Observation and quantification of the hydrogen bond effect on O-H overtone intensities in an alcohol dimer

First overtone (2νOH) transitions of the two OH-stretching modes in isolated 2,2,2-trifluoroethanol dimers are observed for the first time. Their relative intensities provide quantitative evidence for the pronounced IR intensity suppression of two-quantum hydrogen-bonded OH-stretching excitations relative to fundamental transitions by up to three orders of magnitude. The OH anharmonicity is shown to increase by 15% upon hydrogen bonding to another oxygen, whereas it remains constant upon hydrogen bonding to a fluorine atom. The experimental results are supported by reduced-dimensionality anharmonic calculations in up to three dimensions.