ABSTRACT
Recent improvements in mid-infrared short pulse generation technology have significantly advanced the potential of femtosecond time-resolved mid-infrared spectroscopy, leading to detailed structural and dynamical information on molecular geometries and associated interactions. For instance, time evolution of the spectral position and shape the absorption band of a "spectator"-mode gives dynamical information on chemical bonds. Comparison of experimental vibrational patterns with quantum chemical calculations leads to assignments of geometric structures to transient states. Direct information on geometries can be obtained from pump-probe experiments on vibrational bands that exhibit features due to anharmonic coupling between vibrational modes.
In this seminar I will highlight recent results of a time-resolved study of the neutralization reaction dynamics between Brønsted acids and bases. These fast bimolecular reactions, of fundamental importance in chemistry and biology, involve proton transfer, a key process in elementary phenomena such as acid-base neutralization and enzymatic reactions, the ab-normal high proton mobility and the autoionization in water, and proton pumps through membrane protein channels. According to Eigen and Weller [1,2] general acid-base reactions in solution are diffusion assisted and may be modeled by a three-step reaction scheme consisting of a two-stage proton transfer scheme made of (1) diffusional motion, where the acid and base approach each other to form an encounter pair when the mutual distance equals reaction contact radius, and by (2) intrinsic proton transfer. The reaction is completed by subsequent separation of the products by diffusion (3). We investigate this fundamental acid-base neutralization reaction in liquid water by use of femtosecond mid-infrared spectroscopy. We use a photoacid, that upon photoexcitation reacts with a base, and monitor how the bimolecular reaction dynamics proceeds by inspection of appropriate vibrational marker modes of reactants and products [3,4]. We observe bimodal reaction dynamics, that we ascribe to contributions from pre-formed hydrogen bonding complexes and from initially uncomplexed acid and base. Our results demand refinement of the well-established Eigen-Weller model.