Excited molecules, reaction

The assumption of a simple primary dissociation (2) has been cast in doubt by the results of an application of the Cundall technique to the photolysis9, which suggest that excited-molecule reactions are very important at 2537 A. There is clearly a need here for some careful quantum yield measurements to establish the nature of the primary step. 

It is also possible to determine the nature of the excited molecule reaction leading to product formation by kinetic methods. For example, variation in the rate of formation of dimethyluracil hydrate with water concentration in acetonitrile-water mixtures is convex to the water concentration axis (Fig. 15).65 The rate of formation of uracil hydrate under similar conditions is linear with water concentration. The first of these is not the shape of curve to be expected if the function of the water molecules were simply to quench an excited state according to the common mechanism ... 

It is clear that in the gaseous phase the contribution from excited molecule reactions does not exceed that from ion-molecule reactions. This is probably to be attributed to the fact that the electron may more readily escape its parent ion in the gaseous phase and consequently the production of excited states by charge neutralisation is diminished. It would be interesting to examine the effect of let on the relative yields of these processes. 

With the advent of convenient techniques in recent years, it has become popular to measure "lifetimes" of excited states. Decay characteristics are useful adjuncts to quantum yields as they provide further knowledge about the mechanism of reaction, particularly with regard to evaluation of rate constants for excited molecule reactions. Only when the decay is exponential can a unique lifetime be defined. 

The role of the excited-molecule decomposition was neglected for some time but Gorden and Ausloos have shown the importance of excited-molecule reactions by the use of an applied electric field. The observation of CH by Sieck and John-sen led Gorden and Ausloos to expect ethylene formation from CH insertion by reaction (21). The importance of this reaction and of CH2 insertion, with both CH and CH2 arising from excited molecules, was shown by the electric-field study. 

Studies conducted in the presence of radical scavengers such as NO (refs. 383, 245, 409, 410), Oj (ref. 408) or H2S (ref. 246) have shown the importance of free-radical reactions in forming the products isobutane, 2,3-dimethylbutane, -butane, isopentane and others. The ethylene and propene yields are decreased by the presence of the scavengers owing to the disappearance of the fraction of these products that arises from disproportionation reactions. The products which are formed in the presence of inhibitors must arise from molecular or ion eliminations, ion-molecule reactions, excited molecule reactions or charge-neutralization reactions. Work on the inhibited radiolysis has led to a better understanding of the source of these products " . 

The mechanism of the radiolysis of the higher paraffins is generally less clearly understood than that of methane, ethane and propane. The same types of reaction are found, however, and although the system may be more complicated owing to the presence of an increased number of reactive species the pattern of reaction is similar to that with the lower paraffins. Free-radical reactions continue to comprise an important fraction of the overall reaction for the uninhibited radiolysis. In view of the complexity in the presence of free radicals many studies have been made with radical scavengers added to the system so that the ion, ion-molecule and excited-molecule reactions might be studied. 

The importance of ion decomposition and fragmentation, and of ion-molecule reactions, has been established. The contribution of excited-molecule reactions has been shown, but only in a few cases has it been put on a quantitative basis. Often a clear distinction between molecular reactions and bimolecular reactions such as ion-molecule processes has been made through the use of isotopic mixtures. Characterization of the source of many of the olefinic products and of the total hydrogen remains the least understood aspect of the inhibited radiolysis. 

The following reaction scheme includes both free-radical and excited-molecule reactions as well as ionic reactions all of these reactions along with other unknown processes may be occurring in the radiolysis of acetylene. 

The following sequence of events is generally postulated to occur during the radiolysis of saturated hydrocarbons (7). Radiation first produces ions (Reaction 1) or excited molecules (Reaction 2). 

In the case of atom plus molecule reactions, the atom may very efficiently deactivate the excited molecule. Reactions (23) and (24) ... 

Mullin A S and Schatz G C (eds) 1997 Highly Excited Molecules Relaxation, Reaction and Structure (ACS Symp. Ser. 678) (Washington, DC American Chemical Society)... 

