Electronically Excited reactants

By changing from the simplest to larger aliphatic and cyclic ketones, structural factors may be introduced which favor alternative unimolecular primary photoprocesses or provide pathways to products not available to the simple model compound. In addition, both the increase in molecular size and irradiation in solution facilitate rapid vibrational relaxation of the electronically excited reactant as well as the primary products to thermally equilibrated species. In this way the course of primary and secondary reactions will also become increasingly structure-selective. In a,a -unsym-metrically substituted ketones, the more substituted bond undergoes a-cleavage preferentially. 

As a general rule, the rates of those reactions that are endoergic for ground-state reactions are dramatically enhanced when excited-state reactants are employed. Electronic excitation and vibrational excitation may or may not have similar effects. In the case of vibrational excitation, the reactions may occur on a single electronic potential surface. In the case of electronic excitation, a single-surface path between electronically excited reactants and ground-state products may not exist because of the prevailing symmetries. 

The effects of electronically excited reactant ions (AB+) on collision-induced dissociations have been studied in a number of systems, including those for which AB+ =0, N, N0+,C0+, and for which the neutral target is either a rare-gas atom or another neutral molecule. Various reactions of this type that have been investigated are included in Table I. 

In principle, one molecule of a chemiluminescent reactant can react to form one electronically excited molecule, which in turn can emit one photon of light. Thus one mole of reactant can generate Avogadro s number of photons defined as one einstein (ein). Light yields can therefore be defined in the same terms as chemical product yields, in units of einsteins of light emitted per mole of chemiluminescent reactant. This is the chemiluminescence quantum yield which can be as high as 1 ein/mol or 100%. 

It can be assumed that in cycloadditions only one reactant is electronically excited, in view of the short lifetimes of excited species in solution and the consequently low probability of a collision between two excited molecules. Also, the cycloadditions are conducted with light of wavelengths above 2800 A... 

The photoinduced electron transfer step and the back electron transfer step involve three states, excited reactant (R ), ground-state reactant (R) and intermediate (I) ... 

A, B.. . . reactants, Px electronically excited reaction product, P reaction product in the ground state, Xx energy acceptor in electronically excited, X energy acceptor in the ground state... 

If a reaction can yield products in the ground state or in an electronically excited state, the activation energy for the formation of the latter will therefore be less than that required for the formation of the products in the ground state — provided that there is no significant change in the configuration of the excited-state molecules as compared with the reactant molecules. 

B, C reactants Dx electronically excited product A acceptor molecule with high fluorescence efficiency... 

Figure 1 Relative positions of the potential energy (E) surfaces of the electronic states involved in a hypothetical chemiluminescent reaction as a function of intemuclear separation (r). P and P represent the ground and lowest electronically excited singlet states of the product of the reaction, respectively. R represents the ground electronic state of the reactant. AH is the enthalpy of the dark reaction while AHa is its enthalpy of activation. AH is the enthalpy of activation of the photoreaction, hv denotes the emission of chemiluminescence.

When considering the manner in which photochemical reactions occur in Chapter 7, the overall reactions were considered as the sum of a number of elementary steps. It is assumed that absorption of a photon by a reactant molecule, R, produces an electronically-excited-state species, R, which may then react via reactive ground-state intermediate(s), I, to eventually form the product(s), P ... 

It is possible that only a fraction of the radiant energy supplied to the calorimeter would be absorbed by the reaction mixture. Part of that radiation can be reflected (Er) and, if the reaction vessel is transparent, another fraction can be transmitted to the surroundings (Et). Furthermore, the electronically excited states of the reactants may decay by luminescence, so more energy (E ) may be lost to the surroundings. If these three contributions are taken into account, equation 10.1 becomes... 

At the most fundamental level one follows the time development of the system in detail. The reactants are started in a specific initial (quantum) state and the equation of motion are propagated to give the final state. The equation of motion of the system is the time dependent Schroinger equation, or, if the atoms involved are heavy enough (not H or Li) Newtons equation. The starting point is the adiabatic potential energy surface on which the process takes place. For some reactions electronic excitations during the reaction are important and must be included in addition to the electronically adiabatic dynamics. 

As described in Chapter 3, the products of some chemical reactions are initially produced in electronically excited states. If the excited state has a sufficiently short radiative lifetime, it will emit light faster than collisional quenching by air molecules can occur (see Problem 1). The effective concentration of the emitting species (and hence emitted light intensity) is proportional to the concentrations of the reactants. As a result, the chemiluminescence intensity can be used to monitor one of the reactants if the second reactant is kept at a constant (excess) concentration. 

The rates of reaction of the electronically excited iodine atoms have been compared directly with a number of analogous reactions of the ground state atoms in the following manner. The equilibrium constants K2a, K21, and K22 for ground state reactants and products (since the small Boltzmann populations of excited states may be neglected) in the following processes... 

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