Out-of-Plane Vibrations of a Conjugated Hydrocarbon : trans-Butadiene

Abstract

A method of obtaining some of the out‐of‐plane force constants in conjugated hydrocarbons from bond orders and of borrowing the other kinds of these constants from similar molecules is described. The method includes the assumptions that the force constant of torsion around the CC bond is proportional to the product of the π bond order and the overlap integral of the carbon 2pπ atomic orbital and that the interaction constant between wagging motion of a CH group and a torsion around a CC bond to which the CH group is attached is proportional to the diagonal constant of the torsion. The validity of these assumptions is discussed. As an application of the method the nonplanar frequencies of trans‐butadiene are calculated and compared with experimental values. The results are found to be encouraging.