Halogens, photodissociation

In order to probe the importance of van der Waals interactions between reactants and solvent, experiments in the gas-liqnid transition range appear to be mandatory. Time-resolved studies of the density dependence of the cage and clnster dynamics in halogen photodissociation are needed to extend earlier quantum yield studies which clearly demonstrated the importance of van der Waals clnstering at moderate gas densities [37, 111]... 

Although inert in the lower atmosphere (troposphere), the hilly halogenated CFCs and Halons diffuse into the upper stratosphere where they are photodissociated, ie, photolyzed, by the intense ultraviolet radiation. 

Figure 4.34 compares the absorption spectra of the diatomic halogens, F2, Cl2, Br2, and I2. Cl2 is of particular recent interest in the troposphere in that levels up to 150 ppt have been observed in marine areas (Keene et al., 1993 Pszenny et al., 1993 Spicer et al., 1998). Table 4.30 summarizes the absorption cross sections of Cl2, Br2, and BrCl (DeMore et al., 1997 Marie et al., 1994 Hubinger and Nee, 1995). These diatomics all dissociated with a photodissociation quantum yield of 1 (Calvert and Pitts, 1966). 

The photodissociation products of the homonuclear halogens in the visible and ultraviolet are now comparatively well established in view of the detailed spectroscopic studies that have been made. The strongest absorption system observed in this spectral region is associated with a transition to the 3II0u+ state which correlates with X / ) + X(2Pyz). Thus photoexcitation to the continuum associated with this state leads directly to the formation of an excited atom, while excitation to the banded region followed by predissociation will lead only to ground state atoms. 

Photodissociation may occur even on excitation to below a spectroscopic continuum region if a higher energy state intersects the curve at a suitable point creating a dissociative situation (Figure 7.3). The spectra of halogens and halogen acids are instructive in this respect. 

Various transient species containing arsenic in low CN with halogens have been recorded spectroscopically, e.g. Asia from the photodissociation of Aslj.47 The only stable species is the red crystalline iodide, AS2I4 (m.p. 260°C),48 which is decomposed by air or moisture. The... 

The energy balance of photodissociation the importance of stabilization of the free radicals. When chlorobenzene or chloro-Np loses the halogen atom, a phenyl or a naphthyl radical is formed with the odd electron localized in an sp2 orbital which is orthogonal to the aromatic zr orbitals such a radical is not stabilized through resonance, unlike the benzyl- or the methyl-Np radicals for which several resonance structures can be drawn (Figure 4.32). 

In the pulsed molecular beam studies, the results were used to calculate an impact parameter for the photodissociation process. From these impact parameters it was concluded that the recoiling CN fragment did not take the lowest energy path when it was departing from the halogen atom. Rather, because of strong impulsive motion, the trajectory that the CN rsdical... 

A great deal of work has been done on the photodissociation dynamics of the halogen containing compounds. This is partially due to the production of halogen atoms and radicals that can be studied by TOF and partially due to the fact there are intense absorption bands in the ultraviolet and near ultraviolet region. These reasons, coupled with the fact that the spectroscopy of some precursor molecules is fairly well understood, have resulted in many workers looking at these compounds. Finally, some of these compounds have been studied as possible candidates for various types of lasers. 

Aryl and Alkyl Halides. Although the factors governing photodissociation of alkyl halides are not well known, there exists clear experimental evidence for electron exchange between the radical fragments formed by C-X homolysis. For example, the ultraviolet photolysis of 1-octyliodide shows photorearrangement to the 2-halogenated isomer, eq. 58 (181) ... 

The editor has encountered numerous studies of laser ignition, usually of explosives. These demonstrate that the ignition is normally a thermal effect, caused by heating solid particles, not photochemical, and thus the MIE is not lower than other methods. This may not obtain if the light be of a frequency (visible or uv) sufficient to excite the early steps of the explosion reaction, as, for example, photodissociation of halogens to the atomic radicals which are the start of their explosive reactions with fuels. 

In section 3.2, experiments were described in which photodissociation of the HX moiety within a weakly bound complex liberated hydrogen atoms which then went on to react with the other moiety in the complex. Alternatively, photodissociation of HX moieties within weakly bound complexes can be used to prepare halogen atom reactants. This provides advantages which derive mainly from the rapid removal of the hydrogen. 

The van der Waals attraction between Br and I2 is estimated to be 400 cm-1 by analogy with halogen/rare gas complexes (Bieler and Janda 1990 Bieler et al. 1991). This ensures that photodissociation of the HBr moiety cannot produce Br + I2 except via quenching of Br or the unlikely instance in which the hydrogen is trapped efficiently between the heavv particles. With the Br atom unable to escape from the I2 because of the Br-I2 van der Waals attraction, the system is ensured of an essentially unity quantum yield. 

The oxidation scheme for halomethanes not containing a hydrogen atom is similar to that for those which do, except that it is not initiated by tropospheric reaction with hydroxyl radicals, since the fully halogenated methanes are unreactive. Consequently, substantial amounts of CFCs and halons are transported intact up into the stratosphere, where they absorb UV radiation of short wavelength and undergo photodissociation (equation 36) to a halogen atom and a trihalomethyl radical. The halogen atom Y may enter into catalytic cycles for ozone destruction, as discussed in the introduction. 

Besides the photochemical dissociation, ozone decays in oxidation-reduction reactions with different species. The stratospheric 03 reacts rapidly with nitric oxide and products of photodissociation of halogenated hydrocarbons (Figure 9.5). 

As already stated, photodissociation of XeF6 is a convenient route to clean XeF4, and photodecomposition is also an effective tool elsewhere. A facile and high yield (99%) preparation of S2Fio is achieved via photodecomposition of SF5Br [264], in which the S-Br bond is easily cleaved by light from a halogen lamp ... 

In addition to UV/visible flash photolysis and TRIR spectroscopy, other techniques have been used for the detection of transition metal-noble gas interactions in the gas phase. The interaction of noble gases with transition metal ions has been studied in detail. A series of cationic dimeric species, ML" " (M = V, Cr, Fe, Co, Ni L = Ar, Kr, or Xe), have been detected by mass-spectroscopic methods (55-58). It should be noted that noble gas cations L+ are isoelectronic with halogen atoms, therefore, this series of complexes is not entirely unexpected. The bond dissociation energies of these unstable complexes (Table IV) were determined either from the observed diabatic dissociation thresholds obtained from their visible photodissociation spectra or from the threshold energy for collision-induced dissociation. The bond energies are found to increase linearly with the polarizability of the noble gas. 

The unique problem that arises here is that FI 1 and FI 2 are relatively inert chemically and have no natural sources or sinks, as CCI4 does. The lifetimes of these fluorocarbons are controlled by diffusion to the stratosphere, where photodissociation takes place as designated by reactions (166) and (167). The lifetimes of halogen species in the atmosphere are given in Ref. [118]. These values are reproduced in Table 8. The incredibly long lifetimes of FI 1 and FI 2 and their... 

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