Microwave spectra of partially deuterated acetic acid: Internal rotation,potential energy function,and methyl group geometry

The microwave spectra of CH₂DCOOH and CHD₂COOH have been studied by means of microwave-microwave double resonance. For the asy rotamers torsional splittings (5898 and 530 MHz, respectively) and effective rotational constants were determined in the ground state. Effective barrier parameters were provisionally estimated and used to predict excited-state spectra. Here significant interaction between sy and asy rotamers occurred, and a Hamiltonian based on an extension of the IAM method to the case of an asymmetric internal rotor was used to account for the spectra. A few direct sy-asy transitions were observed as well as spectra originating from the second excited torsional state. Effective potential energy coefficients, V₁ through V₆, were determined accurately; apart from V₃ and V₆, which are comparable to values in CH₃COOH and CD₃COOH, large V₂ terms occur (28.5 cm^?1 in CH₂DCOOH and ?25.4 cm^?1 in CHD₂COOH). These terms provide localization in the ground state wave functions, and can be rationalized as arising from the zero-point energies of the other normal vibrations. Also determined were Fourier components of the rotational constants, which were in fair agreement with results from model calculations when geometry relaxation was included. After correction of the ground state inertial moments for effects of the torsion a consistent set of inertial moments was obtained for the various isotopic species, and a complete substitution structure could be determined. The HCH angles in the methyl group were found to differ by 2.7°.