Vibrational spectroscopy is broadly used as a probe of structure and dynamics of chemical systems. In the last decade, nonlinear vibrational techniques, including e.g. pump-probe and two-dimensional photon echo spectroscopies, have provided tremendous new insight into the molecular-level structure and real-time dynamics of liquids and biomolecules in a host of environments. They have led to a much improved understanding of, for example, hydrogen-bond dynamics in water[2,3], solvation shell dynamics around ions in aqueous solution[4–6], hydrogen bonding at the air-water interface and protein conformational dynamics[8,9].
The new directions currently pursued in this active field involve an increased complexity, either in the systems under investigation, with greater heterogeneity, for example water at biomolecular interfaces or liquids in confinement[11,12], or in the techniques and phenomena, for example 3D-IR and vibrational energy transfer. In all these cases, the multidimensional spectral patterns become increasingly intricate, rendering their interpretation challenging and sometimes ambiguous. An advanced theoretical framework together with molecular simulations are thus needed to harness the full power of these experimental techniques.
The numerous advances in nonlinear vibrational spectroscopy have very recently been surveyed in a topical issue of Accounts of Chemical Research (vol. 42, 2009) and we focus here on a selection of a few major pending questions in the application of vibrational spectroscopy to the understanding of complex, heterogeneous systems, and illustrate on these examples how the theory-experiment interplay promoted by this workshop could help resolve the outstanding issues.
Resonant vibrational energy transfer.
The resonant transfer of vibrational energy between the initially excited vibrator and other vibrators had been avoided in most of the first, pioneering nonlinear vibrational spectroscopy studies. These experiments were designed to continuously follow the same vibrator. In the case of water, this was accomplished through isotopic dilution (e.g. HOD in H2O) and allowed the probing of, for example, the reorientation of a single water molecule. In pure H2O, resonant vibrational energy transfer between molecules leads to a much faster anisotropy decay. However, recent studies have found that this resonant energy transfer can be extremely instructive in investigating the hydration dynamics around a solute through the flow of excess vibrational energy from a solute to the surrounding solvent. For example, very recently, results on vibrational energy transfer from DNA bases to the surrounding water solvent have appeared. However, deciphering the pathways followed by the energy flow requires the molecular description provided by simulations. The connection between the experiment and simulations is done through the calculation of the nonlinear spectra including resonant energy transfer, for which several schemes have been proposed (see, e.g.[14,15]) and several challenges remain.
Specifically probing solvent molecules at interfaces.
Identifying how a solute affects the dynamical properties of the surrounding solvent is a long-standing question, both experimentally and theoretically. However, in nonlinear vibrational spectroscopy measurements, discriminating in the collected signal the solvent molecules located at the solute interface from those lying in the bulk is extremely challenging. Several approaches have been used, including e.g. the assumption that the system behaves as a simple two-state mixture (bulk/interface), or the use of concentrated solutions, or the use of systems with environment-dependent vibrational lifetimes, but all these techniques have all been found to lead to artifacts. A combined theoretical and experimental “brainstorming” on this subject would therefore be highly desirable.
Three-dimensional infrared spectroscopy.
Similar to the revolution brought in NMR by the introduction of 2D-NMR, nonlinear vibrational spectroscopy has tremendously increased its power through the advent of 2D-IR (vibrational photon echo) spectroscopy. The introduction of an additional dimension provides, for example, valuable information about the spectral dynamics. However, some research groups are already turning to higher-order techniques, such as 3D-IR spectroscopy. Such high-dimensional spectra then become extremely information-rich but also very hard to interpret if not combined with simulations. We will therefore organize discussions around the power and the difficulties of these new techniques.
Chemical exchange spectroscopy.
2D IR vibrational echo chemical exchange is a recently introduced technique which can measure the equilibrium dynamics for systems undergoing fast structural changes under thermal equilibrium conditions. Such technique, coupled with its polarization-resolved variant, has for example recently provided new insight on the dynamics of water hydrogen-bond acceptor exchange in salt solutions, supporting the mechanism predicted by theory and simulations. The power of this technique and the necessary theoretical developments will be discussed.