Quack M and Tree J 1976 Unimolecular reactions and energy transfer of highly excited molecules Gas Kinetics and Energy Transfer mo 2, oh 5, ed P G Ashmore and R J Donovan (London The Chemical Society) pp 175-238 (a review of the literature published up to early 1976)... 

Once the excited molecule reaches the S state it can decay by emitting fluorescence or it can undergo a fiirtlier radiationless transition to a triplet state. A radiationless transition between states of different multiplicity is called intersystem crossing. This is a spin-forbidden process. It is not as fast as internal conversion and often has a rate comparable to the radiative rate, so some S molecules fluoresce and otliers produce triplet states. There may also be fiirther internal conversion from to the ground state, though it is not easy to detemiine the extent to which that occurs. Photochemical reactions or energy transfer may also occur from S. ... 

Collision-induced dissociation mass spectrum of tire proton-bound dimer of isopropanol [(CH2)2CHOH]2H. The mJz 121 ions were first isolated in the trap, followed by resonant excitation of their trajectories to produce CID. Fragment ions include water loss mJz 103), loss of isopropanol mJz 61) and loss of 42 anui mJz 79). (b) Ion-molecule reactions in an ion trap. In this example the mJz 103 ion was first isolated and then resonantly excited in the trap. Endothennic reaction with water inside the trap produces the proton-bound cluster at mJz 121, while CID produces the fragment with mJz 61. 

It is well known that the electron-impact ionization mass spectrum contains both the parent and fragment ions. The observed fragmentation pattern can be usefiil in identifying the parent molecule. This ion fragmentation also occurs with mass spectrometric detection of reaction products and can cause problems with identification of the products. This problem can be exacerbated in the mass spectrometric detection of reaction products because diese internally excited molecules can have very different fragmentation patterns than themial molecules. The parent molecules associated with the various fragment ions can usually be sorted out by comparison of the angular distributions of the detected ions [8]. 

Weller A 1961 Fast reactions of excited molecules Progress in Reaction Kinetics (Oxford Pergamon) pp 187-214... 

Flynn G W, Michaels C A, Tapalian C, Lin Z, Sevy E and Muyskens M A 1997 Infrared laser snapshots vibrational, rotational and translational energy probes of high energy collision dynamics Highly Excited Molecules Relaxation, Reaction, and Structure ed A Mullin and G Schatz (Washington, DC ACS)... 

This leads to the possibiUty of state-selective chemistry (101). An excited molecule may undergo chemical reactions different from those if it were not excited. It maybe possible to drive chemical reactions selectively by excitation of reaction channels that are not normally available. Thus one long-term goal of laser chemistry has been to influence the course of chemical reactions so as to yield new products unattainable by conventional methods, or to change the relative yields of the products. 

Another useful technique for measuring the rates of certain reactions involves measuring the quantum yield as a function of quencher concentration. A plot of the inverse of the quantum yield versus quencher concentration is then made Stern-Volmer plot). Because the quantum yield indicates the fraction of excited molecules that go on to product, it is a function of the rates of the processes that result in other fates for the excited molecule. These processes are described by the rate constants (quenching) and k (other nonproductive decay to ground state). 

In chemiluminescence, some of the chemical reaction products developed remain in an excited state and radiate light when the excitation is discharged. This is particularly so at low pressures, when the collision frequency is low the excitation is discharged as light radiation. The extra energy bound to the excited molecule can discharge through impact or molecular dissociation. 

Photolytic methods are used to generate atoms, radicals, or other highly reactive molecules and ions for the purpose of studying their chemical reactivity. Along with pulse radiolysis, described in the next section, laser flash photolysis is capable of generating electronically excited molecules in an instant, although there are of course a few chemical reactions that do so at ordinary rates. To illustrate but a fraction of the capabilities, consider the following photochemical processes ... 

The presence of N20 reduces the yield of triplet excited molecules since at least part of these are due to reaction of electrons with either the solvent molecules (S) or solute molecules (D). 

A similar investigation of methane (21) has shown that between 15.5 and about 20 e.v. the fragment ion CH2 + is formed in the ground state, but above 20 e.v. in an excited state that can cause ion-molecule reactions of different kind. 

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