Microwave spectroscopic measurements on the partially deuterated forms of CH3Cl, CH3Br, and CH3I allow a determination of the structural parameters which is less subject to zero-point vibration errors than other methods. In addition, examination of all measured isotopic species reveals that the C—D distance is shorter than C–H by 0.009A, and the D—C—D angle greater than that for H–C–H by about 12 minutes. Best values of structural parameters for the species containing the light isotope of hydrogen only are C–Cl distance 1.781, C–Br 1.939, C–I 2.139, C–H 1.11 (same for all halides), H–C–H angle 110°31′ for CH3Cl, 111°14′ for CH3Br, and 111°25′ for CH3I. Deuteration not only gives a change in the C–H bond but also a well-correlated change in the quadrupole coupling constant of the halogen.
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