In many condensed-phase systems, apparently simple processes have a distribution of rates. Just as multidimensional spectroscopies reveal the origins of broad spectral lines, multidimensional kinetics unravel the causes of broad rate distributions. Two-dimensional (2D) measurements determine the fraction of rate-distribution width that is due to heterogeneity; three-dimensional (3D) measurements measure the lifetime of the heterogeneity, that is, exchange within the rate distribution. Many specific approaches to multidimensional kinetics are possible, depending on the observable and the timescale. Three recent examples illustrate this range. All look at molecular rotation, but on different time ranges. 2D-MUPPETS (multiple population-period transient spectroscopy) has been applied to solute rotation in micelles on the picosecond timescale; 2D and 3D rotational-correlation functions have been calculated from computer simulations of pure o-terphenyl [D. E. Shaw group, J. Phys. Chem. B <b>117</b>, 12898 (2013)] at the mode-coupling temperature (100 ns timescale) and well above it (20 μs timescale); 2D and 3D single-molecule dichroism has been measured from 60 ms – 5 ks near the o-terphenyl glass transition. All these experiments are derived from the same essential concepts. Their results can all be reduced to 2D kinetic-correlation spectra. Projections of these spectra yield quantitative results that are not tied to specific model or mechanism for the dynamics.

Multidimensional kinetics and rate heterogeneity from picoseconds to kiloseconds

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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