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

Abstract

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 from the OH local stretching frequencies reflect the strength of the HB via their red or blue shifts relative to their corresponding values in trans formic acid.

Keywords:

• formic acid dimer
• vibrational spectra
• local vibrational modes
• hydrogen bond stretching frequencies
• hydrogen bond strength
• red and blue shifts of stretching modes

Acknowledgements

This work was financially supported by the National Science Foundation, Grant CHE 1152357. We thank SMU for providing computational resources.

Notes

^a Bending force constants are given in (mdyn Å)/rad².

^b ZPE values (second row) are given as the sum of the contribution of the local mode frequencies and the contribution of the coupling frequencies.

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