Absorption with dissociation

A common class of photolytic reactions consists of those in which the primary photochemical act is absorption of the quantum by a molecule followed by dissociation of the molecule  

Since the fragments produced are often atoms or free radicals, this primary step frequently initiates a chain mechanism, whose occurrence may be indicated by quite large values of the quantum yield, although a chain reaction can occur with very small values of the quantum yield. 

The band converges to a continuum at 499.0 nm. At this wavelength the energy of the transition is sufficient to bring the molecule to that required to dissociate the molecule into two iodine atoms, one of which is in the excited state, Pi/2 Strictly speaking, the transition from to E[o should be spin-forbidden, but because of the large number of electrons in the I2 molecule this prohibition is relaxed. The transitions from the ground state to 

In the case of oxygen the continuum begins at 175.9 nm corresponding to a transition from the state to the state. As indicated, the dissociation produces one normal and one excited oxygen atom. 

The case of HI is unusual in the sense that all the upper states of HI are repulsive. Depending on the wavelength, the iodine atom may or may not be in an excited state. But in all instances the product atoms have a large amount of excess kinetic energy. Because of the great differences in the masses of the H atom and the I atom, most of this excess energy 

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How could absorption with dissociation of a diatomic combined with fragment energy measurements be used to determine dissociation energies ... 

Vp > 1 Chemical absorption with dissociation Pi Partial pressure of the absorbate in the... 

Most other studies have indicated considerably more complex behavior. The rate data for reaction of 3-methyl-l-phenylbutanone with 5-butyllithium or n-butyllithium in cyclohexane can be fit to a mechanism involving product formation both through a complex of the ketone with alkyllithium aggregate and by reaction with dissociated alkyllithium. Evidence for the initial formation of a complex can be observed in the form of a shift in the carbonyl absorption band in the IR spectrum. Complex formation presumably involves a Lewis acid-Lewis base interaction between the carbonyl oxygen and lithium ions in the alkyllithium cluster. 

If the solution contains an appreciable concentration of unhydrated carbonyl compound, the variation of carbonyl u.v. absorption with pH can be used to obtain (Bunnett et al., 1961 Hiiie et al., 1965). Similarly, in the weakly dissociated compounds X.CoH4.C(OH)2.CF3 the variation of the aromatic absorption with pH provides a means of determining Kx or (Stewart and Van der Linden, 1960). 

Absorption and dissociation of the superbly soluble hydrogen cyanide (see chapter 6.5.4.) is clearly superior to chemisorption. Furthermore, the aqueous solution (as solvent) is indispensable for the complex formation and redox reactions of the cyanide with Fe3+. Additionally, the aqueous medium makes the reacting agents mobile, which do not always form at the same location. And finally, the moisture contained in the solid material works as a trap for hydrogen cyanide, because it intensely binds the hydrogen cyanide. Or the other way around the drier a solid material is, the easier hydrogen cyanide, which was ad-/absorbed before, will be released back into the environment. Therefore, a relatively high water content of the solid material will accelerate the reaction. 

Experimental investigations and theoretical computations of SO2 absorption in a spray drier [47] showed that, with an excess of calcium hydroxide absorption is limited only by the gas-phase mass transfer. In addition, the flow in a spray drier could be described by the model of an ideal stirred vessel. Newton et al. [70] considered the subprocesses of mass transfer of SO2 from the gaseous phase to the drop surface, the absorption, the dissociation of SO2, the diffusion of the produced species and the dissolution of calcium hydroxide particles in the drop. 

The recording of absorption spectra has also been applied to studies of acid-base equilibria, since radicals with dissociable functional groups usually show spectral shifts with pH which can be utilized for the determination of p -values. Most of the reported values of dissociation constants have, in fact, been determined in this manner. Several examples of spectral shifts can be found in Table 2 and a detailed discussion of acid-base equilibria of organic radicals is given in Section 6. 

The structures of the pairs have been determined by ab initio calculations. Surprisingly, while the absorption spectrum of the solvated electron presents a single band located around 2250 nm, the absorption spectra of the pairs are blue-shifted and composed of two bands (Fig. 7)7 Those spectra were interpreted as a perturbation of the solvated electron spectrum with the use of an asymptotic model. This model describes the solvated electron as a single electron trapped in a THF solvent cavity and takes into account the effects of electrostatic interaction and polarization due to the solutes that are modeled by their charge distribution. It was shown that the p-like excited states of the solvated electron can be split in the presence of molecules presenting a dipole. So, the model accounts for the results obtained with dissociated alkali and non-dissociated alkaline earth salts in THF since ionic solutes yield absorption spectra with only one absorption band, and dipolar neutral solutes yield absorption spectra with two bands (Fig. 8). ... 

The failure of a simple statistical model is not too surprising, since most evidence indicates alkyl iodide dissociation is a rapid, direct process. The recoil angular distributions which we have measured show that photodissociation occurs prior to significant molecular rotation, and the lack of any vibrational structure in the ultraviolet absorption spectra is also consistent with dissociation before significant vibrational motion. 

Tetrakis(triphenylphosphine)platinum(0) is a pale yellow powder which decomposes in the air to a red liquid at 118-120° and melts in vacuo (1 mm.) to a yellow liquid at 159-160°. The infrared spectrum in Nujol show s absorption maxima at 700(vs), 737(vs), 837(w), 992(s), 1022(s), 1077(vs), 1147(m), 1162(m), 1302(w), and 1432(vs) cm. h The compound is soluble in benzene with dissociation by leaving the benzene solution in the air, a white pow der [the carbonatobis(triphenylphosphine)-platinum(II) formed by action of oxygen and carbon dioxide] separates slowly. The compound reacts wdth carbon tetrachloride, giving cfs-dichlorobis(triphenylphosphine)platinum(II). ... 

Primary process" has been used in accordance with this recently suggested definition (2) "Any continuous sequence of one or more primary steps which starts with the light absorption step." In this sense a primary step is "any one of the elementary transformations of an excited state molecule of the species which absorbs light. The absorption step Itself is also a primary step" (2). Important primary processes of OTM compounds which are described here include (1) absorption, (ii) dissociative reactions, (iii) intramolecular "twisting" isomerizations, (iv) intermolecular energy transfer, (v) inter-molecular electron transfer, (vi) luminescence. Reactions involving OTM compounds as quenchers have also been included. 

Other Photofragmentations - Photodissociation of tert-huty hydroperoxide at 266 nm gives OH radicals with dynamics which are similar to those found for OH from H2O2, and which are consistent with dissociation via a repulsive excited state. Rates of p-scission of the ter/-butoxy radical to acetone and methyl radicals have been determined in flash-photolysis experiments by monitoring its transient UV absorption and its laser-induced fluorescence. ... 

We will use as our example a weak diprotic acid, H2A. We will label the three species by the number of attached, dissociable protons, i.e., H2A will be denoted by the subscript 2, HA by 1, and A2- by 0. The corresponding concentration fractions a2, a1( and a0 are given in (4.8-5) through (4.8-7) respectively. Furthermore we denote two different wavelengths by and " respectively, and associate different molar absorptivities with each species. 

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