Intramolecular H-bond dynamics investigated by THz high-resolution spectroscopy
To a large extent, complex molecules in living matter have structures and properties that are determined by intramolecular hydrogen bonds (HBs). For understanding biological processes, the dynamics of HB formation and breaking is important. For example, proteins are an important class of biomolecules where intramolecular HBs play a fundamental role in stability. The rovibrational signatures of intramolecular HBs have mainly been studied in the mid-IR by probing the local stretching of the R-H donor and acceptor functional groups. Yet, the vibrational modes directly involved in the dynamics of the intramolecular HB lie at lower frequencies in the THz region.
Compared to the mid-IR, the far-IR/THz spectroscopy allows exploration of the dynamics of low-energy vibrations involving the full molecular backbone. That explains why far-IR/THz high-resolution spectroscopy is so efficient in discriminating subtle structural differences (stable conformers, isomers, ...). The low-frequency HB dynamics have been studied directly in the far-IR/THz domain with low or middle resolution vibrational spectroscopy techniques or indirectly, at high resolution, using electronic excitations. The full potential of high-resolution far-IR/THz gas-phase spectroscopy of isolated (bio)molecules with intramolecular HB remains unexplored today.
The complete rovibrational analysis of the free and bonded torsional modes involved in the intramolecular HB presented in this study is unprecedented. We have chosen the catechol molecule which is attractive since the two adjacent hydroxy groups can interchangeably act as donor and acceptor in an intramolecular HB due to the tunnelling between two symmetrically equivalent structures. Using synchrotron radiation, we recorded a rotationally-resolved Fourier Transform far-infrared (IR) spectrum of the torsional modes of the free and bonded -OH groups forming the intramolecular HB. Additionally, the room temperature, pure rotational spectrum was measured in the 70–220 GHz frequency range using a millimeter-wave spectrometer. The assignment of these molecular transitions was assisted by anharmonic high-level quantum-chemical calculations. In particular, pure rotational lines belonging to the ground and the four lowest energy, vibrationally excited states were assigned. Splitting due to the tunnelling was resolved for the free -OH torsional state. A global fit combining the far-IR and millimeter-wave data provided the spectroscopic parameters of the low-energy far-IR modes, in particular those characterizing the intramolecular hydrogen bond dynamics.
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Laboratory of Physical Chemistry of the Atmosphere, University of Littoral, Dunkirk