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In aqueous solution, the basis of all living processes, hydrogen bonding exerts a powerful effect on chemical reactivity. The vibrational energy relaxation (VER) process in hydrogen-bonded complexes in solution is sensitive to the microscopic environment around the oscillator and to the geometrical configuration of the hydrogen-bonded complexes. In this Account, we describe the use of time-resolved infrared (IR) pump-probe spectroscopy to study the vibrational dynamics of (i) the carbonyl CO stretching modes in protic solvents and (ii) the OH stretching modes of phenol and carboxylic acid. In these cases, the carbonyl group acts as a hydrogen-bond acceptor, whereas the hydroxyl group acts as a hydrogen-bond donor. These vibrational modes have different properties depending on their respective chemical bonds, suggesting that hydrogen bonding may have different mechanisms and effects on the VER of the CO and OH modes than previously understood. The IR pump-probe signals of the CO stretching mode of 9-fluorenone and methyl acetate in alcohol, as well as that of acetic acid in water, include several components with different time constants. Quantum chemical calculations indicate that the dynamical components are the result of various hydrogen-bonded complexes that form between solute and solvent molecules. The acceleration of the VER is due to the increasing vibrational density of states caused by the formation of hydrogen bonds. The vibrational dynamics of the OH stretching mode in hydrogen-bonded complexes were studied in several systems. For phenol-base complexes, the decay time constant of the pump-probe signal decreases as the band peak of the IR absorption spectrum shifts to lower wavenumbers (the result of changing the proton acceptor). For phenol oligomers, the decay time constant of the pump-probe signal decreases as the probe wavenumber decreases. These observations show that the VER time strongly correlates with the strength of hydrogen bonding. This acceleration may be due to increased coupling between the OH stretching mode and the accepting mode of the VER, because the low-frequency shift caused by hydrogen bond formation is very large. Unlike phenol oligomers, however, the pump-probe signals of phenol-base complexes did not exhibit probe frequency dependence. For these complexes, rapid interconversion between different conformations causes rapid fluctuations in the vibrational frequency of the OH stretching modes, and these fluctuations level the VER times of different conformations. For the benzoic acid dimer, a quantum beat at a frequency of around 100 cm(-1) is superimposed on the pump-probe signal. This result indicates the presence of strong anharmonic coupling between the intramolecular OH stretching and the intermolecular stretching modes. From a two-dimensional plot of the OH stretching wavenumber and the low-frequency wavenumber, the wavenumber of the low-frequency mode is found to increase monotonically as the probe wavenumber is shifted toward lower wavenumbers. Our results represent a quant...

The vibrational dynamics of the CO stretching mode of 9-fluorenone (FL) in CD3CN and CD3OD were studied by sub-picosecond UV-pump and IR-probe spectroscopy. We found that the CO stretching band is blue-shifted by hydrogen-bond formation in the S1 state but red-shifted in the S0 state. In both CD3CN and CD3OD, the time evolution of the transient absorption spectrum was observed on a picosecond time scale, likely because of vibrational cooling and hydrogen-bond dynamics.

The vibrational energy relaxations (VERs) of the CO stretching mode of acetone and its complexes with alcohols are investigated by sub-picosecond pump-probe spectroscopy and molecular dynamics simulation. The time constants of the vibrational energy relaxation of the free acetone and that of the 1:1 complex are 4.4 and 2.3 ps for methanol solvent and 5.2 and 1.8 ps for 1-proponal solvent, respectively. The VER rate is accelerated a few times by formation of the hydrogen bond. This acceleration of the vibrational energy relaxation is successfully reproduced by the Landau-Teller method calculated from the molecular dynamics simulation. Molecular dynamics simulations reveal that the VER time of acetone with the hydrogen bond is largely affected by the solute polarization induced by solvent molecules.

We have investigated vibrational dynamics of hydrogen-bonded complexes in solutions by sub-picosecond infrared (IR) pump-probe spectroscopy. The anharmonic coupling between the OH stretching mode of benzoic acid dimer and low-frequency mode was studied from analysis of the quantum beat observed in the pump-probe signal. The IR absorption spectrum of 9-fluorenone (FL) in alcohol shows three distinct contributions from the free FL, its complex with one alcohol molecule, and that with two molecules. The time constants of the vibrational energy relaxation (VER) of the CO stretching mode of the free FL and that of the 1:1 complex were obtained. The VER of the OH stretching mode of methanol was studied in the isotopically diluted methanol. The internal isotope effect on the VER is discussed.