Precision measurements of the

J = 1 \leftarrow 0

rotational frequencies and the molecular magnetic moments for various isotopic species of carbon monoxide have been made in order to determine isotopic mass ratios. A correction amounting to several hundred micromass units and evaluated from the magnetic moment has been applied for the fact that the electrons are not spherically distributed about their respective nuclei. It is also shown that the rapidly precessing electronic angular momentum causes a "wobbling" motion of the nuclei which in turn produces a stretching of the molecule ("wobble stretching") inversely proportional to the reduced mass of the molecule, but independent of

J

. This stretching cannot be accurately evaluated from theory and therefore appears to be the ultimate limitation on microwave determinations of mass ratios. This correction amounts to about 20 micro-mass units and is evaluated by the use of the nuclear reaction value for the

C^{14}

C^{12}

mass ratio. The final mass ratios agree very closely with the nuclear reaction values.

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