State-selective bond breaking

The control of the wavefunction of the parent molecule over the outcome of the fragmentation is especially illuminating in the photodissociation of HOD which can break up into two different product channels H + OD(n) 

H and OD. If we put, for example, four vibrational quanta into the HO entity and leave OD unexcited one obviously expects the H-0 bond to break first and therefore the preferential production of OD. 

Because of the appreciable excitation along the H-0 bond in the parent molecule, the bound-state wavefunction ko4 extends much further into the H+OD than into the D+OH channel. The consequence is that it overlaps h+od at relatively low energies where the overlap with the other wavefunction, I,d+oh is essentially zero. The result is a relatively small but finite cross section for the production of OD while the cross section for OH is practically zero. 

The classical picture of photodissociation outlined in Chapter 5 provides an alternative explanation of the preference for the H+OD channel 

If the total energy increases and eventually exceeds the barrier, the two dissociation wavefunctions begin to spread over both channels. This causes the overlap of with d+OH to increase while at the same 

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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.)...

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. 

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. 

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. 

The acid-catalyzed aquation of iron(III)-(substituted)oxinate complexes involves iron oxygen bond breaking and concomitant proton transfer in transition state formation. The latter aspect contrasts with the much slower acid-catalyzed aquation of hydroxamates, where proton transfer seems not to take place in the transition state. Reactivities, with and without proton assistance, for various stages in dissociation of a selection of bidentate and hexadentate hydroxamates, oxinates, and salicylates are compared and discussed—the overall theme is of dissociative activation. ... 

There is a possibility that an FC state will react before complete thermal equilibration. In the case of diatomic molecules, the process is usually known as predissociation — a dissociative state crosses the excited state potential surface. The situation is more complicated in the case of a coordination compound, but one can imagine an FC state relaxing along some nuclear coordinate leading to bond breaking. A state capable of such a process has been called a DOSENCO state, an acronym for Decay On SElected Nuclear Coordinates .21 The same authors use the term DERCOS (DEcay via Random Coordinate Selection) for a thexi state. 

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