Local vibrational modes of the formic acid dimer – the strength of the double hydrogen bond

The 24 normal and 24 local vibrational modes of the formic acid dimer formed by two trans formic acid monomers to a ring (TT1) are analysed utilising preferentially experimental frequencies, but also CCSD(T)/CBS and ωB97X-D harmonic vibrational frequencies. The local hydrogen bond (HB) stretching frequencies are at 676 cm−1 and by this 482 and 412 cm−1 higher compared to the measured symmetric and asymmetric HB stretching frequencies at 264 and 194 cm−1. The adiabatic connection scheme between local and normal vibrational modes reveals that the lowering is due to the topology of dimer TT1, mass coupling, and avoided crossings involving the H⋅⋅⋅OC bending modes. The HB local mode stretching force constant is related to the strength of the HB whereas the normal mode stretching force constant and frequency lead to an erroneous underestimation of the HB strength. The HB in TT1 is stabilised by electron delocalisation in the O=C–O units fostered by forming a ring via double HBs. This implies that the CO apart ...