Ultrafast equilibrium and non-equilibrium chemical reaction dynamics probed with multidimensional infrared spectroscopy

Two-dimensional infrared (2D-IR) spectroscopy provides powerful tools to investigate chemical reaction dynamics in the condensed phase. Correlating excitation and detection frequencies grants access to structural and dynamical information that is hidden in a linear absorption spectrum. Low-barrier reactions naturally can occur on the picosecond time scale, and although they are too rapid to study using nuclear magnetic resonance spectroscopy, the intrinsic ultrafast time resolution of coherent 2D-IR spectroscopy enables direct tracking of equilibrium reactive barrier crossings. 2D-IR chemical exchange spectroscopy can monitor the picosecond dynamics of non-triggered chemical reactions by correlating excited reactant frequencies with detected product frequencies. Solvent and temperature-dependent variations enable comparisons with microscopic rate theories at an unprecedented level of detail. 2D-IR spectroscopy is also emerging as a powerful probe of non-equilibrium light-driven chemical transformations. Transient 2D-IR spectroscopy is able to follow nascent photoproducts caused by electronic excitation or by a temperature jump. Soon it will be possible to study transient species with the full range of 2D observables, such as line shapes and waiting-time dynamics that have motivated the wide adoption of equilibrium 2D-IR spectroscopy. This review summarises the general progress in using 2D-IR spectroscopy to study chemical reactions in solution, focusing on our investigations into reactions of isomerisation of CO2(CO)(8), photodissociation of Mn-2(CO)(10), geminate rebinding in [CpMo(CO)(3)](2) and charge transfer in betaine-30 as viewed from the first solvation shell.