2D IR photon echo spectroscopy reveals hydrogen bond dynamics of aromatic nitriles

The CN vibrations of two aromatic nitriles, cinnamonitrile, PhCHCH–CN and benzonitrile, PhCN, representative of components of common enzyme inhibitors, are examined by two-dimensional infrared spectroscopy. In methanol, these spectra display cross peaks between the two CN components whose evolution exposes the few picosecond (4.5 ps for CIN and 5.3 ps for BN) equilibrium dynamics of hydrogen bond making and breaking. The main features of the 2D IR spectra are reproduced by simulations only when exchange is incorporated. The lowest free energy state is the non-hydrogen bonded form. Both alkyl and aryl nitriles have now shown this picosecond exchange process.