Photodissociation results

The role of CFCs in the destruction of ozone in the stratosphere was something of a surprise to some researchers because those compounds are normally quite stable. In fact, their stability is one of their most desirable properties for many industrial and commercial applications. But, when CFCs escape into the atmosphere and drift upward, they are exposed to ultraviolet radiation in sunlight and, as is oxygen itself, are dissociated by that radiation. In the case of Freon-12 (CCI2F2), photodissociation results in the formation of free chlorine atoms ... 

The restricted motion of molecules and of fragments such as free radicals formed by photodissociation results in interesting differences in the photochemistry of some molecules in solution or as guests in inclusion compounds. To take one example, the aliphatic ketone 5-nonanone can yield fragmentation or cyclization products via the biradical formed through intramolecular hydrogen atom abstraction (Figure 8.18). In the photolysis of the inclusion compound the cyclization is the preferred reaction, and there is a marked selectivity in favour of the ay-isomer of the cyclobutanol. 

The present photodissociation results can be used to help interpret such a picture. For illustration, we choose a definite model. The general energy balance for the reactions is... 

A11 data are in kcal/mol. For the equilibrium reaction 15 in text. Values ( 0.1 kcal/mol) are relative to D (Mg "-MeOH) - 0.00 kcal/mol. Absolute values ( 5 kcal/mol) assigned from photodissociation results, see reference 55. Tetrahydrofuran. 

In the lower stratosphere, 90 % of the O2 photodissociation results from transitions from the ground state to the... 

Figure A3.6.12. Photolytic cage effect of iodme in snpercritical ethane. Points represent measured photodissociation quantum yields [37] and the solid curve is the result of a numerical simnlation [111].

Photochemistry. Vinyl chloride is subject to photodissociation. Photexcitation at 193 nm results in the elimination of HCl molecules and Cl atoms in an approximately 1.1 1 ratio (69). Both vinyUdene ( B2) [2143-69-3] and acetylene have been observed as photolysis products (70), as have H2 molecules (71) and H atoms [12385-13-6] (72). HCl and vinyUdene appear to be formed via a concerted 1,1 elimination from excited vinyl chloride (70). An adiabatic recoil mechanism seems likely for Cl atom elimination (73). As expected from the relative stabiUties of the 1- and 2-chlorovinyl radicals [50663-45-1 and 57095-76-8], H atoms are preferentially produced by detachment from the P carbon (72). Finally, a migration mechanism appears to play a significant role in H2 elimination (71). 

Fig. 15. The optical spectrum of Cu Ar = 1 10 at 10-12 K, (A) showing isolated Cu atoms and Cuj molecules (B), (C) photoaggregation as the result of two 30-min irradiations in the resonance lines of Cu atoms at 302 nm, (D) photodissociation of Cu, resulting from a 30-min irradiation at the 370-nm band of Cu,. The features marked "a are thought to arise from secondary trapping sites of Cu,. Note the scale change between 325 and 400 nm (150).

Easy availability of ultrafast high intensity lasers has fuelled the dream of their use as molecular scissors to cleave selected bonds (1-3). Theoretical approaches to laser assisted control of chemical reactions have kept pace and demonstrated remarkable success (4,5) with experimental results (6-9) buttressing the theoretical claims. The different tablished theoretical approaches to control have been reviewed recently (10). While the focus of these theoretical approaches has been on field design, the photodissociation yield has also been found to be extremely sensitive to the initial vibrational state from which photolysis is induced and results for (11), HI (12,13), HCl (14) and HOD (2,3,15,16) reveal a crucial role for the initial state of the system in product selectivity and enhancement. This critical dependence on initial vibrational state indicates that a suitably optimized linear superposition of the field free vibrational states may be another route to selective control of photodissociation. 

It is our purpose in this review to present a brief summary of some of the main results from the applications of our FOIST based approach to selective control of photodissociation. The formal and computational considerations of this method are summarised in section 2 and in section 3, we discuss some representative results from our applications. Some concluding remarks summarising the main results and avenues for further research are collected in section 4. 

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. 

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