Selective bond breaking

As the excitation energy is increased, vibrational levels above the barrier are populated and the excited state becomes quasi-bound. The earhest Rydberg tagging studies showed that the NH2 product was highly excited rotationally but had little vibrational excitation. This was attributed to the excitation out-of-plane vibrational motion in the excited state, which transforms into NH2 rotation as the H atom departs. Thus, the bending motion in the A state results in a torque, which flips the NH2 radical, causing it to rotate about its u-axis (i.e. perpendicular to the C3 axis in the planar configuration) as the H atom recoils. 

Although the above discussion covers the main aspects of the dynamics of the photodissociation of 

it should be emphasized that a number of other techniques have been used to determine the energy distribution in the NH2 fragment, including LIF and time-resolved FTIR, and these have revealed even more detailed aspects of the dynamics. In particular, work with PTS has revealed a number of interesting vector correlations. A good summary of these findings can be found in a review by Lee (2003). 

In some cases, selective bond breaking is possible even when the bonds have similar electronic character. Eor example, excitation of CH2lBr at 248 nm results predominantly in C-I fission, whilst excitation at 2 = 210 nm results in C-Br fission. However, in other poly-haloaUcanes and most other polyatomic molecules, attempts to achieve wavelength-selective bond fission have failed due to strong non-adiabatic coupling between 

CH17 PHOTODISSOCIAHON OF LARGER POLYATOMIC MOLECULES ENERGY LANDSCAPES 

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Figure 41. Selective bond breaking of H2O by means of the quadratically chirped pulses with the initial wave packets described in the text. The dynamics of the wavepacket moving on the excited potential energy surface is illustrated by the density, (a) The initail wave packet is the ground vibrational eigen state at the equilibrium position, (b) The initial wave packet has the same shape as that of (a), but shifted to the right, (c) The initail wave packet is at the equilibrium position but with a directed momentum toward x direction. Taken from Ref. [37]. (See color insert.)...

Imre, D.G. and Zhang, J. (1989). Dynamics and selective bond breaking in photodissociation, Chem. Phys. 139, 89-121. 

In principle, one can induce and control unimolecular reactions directly in the electronic ground state via intense IR fields. Note that this resembles traditional thermal unimolecular reactions, in the sense that the dynamics is confined to the electronic ground state. High intensities are typically required in order to climb up the vibrational ladder and induce bond breaking (or isomerization). The dissociation probability is substantially enhanced when the frequency of the field is time dependent, i.e., the frequency must decrease as a function of time in order to accommodate the anharmonicity of the potential. Selective bond breaking in polyatomic molecules is, in addition, complicated by the fact that the dynamics in various bond-stretching coordinates is coupled due to anharmonic terms in the potential. 

In conclusion, by proper choice of a molecular system, specific bond breaking can be performed. By using an IET selective bond-breaking procedure, unnecessary parts of a molecule can be cleaved off and thereby active sites can be created. Such molecular fragments can be used as building blocks to join with other specifically tailored species to build a new molecule. 

Seideman, T., Shapiro, M. and Brumer, P (1989). Coherent radiative control of unimolecular reactions Selective bond breaking with picosecond pulses, J. Chem. Phys., 90, 7132-7136. 

Summary. An effective scheme for the laser control of wavepacket dynamics applicable to systems with many degrees of freedom is discussed. It is demonstrated that specially designed quadratically chirped pulses can be used to achieve fast and near-complete excitation of the wavepacket without significantly distorting its shape. The parameters of the laser pulse can be estimated analytically from the Zhu-Nakamura (ZN) theory of nonadiabatic transitions. The scheme is applicable to various processes, such as simple electronic excitations, pump-dumps, and selective bond-breaking, and, taking diatomic and triatomic molecules as examples, it is actually shown to work well. 

The method of electronic excitation by a quadratically chirped pulse mentioned above can be applied to a wavepacket moving away from the turning point, so this technique can be applied to various processes such as pump-dump, wavepacket localization and selective bond-breaking, as we will discuss in the rest of this section. 

