Microwave spectrum of acetaldehyde with asymmetric CH2D and CHD2 internal rotors

Abstract

The microwave spectra of asym-CH₂DCHO and asym-CHD₂CHO have been investigated in detail in the range 8–40 GHz. Tunnelling of the methyl group between the two equivalent conformations causes splitting of the torsional ground state into a symmetric (0₊) and antisymmetric level (0_–). The rotational levels based on these torsional substates are perturbed by coupling between internal and overall rotation, causing deviations from rigid-rotor behaviour and the appearance of intersystem (0₊↔ 0_–) transitions. The spectral assignment was helped considerably by microwave–microwave and radiofrequency–microwave double-resonance techniques. A coupled-level Hamiltonian augmented by centrifugal distortion terms has been used to analyse the spectra. The pure torsional splittings of asym-CH₂DCHO and asym-CHD₂CHO have been determined to be 804.5(1) and 183.6(1) MHz, respectively.

All five quartic centrifugal distortion constants have been determined for each torsional substate for both molecules. These constants agree well with values calculated from the published harmonic force field. Anomalies have been observed in the Q-branch, J_1,J(∓)–J_0,J(±) series of asym-CH₂DCHO which are almost certainly due to near degeneracies with J= 16 and 17, K_a= 3 levels of sym-CH₂DCHO. These accidental resonances indicate a lowering of the zero-point energy of sym-CH₂DCHO relative to asym-CH₂DCHO of 15.55 cm^–1.