Probing the stiffness of the simplest double hydrogen bond: the symmetric hydrogen bond modes of jet-cooled formic acid dimer.

Formic acid dimer is held together and kept planar by two strong hydrogen bonds, which give rise to intermolecular vibrations. Raman active fundamentals, overtones, and combination bands involving out-of-plane bending and stretching vibrations of the hydrogen bonds are recorded under jet-cooled, vacuum-isolated conditions between 100 and 750 cm(-1) and assigned with the help of isotope substitution. Individual anharmonicity effects are shown to be very small (x(i,j) = -(1+/-2) cm(-1)), where they are accessible by experiment. However, they may accumulate to substantial differences between harmonic and anharmonic fundamental excitations. Preliminary experimental evidence for the most elusive fundamental vibration of formic acid dimer, symmetric OH torsion, is presented. A rigorous experimental reference frame for existing and future high level quantum chemical and dynamical treatments of this important prototype system is provided. The effects of clustering beyond the dimer on the low frequency dynamics are found to be small, whereas argon coating gives rise to blueshifts.