Microwave spectrum of the OD-OH₂ complex: A strong deuterium isotope effect on angular momentum quenching in the hydroxyl moiety

Microwave spectra of the hydrogen bonded complex ¹⁶OD-¹⁶OH₂ have been recorded using pulsed-nozzle Fourier transform microwave spectroscopy. The potential splitting, ρ, which describes the partial quenching of the OD orbital angular momentum upon complexation, is determined to be -142.703173(65) cm^-1. Within the spectroscopic model employed, this value implies an energy difference of 202.46 cm^-1 between the ground (²A′) and first excited (²A′′) states of the complex. The observed value of ρ represents a rather large change of 3.85710(11) cm^-1 relative to that in the parent complex and implies a 1.30 cm^-1 decrease in the ²A′-²A′′ energy spacing relative to the parent species. Comparison with previous results for the ¹⁸OH complex suggests that these changes likely arise from changes in vibrationally averaged geometry upon deuteration. Magnetic hyperfine structure from the deuterium and the water protons is analyzed, as is the nuclear electric quadrupole coupling of the deuterium nucleus. Assuming negligible changes in the axial component of the electric field gradient at the deuterium upon complexation, the deuterium quadrupole coupling constant implies an average angular excursion of the OD bond axis from the vibrationally averaged a-inertial axis of the complex of ∼24°.