Rotational Magnetic Moments in Polyatomic Molecules

There is generally present in a molecule a small magnetic moment associated with its rotational degree of freedom. The rotational $g$-factor can be measured by an experimental method based upon the Zeeman effect. Previously, the most successful measurements have been made with the molecular beam resonance technique. More recently, the microwave Zeeman splitting of molecular rotation lines has offered another method for the determination of rotational $g$-factors. With this method, the $g$-factors of several molecules such as N${\mathrm{H}}_3$, ${\mathrm{H}}_2$O, OCS, etc. have been measured. The quantum mechanical theory of the rotational magnetic moment of the symmetric top molecule has been developed, and the experimental results are discussed qualitatively in the light of this theory.