Abstract
The extant theory concerning the interpretation of the infrared and Raman spectrum of dimethylacetylene is summarized. This theory is extended and the symmetry group, symmetry coordinates, and the symmetry properties of the normal coordinates are all discussed. The usual G matrix theory and FG matrix method for the determination of force constants and normal coordinates has to be modified and the G matrix most useful for the calculations is derived. Detailed expressions are obtained for the energy levels of the first excited state of each of the normal vibrations; it is shown that the approximate expressions previously derived in the literature by Howard, Mills and Thompson, Kirtman, and Bunker and Longuet‐Higgins can be obtained from these more exact expressions. Finally the infrared and Raman selection rules are summarized and band shapes are discussed.