Knowledge Bank

The fine structure of the methanol microwave lines $J=0\leftarrow 1$, $K=0\leftarrow 0$ has been observed experimentally by Venkateswarlu, Edwards, and $Gord.1$ both for the normal and for five isotopic molecules. Since the lines corresponding to n = 0, 1, and 2 are found to be split, these data comprise some 36 individual frequencies. The theory of the fine structure has been examined using an approximate Hamiltonian similar to that given by $Kivelson2$ for symmetric hindered rotators but which incorporates the asymmetry existing in methanol. For each isotopic species the formulas contain four constants. Three of these depend solely on the elastic force constants and dimensions of the molecule. In principle they can be calculated exactly, although to obtain the best fit slight adjustments have been made which are not inconsistent with the near-infrared spectrum. The fourth constant describes the dependence of the barrier height upon the normal coordinates and is determined empirically for each species. The thirty line separations may thus be computed with the aid of essentially only six empirical constants. The agreement is generally quite good and is illustrated by the following table for normal methanol. [FIGURE]