Local hydrogen bonding dynamics and collective reorganization in water: Ultrafast infrared spectroscopy of HOD/D2O

Abstract

We present an investigation into hydrogen bonding dynamics and kinetics in water using femtosecond infrared spectroscopy of the OH stretching vibration of HOD in D(2)O. Infrared vibrational echo peak shift and polarization-selective pump-probe experiments were performed with mid-IR pulses short enough to capture all relevant dynamical processes. The experiments are self-consistently analyzed with a nonlinear response function expressed in terms of three dynamical parameters for the OH stretching vibration: the frequency correlation function, the lifetime, and the second Legendre polynomial dipole reorientation correlation function. It also accounts for vibrational-relaxation-induced excitation of intermolecular motion that appears as heating. The long time, picosecond behavior is consistent with previous work, but new dynamics are revealed on the sub-200 fs time scale. The frequency correlation function is characterized by a 50 fs decay and 180 fs beat associated with underdamped intermolecular vibrations of hydrogen bonding partners prior to 1.4 ps exponential relaxation. The reorientational correlation function observes a 50 fs librational decay prior to 3 ps diffusive reorientation. Both of these correlation functions compare favorably with the predictions from classical molecular dynamics simulations. The time-dependent behavior can be separated into short and long time scales by the 340 fs correlation time for OH frequency shifts. The fast time scales arise from dynamics that are mainly local: fluctuations in hydrogen bond distances and angles within relatively fixed intermolecular configurations. On time scales longer than the correlation time, dephasing and reorientations reflect collective reorganization of the liquid structure. Since the OH transition frequency and dipole are only weakly sensitive to these collective coordinates, this is a kinetic regime which gives an effective rate for exchange of intermolecular structures.

Ion–water hydrogen-bond switching observed with 2D IR vibrational echo chemical exchange spectroscopy

“…For this anion, the stretch spectrum of hydroxyls bound to BF 4 Ϫ is substantially shifted to the blue, and the band is relatively narrow (16). The hydroxylanion peak is readily observable on the high-frequency side of the broad hydroxyl-oxygen band (see Fig.…”

“…Here, we present the results of ultrafast 2D IR vibrational echo chemical exchange experiments on an aqueous solution of the anion BF 4 Ϫ from the salt NaBF 4 . For this anion, the stretch spectrum of hydroxyls bound to BF 4 Ϫ is substantially shifted to the blue, and the band is relatively narrow (16).…”

“…A water molecule can have as many as 4 hydrogen bonds with other water molecules, forming an approximately tetrahedral structure. The pure water hydrogen bond network is constantly evolving with a range of time scales from tens of femtoseconds to picoseconds (2)(3)(4)(5). Hydrogen bonds are continually forming and breaking through concerted hydrogen bond rearrangements (6).…”

“…These dynamical processes can be observed on the time scale they occur in considerable detail by using ultrafast infrared spectroscopy. Measurements of spectral diffusion, described in terms of the frequencyfrequency correlation function (FFCF), by using ultrafast 2D IR vibrational echo spectroscopy (3,7,8) as well as other ultrafast IR techniques (4,5) have determined the multiple time scales for the hydrogen bond dynamics. The slowest time component of the FFCF (1.7 ps) is associated with the randomization of the hydrogen bond network through concerted hydrogen bond rearrangements.…”

The relation between the structure of the first solvation shell and the IR spectra of aqueous solutions

“…It is generally believed that the distinctive behavior of water arises from the underlying hydrogen bond network [3,4]. While the nature of aqueous hydrogen bonds has been under considerable debate in the literature, their spectroscopic signatures have been well documented [5][6][7]. The red shift of the O-H stretch upon condensation is a prime example.…”