Photodissociation mechanisms

The photochemistry of Zn+—(CH4) and Zn —(CH3OH) complexes has been studied in detail In that context, the possibility of the formation of a metal-hydroxo insertion complex [HO—Zn—CH3]+ (the isomer of [Zn—(CH30H)]+) has also been discussed . In other series of studies, the mechanism of dimethylzinc zinc monoethyl cation, diethylzinc diethylzinc dimer and dipropylzinc photolysis has been investigated by photoionization techniques. It was the study of Borsella and Larciprete that first observed different gas phase photodissociation mechanisms for Mc2Zn and Et2Zn by using one- and two-color multiphoton ionization combined with TOE MS. 

In comparison with the free ClOOCl molecule, the photodissociation mechanism and outcome do not change noticeably we find very little production of CIO, and mainly we have the almost simultaneous but asymmetric breaking of both Cl — O bonds. Since the chlorine atoms of the adsorbed molecule are found to point towards the ice surface, upon photodissociation their recoil pushes them slightly inwards, while an 02 molecule is projected away. 

In the photochemistry of larger molecules the same physical principles apply and the sequence of events is essentially the same, although descriptions are more complex and less precise. Opposite to diatomic, the polyatomic molecules can yield a multitude of different sets of products. To establish the photodissociation mechanism, the nature of the elementary chemical process undergone by an electronically excited molecular entity (primary photoreaction) yielding primary photoproducts should be known, eg in the case of alkanes both the radical fission... 

Stratospheric ozone protects life on the surface of the Earth by screening the harmful UV light coming from the Sun through a photodissociation mechanism ... 

In the winter of 1984, massive losses of stratospheric ozone were detected in Antarctica over the South Pole (Halley Bay). This ozone depletion is known as the ozone hole. We know now that it also forms over the Arctic, although not as dramatically as in the Antarctic. Stratospheric ozone protects life on the surface of the Earth by screening harmful UV radiation coming from the sun through a photodissociation mechanism (see Chapter 4). 

The photodissociation mechanisms of formaldehyde have been reviewed by Moore and Weisshaar. Photoexcitation of the lines in the 2 4 Si <— So transition belonging to ortho-H2CO were shown to produce no para-H ... 

Molecular beam photofragment translational energy spectroscopy has been used by Hepburn et to elucidate the photodissociation mechanisms of Si glyoxal. They presented evidence for predissociation of S, glyoxal in the absence of collisions and for the existence of three distinct dissociation pathM(ays. The major dissociation mechanisms were shown to lead to the formation of H2CO + CO and 2CO-I-H2, with a third minor channel producing CO 3-an isomer of H2CO, possibly hydroxymethylene. 

U.v-visible MP fragmentation of Cr(CO)g studied by 542 monitoring emission of excited Cr atoms. Photodissociation mechanisms discussed... 

MPD of di- and trinuclear metal clusters under colli- 546 sionless conditions. Photodissociation mechanisms and yields of atomic fragments... 

Elsewhere in this volume Gentry discusses arguments in favor of an indirect photodissociation mechanism for these and other clusters. In such a mechanism the excitation energy would decay first to the van der Waals vibrational modes followed by a slower dissociation step. The direct and indirect mechanisms are very similar in that the energy acceptor modes for the two mechanisms are the same for both pictures. The point of disagreement (if there is one) is whether the intermediate state is more naturally thought of as those of the cluster or those of the products. Our intuition is that such states are so short lived that their identification as states is of limited use in describing the dynamics of the molecule. 

The indirect mechanism hypothesis is based largely on the observation that linewidths for a variety of systems are within an order of magnitude of each other. The data in Figure 2 shows that there is, however, a clear dependence of linewidth on cluster structure. In any case, we are not convinced that an indirect photodissociation mechanism would lead to a uniform set of linewidths any more than would a direct mechanism. Clearly, more work will be required to understand which of the two pictures is more accurate. 

Figure 9.3 Photodissociation Spectra of ABC. A laser pulse transfers the ground state vibrational wavefunction to the repulsive electronically excited potential energy surface. The wavepacket moves and spreads (to, ti, <2, 3 snapshots) on the excited surface, being accelerated in the steepest descent direction (force is negative gradient of the potential) toward A + BCt products (where and f refer respectively to electronic and vibrational excitation). Information about the photodissociation mechanism may be obtained from the (structureless)...

We note that the inclusion of the interactions with potential curves of other multiplicities adds to the complexity of the photodissociation mechanism of this system. 

The main produets formed after irradiation were the trans-cis dimers [tc-3 and tc-4), whieh are also stabilized by N-H 0=C interactions. In contrast to the photoehemistry of the monomers, no dissociative products were observed after 248 nm irradiation of the dimers. The absence of dissociative produets was explained by a proton-transfer mechanism in the excited state that is faster than the photodissociative mechanism. The fact that hydrogen bonding has such a significant effect on the photochemical stability of JV-methylformamide has important implications to understand the stability of peptide-bonded systems to UV irradiation. 

The photodissociative pathway was confirmed by Meyer and Hammond, who foimd that essentially no o- or p-hydroxyacetophenone was formed in the photolysis of phenyl acetate in the gas phase. Instead, all products could be rationalized by recombination of phenoxy and methyl radicals (formed from decarbonylation of acyl radicals). Similarly, a photodissociative mechanism for the photo-Claisen reaction was supported by observation of products expected from the recombination of radicals produced by photodissociation of 3-methyl-l-phenoxybut-2-ene (113, Table 12.6). In addition to phenol, products of the reaction are the rearranged ether 114, the two y,y-dimethylallyl phenols 115 and 116, and the two rearranged allyl phenols... 

Photodissociation mechanisms in aliphatic and aryl halides has been systematically studied in the last decade at the CASSCF/CASPT2 level of theory. In this section, we will discuss the recent work on the I-C bond cleavage in iodotoluene. 

The direct irradiation of molecule sets up the molecule in vibrationally excited state. When the energy of photon is sufticient to overcome bond dissociation energy, the fragmentation will occur at the excited bond. The photodissociative mechanisms are best represented with the help of potential energy curves for diatomic molecules [likeCl2 and HI]. 

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