Sean Garrett-Roe - What can we learn from 3D-IR spectroscopy

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  Publication Details (including relevant citation   information):

  Garrett-Roe, S. and Hamm, P. Acc. Chem. Res.  42, 1412--1422 (2009).


  The low-frequency part of the vibrational spectrum of a liquid is   dominated by intermolecular degrees of freedom. Hence, it reports   on the motion of solvent molecules with respect

  to each other rather than on the intramolecular details of   individual molecules. In hydrogen-bonded liquids, in particular   water, a detailed understanding of the low-frequency spectrum

  is enormously complicated because of the complex hydrogen-bond   network, which constantly rearranges on an ultrafast femtosecond   to picosecond time scale. Many of the peculiar

  properties of water have their origin in these processes.   Conventional far-infrared (far-IR) or Raman spectroscopy, as well   as two-dimensional IR (2D-IR) spectroscopy, are all linear with

  respect to the intermolecular (solvent) degrees of freedom. These   spectroscopies tell us much about the density of states in the   low-frequency range but little about the dynamics of the   hydrogen-bond making and breaking.

  In this Account, we propose three-dimensional IR (3D-IR)   spectroscopy as a novel tool that is nonlinear with respect to   these low-frequency degrees of freedom; hence, it may provide   much more detailed insights into intermolecular dynamics. The   first experimental realizations of 3D-IR spectroscopy have been   demonstrated in the literature; the information it affords is   similar to that of 2D-Raman spectroscopy. Three-dimensional IR   spectroscopy will, for the first time, reveal whether the   low-frequency part of the vibrational spectrum of liquids has to   be considered mostly homogeneously or inhomogeneously broadened.   Alternately, we may find that either of these classifications is   completely wrong because the normal mode picture fails when   thermal energy is of the same order of magnitude as the   ruggedness of the intramolecular potential energy surface.

  We briefly introduce the theoretical background of 3D-IR   spectroscopy and discuss two of its most promising applications:   (a) the more thorough characterization of non-Gaussian stochastic   processes such as the hydrogen-bond dynamics of water and (b)   non-Markovian ultrafast exchange processes. In the ultrafast   regime, many of the otherwise valid simplifying assumptions of   nonequilibrium statistical mechanics (for example, linear   response and Markovian dynamics) are likely to fail; 3D-IR   spectroscopy will allow us for the first time to experimentally   explore their range of validity.

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