;Uwas 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.