Energy selectivity mode-selective chemistry

In the early 1970s it was demonstrated that vibrational excitation along the reaction coordinate would be more efficient than translational motion in promoting endoergic reactions of the so-called late barrier type (J.C. Polanyi, 1972). This refers to those reactions whose transition-state region occurs late en route from reactants to products. 

The first experiment showing the vibrational enhancement of a chemical reaction was reported for the crossed-beam reaction K + HCl KCl + H. An HCl chemical laser was employed to excite the HCl reactant resonantly, inducing the vibrational transition V = 0 1. It was estimated that an enhancement of two orders of magnitude in the KCl yield upon HCl vibrational excitation from v = 0 to v = 1 took place. 

An interesting example of mode-selective chemistry by vibrational excitation is that of the reaction H -f HOD, which can produce (a) H2 + OD or (b) HD + OH. The isotopic variant reaction H + H2O H2 -H OH has a reaction barrier of 7580 cm . It was proposed that the excitation of the OH stretching mode could enhance the H + H2O reaction rate. 

The reagent HOD is a perfect candidate for mode-selective chemistry because the H-OD and 

HO-D stretching frequencies are 3800cm and 2800cm respectively, i.e. they are quite different and represent almost pure vibrational modes. Is it possible to control the outcome of this reaction by exciting each of these modes separately In other words, can the H + HOD reaction be controlled to trigger one of the two following reactions  

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Modem photochemistry (IR, UV or VIS) is induced by coherent or incoherent radiative excitation processes [4, 5, 6 and 7]. The first step within a photochemical process is of course a preparation step within our conceptual framework, in which time-dependent states are generated that possibly show IVR. In an ideal scenario, energy from a laser would be deposited in a spatially localized, large amplitude vibrational motion of the reacting molecular system, which would then possibly lead to the cleavage of selected chemical bonds. This is basically the central idea behind the concepts for a mode selective chemistry , introduced in the late 1970s [127], and has continuously received much attention [10, 117. 122. 128. 129. 130. 131. 132. 133. 134... 

Figure 8. (a) Pulse sequence resulting from optimization of the control field to generate H in the same reaction as studied in Fig. 6. (6) The Husimi transform of the pulse sequence shown in (a). (c) Time dependence of the norms of the ground-state and excited-state populations as a result of application of the pulse sequence shown in (a). Absolute value of the ground-state wave function at 1500 au (37.5 fs) propagated under the pulse sequence shown in (a), shown superposed on a contour diagram of the ground-state potential energy surface. (From D. J. Tannor and Y. Jin, in Mode Selective Chemistry, B. Pullman, J. Jortner, and R. D. Levine, Eds. Kluwer, Dordrecht, 1991.)...

Mode-selective chemistry The use of laser beams to control the outcome of a chemical reaction by exciting specific energy states of the reactants. 

The central concept of mode-selective chemistry is illustrated in Fig. 1, which depicts the ground and excited state potential energy surfaces of a hypothetical triatomic molecule, ABC. One might wish, for example, to break selectively the bond between atoms A and B to yield products A+BC. Alternatively, one might wish to activate that bond so that in a subsequent collision with atom D the products AD+BC are formed. To achieve either goal it is necessary to cause bond AB to vibrate, thereby inducing motion along the desired reaction coordinate. 

See, for example, D. L. Bunker, /. Chem. Phys., 40,1946 (1963). Monte Carlo Calculations. IV. Further Studies of Unimolecular Dissociation. D. L. Bunker and M. Pattengill,/. Chem. Phys., 48, 772 (1968). Monte Carlo Calculations. VI. A Re-evaluation erf Ae RRKM Theory of Unimolecular Reaction Rates. W. J. Hase and R. J. Wolf, /. Chem. Phys., 75,3809 (1981). Trajectory Studies of Model HCCH H -P HCC Dissociation. 11. Angular Momenta and Energy Partitioning and the Relation to Non-RRKM Dynamics. D. W. Chandler, W. E. Farneth, and R. N. Zare, J. Chem. Phys., 77, 4447 (1982). A Search for Mode-Selective Chemistry The Unimolecular Dissociation of t-Butyl Hydroperoxide Induced by Vibrational Overtone Excitation. J. A. Syage, P. M. Felker, and A. H. Zewail, /. Chem. Phys., 81, 2233 (1984). Picosecond Dynamics and Photoisomerization of Stilbene in Supersonic Beams. II. Reaction Rates and Potential Energy Surface. D. B. Borchardt and S. H. Bauer, /. Chem. Phys., 85, 4980 (1986). Intramolecular Conversions Over Low Barriers. VII. The Aziridine Inversion—Intrinsically Non-RRKM. A. H. Zewail and R. B. Bernstein,... 

Studies in kinetics in the infrared include vibrational energy transfers in molecules. This opens up a new class of experiments to study mode-selective chemistry that requires high-power short pulses that excite molecular vibrations. 

Energy Redistribution in Isolated Molecules and the Question of Mode-Selective Laser Chemistry Revisited, N. Bloembergen and A. H. Zewail, J. Phys. Chem. 88, 5459 (1984). 

Besides its practical importance, photodissociation — especially of small polyatomic molecules — provides an ideal opportunity for the study of molecular dynamics on a detailed state-to-state level. We associate with molecular dynamics processes such as energy transfer between the various molecular modes, the breaking of chemical bonds and the creation of new ones, transitions between different electronic states etc. One goal of modern physical chemistry is the microscopical understanding of molecular reactivity beyond purely kinetic descriptions (Levine and Bernstein 1987). Because the initial conditions can be well defined (absorption of a single monochromatic photon, preparation of the parent molecule in selected quantum states), photodissociation is ideally suited to address questions which are unprecedented in chemistry. The last decade has witnessed an explosion of new experimental techniques which nowadays makes it possible to tackle questions which before were beyond any practical realization (Ashfold and Baggott 1987). 

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