![]() Therefore, only those exchange processes that can compete with other dephasing mechanisms can contribute to the IR spectral shape: this is the fast exchange or coherence transfer condition. In linear IR spectra there is only one time interval involved, a detection time, and the IR spectrum displays the Fourier transform of the free decay on this time axis. The population exchange kinetics reported previously 5 depends on different dynamical constants in principle. We use the term coherence transfer because it is the 0–1 coherence that is transferred back and forth between the H and F states. ![]() It will be seen that transfer between the vibrational coherences of the hydrogen-bonded ( H) and non-hydrogen-bonded ( F) sub-populations is occurring. The example used is the CN stretching mode of a dilute solution of acetonitrile in methanol. By fast exchange is meant the same as in NMR, 4 namely that the exchange competes effectively with the other mechanisms of vibrational dephasing of the transition. In this paper we consider the comparison of 2D IR and linear IR spectra of a system undergoing fast hydrogen-bond exchange that gives rise to coherence transfer. Furthermore in 2D IR spectra, the overlapping populations of solvent-solute structures are more likely to be separately observed because of the separation of homogeneous and inhomogeneous contributions to the spectral bands. On the other hand, the two-dimensional infrared (2D IR), spectra 1– 3 are composed from many more independent data points, so they can establish these parameters with improved confidence limits. The conventional IR spectral line shapes contain all the details of the dynamics of the modes, but it is often impossible to establish a physically meaningful set of dynamical parameters from the linear spectrum.
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