There are two ways to achieve selective bond-breaking (1) move the initial wavepacket to another FC region located in the target dissociative channel (bond-selective force case [46,47]) and (2) prepare the initial wavepacket with a finite momentum directed to the desired channel (bond-directed momentum case in [48]). These two methods may be combined to improve the efficiency, but on the other hand, appropriate FC regions may not be easy to find in general. In the case of H2O, for instance, it is found that the required laser is rather intense over a broad region of the system except for a small domain near the equilibrium position of Vg. This is due to the very steep potential energy surfaces of both the X and the A states and the exponential decay of the transition dipole moment away from the equilibrium position.We hope, however, that this does not represent a very common case. 

Fig. 5.9. Selective bond-breaking of H2O by means of the quadratically chirped pulses with initial wavepackets a, b and c, as described in Table 5.1. The left-hand, middle and right-hand columns correspond to the cases for the initial wavepackets a, b and c, respectively. The laser-driven dynamics of the wavepackets moving on the excited potential energy surface Ve are illustrated by the density. The time is taken from the center of the pulse (i.e., tp = 0)...

There are several unresolved issues in the problem of coherent vibrational pumping by shock fronts. These include (1) to what degree can a shock front be viewed as a coherent phonon source (2) can a shock front coherently drive vibrational excitations, and (3) could shock front coherent pumping cause selective bond breaking, especially bonds other than those broken by ordinary thermochemical reactions One way to look at the first issue is to look at the shock front as a superposition of phonons. Since phonons form a complete set... 

Figure 7. Selective bond breaking of HOD. Dissociation yields (upper panels), bond-length expectation values (middle panels), and LCT fields (lower panels) are shown for two objectives. The left-hand panels correspond to the case where the kinetic energy of the D atom is steadily increased, leading exclusively to D + OH dissociation. The results for the H + OD selective excitation and fragmentation are shown on the right-hand side of the figure.

Ir/aluittina, hydrogenolysis is by selective bond breaking only,... 

These frameworks therefore demonstrate breathing behaviour that results from rotations and twists of structural units in order to achieve better coordination to bound species. A second type of flexible framework appears to be able to break bonds to permit the uptake of adsorbate molecules. In Ni2(4,4 -bipy)3(N03)4, for example, it is found that the windows observed in the X-ray crystal structure do not limit the size of the molecules that can be adsorbed into internal cavities they must be able to open to permit guest passage. This type of behaviour, which must require selective bond breaking and reforming, begins to resemble that exhibited by some biological systems. These effects of structural response to adsorption are discussed further in Chapter 7. 

But it needs to be emphasized that caution must be exercised in drawing too many strict (and/or inflexible) conclusions from activation energy data. This, of course, must also throw some doubt on, and add caution to, the inconsiderate use of model compounds as materials from which to project coal behavior during coal pyrolysis. However, on a positive note, the use of model compounds does offer valuable information about pyrolysis mechanism it is the means by which the conclusion is drawn with respect to coal that can hurt the effort. Finally, the concept of induced bond scission (McMillen et ah, 1989) also opens up the area of coal pyrolysis to the additional concept of selective bond breaking by addition of suitable reagents. 

Selective bond breaking has been demonstrated with HOD by first exciting the fourth overtone (local mode) of the OH bond and then photodissociating the molecule via the A X transition. The A <— X transition is red shifted (hot-band absorption) into the 240-270 nm region and the dissociation of the OH bond, relative to the OD bond, is enhanced by a factor of 15. This type of process is referred to as vibrationally mediated photodissociation and can be a very effective approach, provided the initial vibrational excitation remains localized in one chemical bond for a sufficient length of time to allow further excitation and dissociation. In the case of HOD it is clear that randomization of the vibrational energy is slower than the photodissociation step, and this further emphasizes the direct and impulsive nature of dissociation on the A Bi-state PES. 

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