Anomalous isotope effect in the H bonds of acetic acid dimers

Measurements of the integrated intensities of the νO–H ... O, νO–H, and νC=O bands of acetic acid CH3COOH in the vapor phase (where H-bonded dimers are in equilibrium with non H-bonded monomers), together with measurements of the intensities of the corresponding bands in CH3COOD, have been performed at various pressures and temperatures. It has been found that the transition probability P O–H ... O for the 0 → 1 transition of the stretching motion νO–H ... O of the H atoms in the H-bonds of (CH3COOH)2 was 2 times greater than the corresponding probability νO–D ... O in (CH3COOD)2. This value was determined with a precision better than 10%. The ratio PC=O ... H/PC=O ... D of the transition probabilities for the νC=O ... H and νC=O ... D modes in (CH3COOH)2 and (CH3COOD)2 was found to be equal to 1.25, with a precision better than 10%. The same ratios were found in CD3COOH and CD3COOD. These results are quite puzzling and have not yet been interpreted (by comparison, the ratio P O–H/P O–D of the transition probabilities for the free νO–H and νO–D modes in the monomers was found to be equal to 1.3). This ``anomalous'' ratio P O–H ... O/P O–D ... O seems hardly to be understandable on the basis of anharmonicity considerations alone, and it is suggested, after a critical examination of the possible origins of this effect, that the H bonds are not adiabatic. This would imply that the motion of the H atoms along the H bonds induces electronic transitions.