Application of Emerging Ultrafast Spectroscopy to PCET

Application of 2D IR spectroscopy to PCET models of Section 17.3.2 is a logical starting point for this type of investigation. 2D methods can unravel the correlated nuclear motion in a PCET reaction and in principle decipher how vibrational coupling in the Dp/Ap interface couples to the ET event between the Ae/De sites. These data can identify the structural dynamics within the interface that promote PCET reactions in much the same way that local hydrogen bonding structure and dynamics mediate excited state PT reactions [239, 240]. In these experiments, the PCET reaction can be triggered by an ultrafast resonant visible laser pulse (as in a standard TA experiment) and a sequence of IR pulses may be employed to build a transient 2D IR spectrum. These experiments demand that systems be chosen so that the ET and PT events occur on an ultrafast timescale. 

Another novel ultrafast methodology that has rapidly matured and is poised for PCET applications is time-domain nonresonant third-order Raman (TOR) spectroscopy [241-243], which can directly detect the low-frequency response of liquids. Blank and coworkers have successfully applied this technique to probe the 

PCET rate formalisms are cast primarily in terms of solvent coordinates for both the electron and proton, since both are charged particles that couple to the solvent polarization [5, 24]. In a concerted PCET reaction, the coupled transfer must occur via a common transition state and a common solvent configuration on both solvent coordinates. An ultrafast PCET reaction could be photoinitiated with resonant excitation, and a TOR probe would subsequently reveal the evolution of the two-dimensional reaction coordinate via the solvent response. Working in concert, these experiments would offer a powerful means to evaluate the coupling between the two coordinates in different types of PCET reactions and thus enable the PCET trajectories within the 2D space of Fig. 17.2 to be determined with much greater clarity. 

thanks the Fannie and John Hertz Foundation for a pre-doctoral fellowship. The work on PCET and the activation of small molecules by PCET has been supported by grants from the NIH (GM47274) and the DOE DE-FG02-05ER15745. 

10 Electron Transfer in Chemistry, V. Balzani (Ed.), Wiley-VCH, Weinheim, 2001-2002, Vol. 1-5. 

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