Reactions of Excited Species

Photolysis and chemical reaction can produce species that are vibrationally or electronically excited. These molecules process more energy than in the ground state. The most important exicted species in atmospheric chemistry is the first electronically excited state of the oxygen atom, 0( D). The major source of O( D) below 40km altitude is the photolysis of O3  

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Finally, compared to the chemical reactions discussed in the previous chapter, photochemical transformations of organic compounds usually exhibit a much weaker temperature dependence. Reactions of excited species in aqueous solutions have activation energies of between 10 and 30 kJ.mol-1 (Mill and Mabey, 1985). Hence, a 10°C increase (decrease) in temperature accelerates (slows down) a reaction only by a factor of between 1.15 and 1.5 (see Table 3.5). 

In recent wrork particular emphasis has been given to studies of flame spectra and the evidence as to the formation and reaction of excited species such as C2, CH, OH, and HCO from acetylene and oxygen (17, 29, 31, 41, 43, 54). The occurrence of excited hydrocarbon flame bands attributable to HCO radicals led Herman, Hombeck, and Laidler (31) to suggest the reaction... 

The study of the reactions of excited species is becoming an increasingly important area of research in kinetics [49, 50]. The excitation may take the form of enhanced translational, rotational, vibrational or electronic energy. Reactions with translational excitation are most commonly studied under molecular beam conditions using seeded nozzle beams or other types of sources to provide the enhanced energy [51, 52]. Translationally hot atoms may also be generated by nuclear recoil [53] or photodissociation [ 54 ]. 

The obvious goal in this case could be an attempt to describe all phenomena ever observed in experiment within the framework of some broad, but uniform, mechanism. If this goal is reached, the model can be used to predict the system behavior even in the range of parameters far beyond those accessible in any realistic experiment. Such a range can be limited only by the physical and chemical sense of the kinetic scheme content and kinetic parameters. For instance, if we do not consider elementary reactions of excited species (e.g., carrying vibrational and/or electronic excitation), the scheme cannot be extrapolated to relatively high temperatures at which the effect of these factors becomes substantial. 

Finally in this Section on infrared photochemistry, the i.r. laser-induced reaction of SF with a Si surface has been described. Pulsed COj laser radiation in the absence of SF causes momentary heating of a Si target, but no Si removal in the presence of a few Torr of SF, etching of the Si is observed. The mechanism for the process is yet to be established, but it appears that both the reactions of excited species produced by the laser, and the effect of the laser radiation at the gas-surface interface are of importance. ... 

Bimolecular reactions of excited species A with substrate molecules B (which may be identical with A) may be classified as energy transfer reactions leaving the A-molecule intact and photoreactions leading to chemically different reaction products. B-molecules act as quenchers when radiative transitions A —> A + hv compete with the bimolecular process. Since the emission can also be studied in the absence of quenchers, it may be used as a probe for investigating the bimolecular reaction. Photoreactions require a contact between A - and B-molecules, i.e. diffusion energy transfer of the Fbrster type (48-52) can be fast in comparison with relevant diffusion times. 

The reactivity of the excited species will be influenced by both its energy level and rearranged electron configuration. Excited species are usually both better electron donors and electron acceptors than their parent molecules. The excited electron would be more readily donated while the low-energy vacancy left by the excited electron would more readily accept an electron. The redistribution of electrons in molecular orbitals may influence shape and dipole moments. Hydrogen abstraction is a common reaction of excited species. For example, benzophenone in the presence of a suitable donor solvent such as ethanol is readily reduced to the alcohol. 

M denotes a highly excited species that can emit a photon differing in energy to that emitted by M or can undergo ionization or bond breakage. Annihilation is a self-reaction of excited species that may be singlets or triplets. 

Which NH2 source to use depends on the kinetic problem and the specific facilities available. In flow reactors, heterogeneous depletion of NH2 and other intermediate radicals can not be avoided, thus giving erroneous kinetic parameters, if the y values (as defined above) depend on the reactant concentrations. The photolytic NH2 production is disadvantageous, if the reactant also absorbs the photolysis light in the case of 1,3-butadiene [127], for example, the reaction of excited species or of photofragments may interfere. NH2 production in flames is part of a complex system here it is difficult to identify an elementary process, for which the kinetic parameters are to be determined. 

Inelastic energy transfer processes Instrumental methods for experiments hemical processes at microscopic level photodissociation, energy distribution in products of elementary reactions, reactions of excited species Photodissociation processes, intramolecular processes Dynamics of intramolecular processes... 

The absorption of a light pulse instantaneously generates reactive species in high concentrations, either tlirough the fomiation of excited species or tlirough photodissociation of suitable precursors. The reaction can... 

