Vibrational exciton coupling in homo and hetero dimers of carboxylic acids studied by linear infrared and Raman jet spectroscopy.

The jet-cooled band positions of the C=O stretching vibrations in the three hetero dimers composed of formic, acetic, and pivalic acid have been determined. Resonance patterns in the symmetric stretching modes have been corrected for by assuming a single bright state. An analysis of their Davydov or vibrational exciton splitting shows that the hetero dimer values can be averaged from the respective homo dimer splittings (ranging from 56 cm-1 for the acetic to 75 cm-1 for the formic acid dimer) with an error of ≤7%. The set of 6 exciton splittings and 6 independent downshifts caused by double hydrogen bonding serves as a reference data base for the benchmarking of computational methods. B3LYP is shown to be unable to describe the difference between the formic and acetic acid monomer but is otherwise satisfactory, if one assumes that exciton splittings are only weakly affected by anharmonic effects beyond the deconvoluted local resonances. However, a vibrational perturbation theory test points at significant diagonal anharmonicity effects for the exciton splitting. Spin-component-scaled and canonical MP2 fail in reproducing experimental dimer shifts and splittings in the harmonic approximation, but anharmonic corrections are expected to improve the performance. Harmonic PBEh-3c reproduces the experimental data set well after scaling. The experimental data set the stage for more rigorous anharmonic treatments of the multidimensional coupling of C=O oscillators in carboxylic acid dimers and trimers. In addition, we report the first vibrational jet spectrum of cis-formic acid in the C=O stretching region by heating the nozzle and the nozzle feed line of the Raman setup.