Reaction coordinate photodissociation

An issue of considerable interest is whether it would be possible to control or manipulate the photodissociation pathways of a particular molecule by initial vibrational preexcitation. A main concern in achieving this control is the difficulty in preparing states with vibrational energy that is localized in a specific mode that resembles the reaction coordinate. This is due to intramolecular vibrational... 

Unlike the case of simple diatomic molecules, the reaction coordinate in polyatomic molecules does not simply correspond to the change of a particular chemical bond. Therefore, it is not yet clear for polyatomic molecules how the observed wavepacket motion is related to the reaction coordinate. Study of such a coherent vibration in ultrafast reacting system is expected to give us a clue to reveal its significance in chemical reactions. In this study, we employed two-color pump-probe spectroscopy with ultrashort pulses in the 10-fs regime, and investigated the coherent nuclear motion of solution-phase molecules that undergo photodissociation and intramolecular proton transfer in the excited state. 

Figure 53. Schematic diagram depicting the time-resolved probing of the photodissociation and coherence retention of t-stilbene-FIe complexes. iA and iK are the polarization vectors for absorption and nascent product emission, respectively. D is the binding energy of the complex. The optically excited stilbene-type mode of the complex is represented on the left side of the figure. Energy reaches the reaction coordinate (right half) via IVR. After dissociation (far right) the available energy is partitioned between translation ( fr) and internal excitation of stilbene...

The depiction in Figure 12.55 is a very simple model of a photodissociation. As noted in Chapter 11, states of the same symmetry may not cross on a potential energy surface based on just one nuclear coordinate (the reaction coordinate). When more than one set of nuclear coordinates is included, however, then states of the same symmetry may cross. " Thus, photodisso-... 

Indirect photodissociation can be the result of more than one type of delay. First, the energy may not be made directly available for the motion along the reaction coordinate. If there is time for energy scrambling tiien the dissociation will be a la RRKM, as will be discussed further in Sections 111-1.1 A. There is however another type of delay. It is when electronic energy is made available as vibrational energy of a lower-lying electronic state. 

Coherence in the nascent products is one manifestation of the limitation of a one-dimensional point of view because it refers to motion not along the reaction coordinate. Will the coherence survive for more complex systems or for situations such as reactions in solution where the coupling to the environment is a key The answer to both questions is yes. In solution, the coherence of Hg—I motion following ultrafast photodissociation of Hgl2 is observable. Figure 8.4. 

Waschewsky et al. (1994) used crossed laser-molecular beam experiments to monitor the competition between photodissociation pathways in chloroacetone. Their studies proved that C—C bond fission, process (II), competes with C—Cl bond fission, process (1), in C1CH2C(0)CH3 photolysis at 308 nm, although the latter process is favored. Several kJ of translational energy were evident in the translation of the initial products of the two competing dissociation pathways. The authors point out that this may indicate that, for both channels, dissociation proceeds via a reaction coordinate that has a significant exit barrier (barrier to reverse reaction), so fragments exert a repulsive force on each other as they separate. The pathway to dissociation probably does not involve... 

The conformation dependence of the ratio of C—C to C—Br branching ratio is not determined by the C—Br fission reaction coordinate. In the molecular beam experiments with nozzle temperatures of 100° and 400°C, the experimentally observed ratio was < i/< >ii = 5.7 and 3.5, respectively. It is uncertain how these results obtained at low pressures and somewhat elevated temperatures would compare with those for l-bromo-2-propanone photodissociation at the lower temperatures of the troposphere with 1 atm. of air. No evidence for the occurrence of the processes (IB) and (IV) was obtained, but by analogy with the photolysis of acetone, these processes are expected to occur to some extent in photolysis at the shorter wavelengths of absorbed light. 

The degree of vibrational excitation in a newly formed bond (or vibrational mode) of the products may also increase with increasing difference in bond length (or normal coordinate displacement) between the transition state and the separated products. For example, in the photodissociation of vinyl chloride [9] (reaction 7), the H—Cl bond length at the transition state for four-center elimination is 1.80 A, whereas in the three-center elimination, it is 1.40 A. A Franck-Condon projection of these bond lengths onto that of an HCl molecule at equilibrium (1.275 A) will result in greater product vibrational excitation from the four-center transition state pathway, and provides a metric to distinguish between the two pathways. 

It has already been mentioned that one of the key points in the theory of photodissociation reactions is understanding how, as a bond stretches and breaks, there is established a continuous connection between the translational coordinate along which fragment separation occurs and the vibrational motions of the molecule. There is an analogous problem in... 

Figure 1. Simplified Jablonski-type diagram of photoinduced catalytic (I), photoassisted (II), and sensitized photoreaction (III) as well as catalysed photolysis (IV). (MLn i + L) represents coordinatively unsaturated species, free ligands as well as ligands and/or coordination compounds with changed formal oxidation number generated by photo-redox and/or photodissociation reactions.

Iron pentacarbonyl exhibits efficient photodecomposition (in the absence of ligands) because the bimolecular reaction between Fe(CO)3 and photogenerated Fe(C0)4 yields insoluble Fe2(C0)g. Sterically unhindered Fe(CO)3[l,4-Me2N4] mimics the behavior of Fe(CO)3 in forming a cluster on irradiation in the absence of ligands. Furthermore selective photodissociation of CO from the tetraazabutadiene complex produces a coordinatively unsaturated species that reacts (like photogenerated Fe(C0)4) with Fe(C0)3 to form a dimer, Equation 10. 

In contrast to the subsystem representation, the adiabatic basis depends on the environmental coordinates. As such, one obtains a physically intuitive description in terms of classical trajectories along Born-Oppenheimer surfaces. A variety of systems have been studied using QCL dynamics in this basis. These include the reaction rate and the kinetic isotope effect of proton transfer in a polar condensed phase solvent and a cluster [29-33], vibrational energy relaxation of a hydrogen bonded complex in a polar liquid [34], photodissociation of F2 [35], dynamical analysis of vibrational frequency shifts in a Xe fluid [36], and the spin-boson model [37,38], which is of particular importance as exact quantum results are available for comparison. 

Photodissociation of coordinated ligands has also lead to the synthesis of new complexes by oxidative addition to the coordinatively unsaturated intermediates as in reactions (17)—(19).85-87 These types of reactions have been invoked in transition metal complex photoassisted and photocatalyzed reactions. 

---