Raman Spectroscopy of Carboxylic Acid and Water Aggregates

In the present work the carboxylic acid dimers and water clusters are chosen as model systems of strong and moderately strong hydrogen bonds and studied with spontaneous Raman spectroscopy coupled with a supersonic jet expansion. The spectra of several carboxylic acids are measured and assigned in the entire fundamental wavenumber region. An overview of their vibrations is achieved by comparing our experimental results with former experimental results as well as with quantum chemical calculations. For the first time, we know from experiment how anharmonic the hydrogen bond vibrations of formic acid are and it is proved that the hydrogen bond becomes stiffer when moving from formic acid to acetic acid. Now there is a very complete spectroscopic data base for formic acid dimer, which can be used to rigorously test quantum chemical calculations. Besides, we finally know from experiment how strongly the OH oscillators are coupled to each other in the simplest ring clusters of water. All this was only possible by the use of spontaneous Raman scattering in supersonic expansions, which is a really powerful tool to investigate hydrogen bonded systems.