The hydrogen-bonded 2-pyridone dimer model system. 2. Femtosecond mid-infrared pump-probe study.

2-Pyridone (PD) tautomerises to 2-hydroxypyridine (HP) in liquid solution, the equilibrium of which is solvent dependent. Dimerization of PD and HP leads to the cyclic dimers (PD)(2), (HP)(2), and (PD-HP). A combined NMR and FT-IR study [Szyc, Ł.; et al. J. Phys. Chem. A 2010, 114, 7749-7760] has shown that solutions of 2-pyridone in CD(2)Cl(2) constitute mainly PD-CD(2)Cl(2) solute-solvent complexes and cyclic dimers (PD)(2). Because of a lack of specific marker modes, a contribution of the cyclic dimer (HP)(2) to the NH/OH stretching absorption between 2400 and 3300 cm(-1) could not be fully ruled out. Here, we present the first ultrafast infrared (IR) pump-probe experiments on the NH/OH stretching region of a solution of 2-pyridone in CD(2)Cl(2). The temporally and spectrally resolved data reveal different rate-like relaxation processes with time constants between 150 fs and 20 ps as well as coherent low-frequency oscillations due to hydrogen bond modes. An analysis shows that the transient behavior is dominated by a single hydrogen bonded species. We compare the low-frequency wavepacket motions, observed with 99 and 150 cm(-1) frequencies, with literature values as well as our quantum chemical calculations and conclude that this single molecular species is cyclic (PD)(2).