Excited states production

Schuster, G. B. (1979). Chemiluminescence of organic peroxides. Conversion of ground-state reactants to excited-state products by chemically initiated electron-exchange luminescence mechanism. Acc. Chem. Res. 12 366-373. 

Hydroxyl radical (OH) is a key reactive intermediate in combustion and atmospheric chemistry, and it also serves as a prototypic open-shell diatomic system for investigating photodissociation involving multiple potential energy curves and nonadiabatic interactions. Previous theoretical and experimental studies have focused on electronic structures and spectroscopy of OH, especially the A2T,+-X2n band system and the predissociation of rovibrational levels of the M2S+ state,84-93 while there was no experimental work on the photodissociation dynamics to characterize the atomic products. The M2S+ state [asymptotically correlating with the excited-state products 0(1 D) + H(2S)] crosses with three repulsive states [4>J, 2E-, and 4n, correlating with the ground-state fragments 0(3Pj) + H(2S)[ in... 

Here at least nine dissociative channels are theoretically accessible below the ionization threshold. The dissociation products are NH2 -NH + H2, NH2 + H, or NH + 2H (final state), of which NH and NH2 may exist either in the ground or an excited state. Production of molecular hydrogen is negligible at low excitation energies, but it can account for 15% or more of the dissociation processes when the excitation exceeds -7 eV Note that the lowest excited state of NH3 ( 4 eV) does dissociate into NH and H2 but is spin forbidden. 

In the crossing over of product molecules to the excited state the geometry of the excited-state product as compared with that of the reactants is of decisive importance. This is demonstrated by a theory of... 

A.W. Adamson, University of Southern California You spoke of an excited state product. We have never been able to... 

As in the condensed phase, gas-phase chemiluminescence consists of a chemical reaction forming an excited-state product that then undergoes one or more relax-... 

On the basis of mechanistic studies, mainly on these isolable cychc four-membered peroxides (1 and 2), two main efficient chemiexcitation mechanisms can be defined in organic peroxide decomposition (i) the unimolecular decomposition or rearrangement of high-energy compounds leading to the formation of excited-state products, exemplified here in the case of the thermal decomposition of 1,2-dioxetane (equation i)". 5,i9. 

This biradical-like concerted mechanism, in which the kinetic features reflect the biradical character and the formation of excited-state products can best be rationalized by the concerted namre of the complex reaction coordinate, was proposed to optimally reconcile the experimentally determined activation and excitation parameters of most 1,2-dioxetanes studied and has been called the merged mechanism . Specifically, bofh fhermal sfabil-ity and singlel and friplef quanfum yields in fhe series of mefhyl-subsliluled 1,2-dioxelanes, including fhe parenf 1,2-dioxefane" , could be readily rationalized on the basis of the merged mechanism and qualitative quanmm mechanics considerations . 

In the five years since the first volume was published, there has been increased interest in the chemistry within gas lasers and the chemistry induced by laser radiation, the kinetics and photochemistry within fusion and industrial plasmas, as well as in the normal and perturbed lower and upper atmosphere. And. since the Three Mile Island accident there has been renewed interest in radiation damage to living and nonliving things. This state of affairs has not only precipitated a variety of spectroscopic studies, but has also brought more attention to the nonspectroscopic aspects of excited state production and the interaction of excited species. The latter topic was stressed in the earlier volume and the emphasis is retained here. 

Complex, its photoreactive excited state Products Conditions and parameters of reactions Notes Ref. 

The energy requirement of (3) is a necessary but not a sufficient condition for a reaction to be chemiluminescent (Richardson, 1980). While few organic reactions meet this requirement, and this mainly accounts for the rarity of chemiluminescence, there are other important factors which influence first whether a reaction will be chemiluminescent and thereafter the efficiency of chemiexcitation. Provided the energy requirement is met, there still must be a reason for the rather remarkable non-equilibrium formation of excited state products. In this section we will discuss those factors which are thought to favor the selection of an excited state over a ground state product. 

---