Infrared spectra of CHD2Cl and CHD2CCH and the geometries of methyl chloride and propyne

Five type B or type C bands in the gas phase infrared spectrum of CHD₂Cl and two type C bands of CHD₂CCH have been analyzed with a resolution of 0.2 cm^?1. All other fundamental vibrations have been located. Accurate determinations of ($\text{A}{}_0\text{-}\text{B}{}_0$) for each molecule are obtained from ground state combination differences, from which A₀ values are derived using microwave estimates of B₀ and C₀. The combination of one A₀ rotational constant with B₀ and C₀ constants for all other isotopic species enables the ground state molecular geometry to be fixed within narrow limits, assuming that zero-point energy effects cause a shortening of the CH bond length by 0.002 Å on deuteration. The CH bond lengths are in almost precise agreement with the values predicted from the position of the sole CH stretching vibration frequency for each molecule.