Volume 1 Volume 2 Volume 3 The Practice of Kinetics The Theory of Kinetics The Formation and Decay of Excited Species Section 2. HOMOGENEOUS DECOMPOSITION AND ISOMERISATION REACTIONS... 

Energetic electron transfer reactions between electrochemically generated, shortlived, radical cations and anions of polyaromatic hydrocarbons are often accompanied by the emission of light, due to the formation of excited species. Such ECL reactions are carried out in organic solvents such as dimethylformamide or acetonitrile, with typically a tetrabutylammonium salt as a supporting electrolyte. The general mechanism proposed for these reactions is as follows. 

In thermal reactions heat is applied to reactants, reaction media and products in an indiscriminate manner. In photochemical reactions a high concentration of excited species can be produced selectively by using monochromatic light of the correct energy at low temperature to produce monoenergetic products. 

Quantitative data on rates of reaction have been obtained for some of the triplet reactions. Assuming triplet quenching to be approximately diffusion controlled, the rate constants for the reactions between excited species and nucleophile are 10 -10 1 mole s . The data show that in comparing and interpreting quantum yields—even in the case of related systems—one should proceed to determine separately rate constants as well as intersystem crossing efficiencies and lifetimes of the reacting excited species. 

As indicated in Fig. 16.2, in addition to energy transfer, chemical reactions of excited UCs ( UC, 3UC ) may lead to the formation of other reactive oxygen species (ROS) that may react with organic pollutants. Such ROS include DOM-derived oxyl- and peroxyl radicals (RO , ROO ), superoxide radical anions (02 ) that may be further reduced to H202, and hydroxyl radicals (HO ). In the case of HO , however, DOM is a net sink rather than a source. Finally, some of the 3UC may react directly with certain more easily oxidizable pollutants (see below). 

Lasers have applications in studying chemical reactions. One can use a monochromatic laser pulse to populate a specific vibrationally excited state of a molecular species, and then follow the kinetics of the reactions the excited species undergoes. 

Several uncertainties and apparent inconsistencies arise in the gas-phase data and they may result to some extent from the different experimental techniques used in various laboratories. In almost every case, flow methods have been used for the laboratory study of excited oxygen, and the use of flow systems is implied in the discussions which follow, unless specific mention is made of another technique. Several methods have been used for the detection and estimation of the excited species, and the main discussion of the information about the production and reaction of excited oxygen is therefore preceded by a description of the various methods. 

The reaction of excited molecular oxygen with ozone may also offer a further method of following the concentration of the excited species. For example, March et al.26 followed ozone concentrations in a discharge-flow experiment by means of its optical absorption at A = 2537 A. The ozone concentration continued to decrease with time even when no Oa(12s+) was present in the system, and the effect was ascribed to the reaction... 

The problem of accurately determining rates of quenching is important not only for understanding energy transfer but also for estimating rates of physical and chemical reactions of excited triplet species. Quenching studies of the Stern-Volmer type184 yield values of kQrT, where rT is the lifetime of the triplet species and kq is the rate constant with which some compound quenches it. Since quantum-yield and product-yield measurements allow rT to be factored into rate constants for individual reactions, absolute values of these reaction rate constants can be determined provided that the absolute value of... 

These reactions have a characteristic free energy which implies the minimal voltage required. As discussed in section 4.1, an excited molecule is at the same time more easily oxidized and reduced than the ground state species. Reactions of excited molecules at electrodes are however practically unknown because their short lifetimes preclude the contact with the electrode when irradiation takes place in the bulk of the liquid. In practice the photoelectro-chemical reactions at non-excited electrodes are simply the thermal reactions of photoproducts. We shall give here two examples of such reactions. 

It was not necessary to include reactions of excited molecules. A similar correlation between mass spectral and radiolytic results was obtained for n-hexane22. If these correlations are significant it would indicate that, in these systems at least, excited species that may be produced decay without chemical decomposition. 

Combination and addition reactions have been used effectively for the study of excited species. In effect, chemi-excitation reactions have been used for synthesis of reagents of known excitation energy1,72-81. A major effort has been made to use such excited molecules as tools for the exploration of the details of uni-molecular decomposition reactions (see Rabinovitch and Setser82). 

There have been numerous studies of low-temperature chemical reactions of matrix-isolated reactants (see, for example, the review by Perutz [1985]). Two of the most interesting from the standpoint of this volume are the reactions of NO with 02 and 03 studied by Smith and Guillory [1977] and Lucas and Pimentel [1979]. The cis dimer (NO)2 has been formed in solid oxygen at 13-29 K. Reaction of this species with the 02 matrix forms the product N204 in an electronically excited state. The transition state structure is reportedly of the form... 

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