Vibrationally mediated reaction

The dynamics of a reaction that proceeds directly over the transition state is expected to be qualitatively different from that of a resonance-mediated reaction. In particular, one expects that the branching ratios into the product rovibrational states will be very different between the direct and the resonant mechanisms. For example, if a given Feshbach resonance corresponds to trapping on the v = 1 vibrationally adiabatic curve, then one might expect that the population of the v = l vibrational state of the product molecule may be greatly enhanced by the resonant mechanism. Similarly, the rotational product distribution resulting from the fragmentation of a resonance molecule may show a quite distinct pattern from that of a direct reaction. Indeed, Liu and coworkers [94], and Nesbitt and coworkers [95] have noted distinct rotational patterns in the F+HD resonant reaction. 

Direct excitation to the continuum usually (but not always, vide infra) results in rupture of the weakest bond. In order for the experimenter to have control over which bond is broken, it is helpful first to excite motion along the bond of interest. This process, known as vibrationally mediated photodissociation, preselects the desired degree of freedom before the reaction takes place. This method is illustrated in Fig. 1, where a low energy photon excites... 

Complex-mediated reaction An elementary bimolecu-lar reaction proceeding via a long-lived collision complex having lifetimes ranging from several vibrational periods (100 fs) to many rotational periods (>10 ps). If complex lifetimes exceed several rotational periods, product angular distributions from crossed beam reactions exhibit forward-backward symmetry in the center-of-mass frame of reference. 

Figure 3.9 shows the resonance-mediated reaction mechanism. The HF(v = 3)-H VAP on the new PES is very peculiar with a deeper vibrational adiabatic well close to the reaction barrier and a shallow van der Waals (vdW) well. The ID wave function for the ground resonance state in Fig. 3.9 shows that this state is mainly trapped in the inner deeper well of the HF(v = 3)-H VAP with a considerable vdW character, whereas the excited resonance wave function is mainly a vdW resonance. Because of the vdW characters, these two resonance states could likely be accessed via overtone pumping from the HF(v = 0)-H vdW well. 

Fig. 3.9 Schematic diagram showing the resonance-mediated reaction mechanism for the F -I- H2 reaction with two resonance states trapped in the peculiar FIF(v = 3)-FI vibrational adiabatic potential well. The ID wave functions of the two resonance states are also shown. From [24], reprinted with permission from AAAS...

Calcined barium hydroxide is an efficient catalyst for a number of base-mediated reactions. Among these, the Claisen-Schmidt condensation of acetophenones with benzaldehydes occurs in times as short as 10 min at room temperature (Eq. 15). The acetophenone enolate (detected by IR spectroscopy) is formed on the catalyst surface, where the reaction with the aldehyde takes place. The higher activity of the ketone enolate is interpreted by the authors as the result of "an increased vibrational state of the lattice", a formulation close to the mechano-chemical explanation. With the help of selective poisoning experiments, it is concluded that the enolate forms via a SET mechanism. 

A problem central to chemical reaction dynamics is that of IVR [156, 157], the flow of energy between zeroth-order vibrational modes. Indeed, IVR generally accompanies (and mediates) nonadiabatic dynamics, such as internal... 

The transfer and storage of vibrational energy in large and small molecules mediate a variety of molecular processes. A central motivation for the study of vibrational energy flow in molecules has long been its influence on chemical reaction kinetics in gas and condensed phases [1-11], as well as its role in... 

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. 

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