Gas phase observation of the first overtone of the H—F stretching fundamental in hydrogen bonded complexes

First overtones of H—F and D—F stretching vibrations in hydrogen bonded complexes with dimethylether, diethylether, and acetone have been measured in the gas phase. The assignment of the observed bands is discussed together with previously made observations on the fundamentals. Anharmonicity constants of the order of −200 and −100 cm−1 are determined for the most probable assignments from the available data for the H—F and D—F stretching vibrations (ν1) while their values in the "free" H—F and D—F molecules are −90 and −46 cm−1 respectively. The spectra are compatible with a coupling constant between the ν1 and the bridge stretching vibration (ν3) of the order of +70 cm−1.