Metastable intermediates, chemical

Figure 5.42 Sequence and relative stability in growth pattern of bile showing formation of metastable intermediates as function of time after supersaturation. Less stable structures have higher chemical potential. Solid and dotted arrows represent observed and presumed transitions, respectively. Reprinted with permission from Ref. 161. Copyright 1993 by the National Academy of Sciences, U.S.A.

The chemical deactivation of photoexcited anthracenes by dimerization usually proceeds by 4re + 4re cycloaddition [8]. However, exceptions to this rule have become known in recent years [8], and a multitude of steps, including the formation of metastable intermediates such as excimers, may actually be involved in a seemingly simple photochemical reaction such as the dimerization of 9-methylanthracene [9, 10]. Moreover, substitution of the anthracene chromophore may affect and alter its excited state properties in a profound manner for a variety of reasons. For example, in 9-tert-butylanthracene the aromatic ring system is geometrically distorted [11,12] and, consequently, photoexcitation results in the formation of the terf-butyl-substituted Dewar anthracene [13-15], The analogous photochemical isomerization of decamethylanthracene [16] probably is attributable to similar deviations from molecular planarity. 

In the sequence of metastable intermediates between starting materials and products of the free-radical reactions we have studied in crystals, many structures differ only in the arrangement of an identical set of molecular fragments. The physical reactions that connect them involve motion without a change in chemical bonding, but these steps are as well-defined kinetically and as important to the overall mechanism as chemical steps. 

Despite high probabilities for triplet formation and for triplet reaction, many overall photoreactions proceed in low quantum efficiency. In the absence of competing chemical reactions, the factor most often responsible for low quantum yields is the revertibility 8> of primary triplet reactions. Metastable intermediates such as radicals, biradicals, and charge transfer complexes (either excited or ground state) are the usual photoproducts from excited triplet reactions. These intermediates generally can revert to ground state reactant, thus providing a chemical path for radiationless decay , as well as proceed to stable products. Hence, the factor Pp is necessary to describe the probability that the intermediate will form product. 

The recent developments of electron paramagnetic resonance (EPR) spectroscopy and nuclear magnetic resonance (NMR) spectroscopy have made available two new tools for the exploration of free radicals and metastable intermediates in chemical reactions. Atoms or radicals with unpaired electron species will absorb microwaves of the proper frequency when placed in a uniform magnetic field. Concentrations of radicals the order of 10 M can be detected in samples as small as 0.1 cc, and many radicals and paramagnetic species have been observed by this method. ... 

Figure 6.11 Degenerate stepwise HH-transfer involving metastable intermediates, (a) Chemical reaction network as base for a description in terms of formal kinetics, (b) Corresponding free energy diagram. Reproduced with permission from Ref [26].

The high acidity and the extremely low nucleophilicity of the counterions of superacidic systems are especially useful for the preparation of stable, electron-deficient cations, including carbocations. Many of these cations, which were formerly suggested only as fleeting metastable intermediates and were detectable only in the gas phase in mass spectrometric studies, can be conveniently studied in superacid solutions. New chemical transformations and syntheses that are not possible using conventional acids can also be achieved with superacids. These include transformations and syntheses of many industrially important hydrocarbons. The unique ability of superacids to bring about hydrocarbon transformations, even to activate methane (the principal component of natural gas) for electrophilic reactions, has opened up a fascinating new field in chemistry. 

Isolating the less thermodynamically stable of two interconvertible forms of the same composition (whether it be coordination compounds, allotropes of elements, or any other chemical composition) can be done only if the rate of reaching equilibrium is so slow as to render the conversion of the metastable isomer to the stable one very protracted. This is a form of Ostwald s law of metastable intermediates. Such rates are slow (minutes < ti/2 < years) for the equilibrations of coordinated cobalt(III), chromium(III), a few (spin-paired) d ferrous compounds, such as salts of the ferroin 22a, tris-l,10-phenanthrolineiron(II) cation, and a few octahedral nickel(II) species with strong field ligands (3d, e.g., the tris-l,10-phenanthrolinenickel(II) ion 22b). 

A concerted reaction is one in which the conversion of reactants (R) into the products (P) occurs directly by way of a single transition state (T.S.). An exothermic concerted reaction is represented by the potential energy profile of Fig. 3.1(a). When the conversion of reactants into products proceeds by way of more than one transition state, such that one or more intermediates (I) are formed, the processes are accordingly non-concerted. A two-step process involving one (metastable) intermediate is represented by Fig. 3.1(b). However, since each elementary step of any chemical reaction must be concerted, by definition, then case (b) may be divided into the two concerted sequences ... 

X-Ray irradiation of quartz or silica particles induces an electron-trap lattice defect accompanied by a parallel increase in cytotoxicity (Davies, 1968). Aluminosilicate zeolites and clays (Laszlo, 1987) have been shown by electron spin resonance (e.s.r.) studies to involve free-radical intermediates in their catalytic activity. Generation of free radicals in solids may also occur by physical scission of chemical bonds and the consequent formation of dangling bonds , as exemplified by the freshly fractured theory of silicosis (Wright, 1950 Fubini et al., 1991). The entrapment of long-lived metastable free radicals has been shown to occur in the tar of cigarette smoke (Pryor, 1987). 

The F + H2 — HF + FI reaction is one of the most studied chemical reactions in science, and interest in this reaction dates back to the discovery of the chemical laser.79 In the early 1970s, a collinear quantum scattering treatment of the reaction predicted the existence of isolated resonances.80 Subsequent theoretical investigations, using various dynamical approximations on several different potential energy surfaces (PESs), essentially all confirmed this prediction. The term resonance in this context refers to a transient metastable species produced as the reaction occurs. Transient intermediates are well known in many kinds of atomic and molecular processes, as well as in nuclear and particle physics.81 What makes reactive resonances unique is that they are not necessarily associated with trapping... 

In the following sections, studies of isomeric ions are reported in which the ions are reactively probed. Where calculations are available, information on potential energy surfaces is given. This is usually the structure of the stable isomeric forms and transition states and their relative energies thus only points on the potential surface are known. The detailed form of the potential surface is almost never available nor is the connectivity between the various states usually established theoretically (chemical intuition is often used to connect the states). Pertinent experimental data on CID and metastable ions, isomers produced in binary reactions, and potential surfaces probed by binary reactions (with the excited isomeric ion as the reaction intermediate) are also given. 

Figure 16. Metastable ion cyclotron resonance (MICR) spectra for the unimolecular dissociation of the chemically activated adduct ion derived from association of the methoxymethyl cation with pivaldehyde during a 2-s reaction delay at a pressure of pivaldehyde of 1.0 x 10 torr. The three spectra correspond to values of rf amplitude appropriate to eject transient intermediates with lifetimes longer than (a) 60 ps, (b) 80 ps, and (c) 1 70 ps. A partial pressure of CH4 of 1.0 x 10 torr was also present to thermalize ions. The peak at m/z 125 is a secondary reaction product of m/z 85.

Traditional Sequence of Differently Bonded Intermediates. Organic chemists have traditionally considered a reaction mechanism, in its most primitive form, to consist of a sequence of differently bonded intermediates on the path between starting materials and products. In these terms, a mechanism may be considered understood once these chemically distinct species have been correctly identified. For purposes of understanding reaction rates and stereochemistry, it is necessary to expand this set of metastable reaction intermediates to include transition structures at the saddle points between intermediates on a potential energy surface. For photochemistry one must also consider transitions between potential energy surfaces. 

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