H. J. Bakker

Orientational relaxation of liquid water molecules as an activated process

Femtosecond mid-infrared pump–probe spectroscopy is used to study the orientational relaxation of HDO molecules dissolved in liquid D2O. In this technique, the excitation of the O–H stretch vibration is used as a label in order to follow the orientational motion of the HDO molecules. The decay of the anisotropy is nonexponential with a typical time scale of 1 ps and can be described with a model in which the reorientation time depends on frequency and in which the previously observed spectral diffusion is incorporated. From the frequency and temperature dependence of the anisotropy decay, the activation energy for reorientation can be derived. This activation energy is found to increase with increasing hydrogen bond strength.

Hydration strongly affects the molecular and electronic structure of membrane phospholipids

We investigate the structure and electronic properties of phosphatidylcholine (PC) under different degrees of hydration at the single-molecule and monolayer type level by linear scaling ab initio calculations. Upon hydration, the phospholipid undergoes drastic long-range conformational rearrangements which lead to a sickle-like ground-state shape. The structural unit of the tilted gel-phase PC appears to be a water-bridged PC dimer. We find that hydration dramatically alters the surface potential, dipole and quadrupole moments of the lipids and consequently guides the interactions of the lipids with other molecules and the communication between cells.

Strong frequency dependence of vibrational relaxation in bulk and surface water reveals sub-picosecond structural heterogeneity

Because of strong hydrogen bonding in liquid water, intermolecular interactions between water molecules are highly delocalized. Previous two-dimensional infrared spectroscopy experiments have indicated that this delocalization smears out the structural heterogeneity of neat H2O. Here we report on a systematic investigation of the ultrafast vibrational relaxation of bulk and interfacial water using time-resolved infrared and sum-frequency generation spectroscopies. These experiments reveal a remarkably strong dependence of the vibrational relaxation time on the frequency of the OH stretching vibration of liquid water in the bulk and at the air/water interface. For bulk water, the vibrational relaxation time increases continuously from 250 to 550 fs when the frequency is increased from 3,100 to 3,700 cm−1. For hydrogen-bonded water at the air/water interface, the frequency dependence is even stronger. These results directly demonstrate that liquid water possesses substantial structural heterogeneity, both in the bulk and at the surface.

Interfacial water facilitates energy transfer by inducing extended vibrations in membrane lipids.

We report the complete assignment of the vibrational spectrum of dipalmitoylphosphatidylcholine (DPPC), which belongs to the most ubiquitous membrane phospholipid family, phosphatidylcholine. We find that water hydrating the lipid headgroups enables efficient energy transfer across membrane leaflets on sub-picosecond time scales. The emergence of spatially extended vibrational modes upon hydration, underlies this phenomenon. Our findings illustrate the importance of collective molecular behavior of biomembranes and reveal that hydrated lipid membranes can act as efficient media for the transfer of vibrational energy.

Dynamical Studies of Zeolitic Protons and Adsorbates by Picosecond Infrared Spectroscopy

Abstract The application of (picosecond) nonlinear infrared spectroscopy to investigate zeolite catalysts and adsorbates is reviewed. In these time-resolved experiments, one specific vibration in the zeolite system (i.e., a zeolite or adsorbate vibration) is selectively excited with an ultrashort (tunable) mid-infrared pulse. The effect of this excitation and the subsequent energy relaxation can be monitored real time, providing information on the structure of the bare zeolite and adsorption complexes. More importantly, with this technique the picosecond energy flow at the catalytic site and the dynamics of the catalyst-adsorbate interaction can be investigated: Short-lived transient species (e.g., reaction intermediates) are observed and the picosecond relaxation rates and pathways at the catalytic site render insights into the dynamics of the interaction between the zeolite catalyst and its adsorbates at a molecular level. This illustrates the potential of time-resolved infrared spectroscopy in the inve...

Solvent-dependent vibrational relaxation pathways after successive resonant IR excitation to υ = 2

Abstract With two subsequent resonant intense picosecond infrared pulses, we have succeeded in pumping a significant fraction of iodoform molecules in solution to the second vibrationally excited state of the CH stretching mode. Transient populations of the vibrational levels are monitored with weak probe pulses. From these pump-pump-probe experiments, we find that the subsequent relaxation route depends critically on the solvent. In a strongly polar solvent (acetone) relaxation from υ = 2 to υ = 0 occurs predominantly via the υ = 1 state, with time constants of T 1 2→1 = 10 ± 5 and T 1 1→0 = 60 ± 5 ps, respectively. In contrast, in a less polar solvent (chloroform) direct decay to the ground state is observed, with a time constant ( T 1 2→0 = 80 ± 20 ps), comparable to the energy lifetime of the first excited state ( T 1 1→0 = 125 ± 5 ps).