Hydrogen Bonding and Vibrational Energy Relaxation in Water−Acetonitrile Mixtures

Abstract

We present a study of the effect of hydrogen bonding on vibrational energy relaxation of the OH-stretching mode in pure water and in water-acetonitrile mixtures. The extent of hydrogen bonding is controlled by dissolving water at various concentrations in acetonitrile. Infrared frequency-resolved pump-probe measurements were used to determine the relative abundance of hydrogen-bonded versus non-hydrogen-bonded OH bonds in water-acetonitrile mixtures. Our data show that the main pathway for vibrational relaxation of the OH-stretching mode in pure water involves the overtone of the bending mode. Hydrogen bonding is found to accelerate the population relaxation from 3 ps in dilute solutions to 700 fs in neat water, as a result of increasing overlap between donor and acceptor modes. Hydroxyl groups that initially are not hydrogen bonded have two relaxation pathways: by direct nonresonant relaxation to the bending mode with a time constant of 12 ps or by making a hydrogen bond to a neighboring water molecule first (∼2 ps) and then relaxing as a hydrogen-bonded OH oscillator.

Probing the hydrogen-bond network of water via time-resolved soft X-ray spectroscopy

“…Static x-ray spectroscopy has revealed quantitative information on local atomic structure 4,5 whereas time-resolved vibrational spectroscopy has produced information on timescales of structural dynamics as short as 50fs. 6,7,8,9 Time-resolved vibrational spectroscopy has concentrated on the infrared spectrum of HOD in D 2 O and H 2 O 10,11,12,13,14,15,16 and to a lesser extent on pure H 2 O, 17,18 limited by the (sub-)micron absorption length. The latter also poses a challenge for x-ray absorption spectroscopy.…”

Ultrafast anisotropy dynamics of water molecules dissolved in acetonitrile

“…4,5 In the last years, the advances in laser technology, namely, in the generation of ultrafast IR pulses, [6][7][8][9][10] made it possible to monitor processes that occur on time scales as short as 100 fs. Such experiments opened new perspectives on the investigation of the structure 11 and dynamics of the hydrogen bond network in liquid water. [12][13][14][15] The complexity of liquid water properties calls for simplifying approaches which are able to reveal different aspects of the physics involved.…”

“…[11][12][13]16 Another possibility, which also mimics biological situations, 17 is to alter the complexity of the hydrogen bond network by studying water at interfaces, 18,19 in reverse micelles, [20][21][22][23][24][25][26][27] or in mixtures with various solvents. 11,28,29 The last setting is particularly advantageous because it allows investigating spatially separated water molecules and, thus, provides a unique opportunity to study purely intramolecular properties, which are not screened by interactions with other water molecules. In addition, by a careful choice of solvents with different hydrophilic characters, the influence of the surrounding on the water molecule can be investigated in a progressive manner, from weakly hydrogen-bonded situations 28,29 to rather strongly bonding environments.…”

“…In addition, by a careful choice of solvents with different hydrophilic characters, the influence of the surrounding on the water molecule can be investigated in a progressive manner, from weakly hydrogen-bonded situations 28,29 to rather strongly bonding environments. 11,28,29 An interesting model system, which comprises both hydrogen-bonded and free hydroxyl groups, can be obtained by dissolving water in a mixture of acetone and carbon tetrachloride. 30 From IR pump-probe measurements it has been concluded that the ultrafast dynamics in such a ternary mixture reflect the breakage and reformation of the hydrogen bonds.…”

“…11,31,32 The hydrogen bonds formed between water and acetonitrile are much weaker than the water-water hydrogen bonds. Nevertheless, they considerably improve the miscibility of the two liquids, allowing them to mix in any ratio.…”