Metastable complex

Radiative association is the combination of ion and molecule to form a metastable complex that is then stabilized by emission of a photon ... 

Four microscopic rate processes are considered bimolecular formation of metastable complexes with rate constant unimolecular redissociation of complexes to... 

We turn to the chemical behavior of cycloalkane holes. Several classes of reactions were observed for these holes (1) fast irreversible electron-transfer reactions with solutes that have low adiabatic IPs (ionization potentials) and vertical IPs (such as polycyclic aromatic molecules) (2) slow reversible electron-transfer reactions with solutes that have low adiabatic and high vertical IPs (3) fast proton-transfer reactions (4) slow proton-transfer reactions that occur through the formation of metastable complexes and (5) very slow reactions with high-IP, low-PA (proton affinity) solutes. 

Class (3) reactions include proton-transfer reactions of solvent holes in cyclohexane and methylcyclohexane [71,74,75]. The corresponding rate constants are 10-30% of the fastest class (1) reactions. Class (4) reactions include proton-transfer reactions in trans-decalin and cis-trans decalin mixtures [77]. Proton transfer from the decalin hole to aliphatic alcohol results in the formation of a C-centered decalyl radical. The proton affinity of this radical is comparable to that of a single alcohol molecule. However, it is less than the proton affinity of an alcohol dimer. Consequently, a complex of the radical cation and alcohol monomer is relatively stable toward proton transfer when such a complex encounters a second alcohol molecule, the radical cation rapidly deprotonates. Metastable complexes with natural lifetimes between 24 nsec (2-propanol) and 90 nsec (tert-butanol) were observed in liquid cis- and tra 5-decalins at 25°C [77]. The rate of the complexation is one-half of that for class (1) reactions the overall decay rate is limited by slow proton transfer in the 1 1 complex. The rate constant of unimolecular decay is (5-10) x 10 sec for primary alcohols, bimolecular decay via proton transfer to the alcohol dimer prevails. Only for secondary and ternary alcohols is the equilibrium reached sufficiently slowly that it can be observed at 25 °C on a time scale of > 10 nsec. There is a striking similarity between the formation of alcohol complexes with the solvent holes (in decalins) and solvent anions (in sc CO2). 

The solvated C02 anion radical has been observed both in the gas phase and in condensed matter (Holroyd et al. 1997) and has been well characterized by ESR spectroscopy (Knight et al. 1996). Being solvated, C02 anion radicals form complexes that yield quasi-free electrons upon photoexcitation. Gas-phase studies (Saeki and others 1999) and ab initio calculations (Tsukuda et al. 1999) indicate that static ion-dipole interactions stabilize the [(C02)n m(R0H)m] type of small clusters. In supercritical carbon dioxide, monomers and dimers of water, acetonitrile, and alcohols also form metastable complexes (Shkrob Sauer 2001a,b). Such complexation should be taken into account in studies of electron-transfer kinetics in reactions with the participation of C02. ... 

Thus, one of the possible channels to the final products of the reaction of silyl-type radicals with nitrous oxide (oxy radical and molecular nitrogen) is the formation of a metastable complex and its further isomerization and decomposition. 

Table 2. Losses of H20 and HDO from metastable complexes of 2-butanols with Mn+ cations in the gas phase [21]a,b...

It seema quite reasonable to suppose that this is the way in which reaction 3 occurs, since there is independent evidence for the metastable complex O—N—0—O. Alterna-... 

Figure 34. Square scheme illustrating the response of the catenate Cu-lS" " to an electrochemical signal. Oxidation and reduction generates metastable complexes which rearranges to adopt the best coordination mode for the new oxidation state of the copper center. Cu is a black circle and Cu is an open circle. (For the notation used here, see Figure 33.)...

Ohman, L.-O. (1988). Equilibrium and structural studies of silicon(IV) and alu-minum(III) in aqueous solution, 17 Stable and metastable complexes in the system H -AP+—citric acid. Inorg. Chem. 27, 2565-2570. 

At first the rules concentrated on product momenta. Recently it has become clear that the ability of the metastable complex to excite product angular momentum channels can also be important. There is a growing body of information, not discussed in this paper, on the propensity for intramolecular V-V transfer to govern relaxation rates. The goal of these studies, then, is to isolate as much as possible the effects of the intramolecular potential, vibrational symmetry and product channel availability on the overall rates. Maturally, all of these effects are correlated with each other. By judicious choice of examples, however, the relative importance of individual effects can be demonstrated. 

Recently, it was demonstrated that the scavenging of trans-decalin holes by some alcohols proceeds through the formation of a metastable complex [26]... 

Oxygen is known to form metastable complexes with olefins and some aromatic hydrocarbons which are capable of absorbing sunlight UV wavelengths, unlike the parent hydrocarbons (Khalil and Kasha, 1978). In some cases, the existence of these complexes may lead to reactions of the substrate hydrocarbons or formation of singlet oxygen (see Section 4.A.3). 

These metastable complexes are capable of passing into the composition of both the solution and the mineral. It plays an important role in reversible and irreversible processes of nucleation, coprecipitation, precipitation, solution, etc. 

In nonpolar liquids, both species display a strong tendency to form metastable complexes with polar molecules, such as alcohols and nitriles, in which the charge is electrostatically bound to the solute dipole. With respect to this propensity, the high-mobility ions are similar to solvated electrons in saturated hydrocarbons. Even in polar solvents, solvent anions (e.g., the dimer anion in acetonitrile) are protonated only after formation of a complex with the alcohol monomer the transfer occurs when a second alcohol molecule encounters the complex [30]. 

The oxidation of aminopolycarboxylic acids by Ce(IV) produces numerous species which can be used to infer mechanistic pathways. A more well-defined aspect of these systems is revealed in the initial steps of this class of reactions. Prior to the oxidation steps, Ce(IV) forms complexes with these compounds as demonstrated in the studies of Trubacheva and Pechurova (1981) and Hanna and Moehlenkamp (1983). The experimental constraints in the former study with ethylenediaminedisuccinic acid as the substrate resulted in a metastable complex formulated as Ce(OH)(EDDS) with log X,lab = 16.46. The subsequent redox reaction is extremely slow and exhibits a complex hydrogen ion concentration dependence. The latter system, wherein the substrate is A -benzyliminodiacetic acid (BIDA), was described in terms of the sequential reaction ... 

In Feshbach resonance system, the reaction coordinate couples with the other freedom degrees. The system can form a quasi-bound or metastable complex, which is Feshbach resonance, also known as dynamic resonance. Even in pure repulsion potential energy surface, this resonance state can be formed. Quasi-bound complex can react with other adiabatic potential surface, producing non-adiabatic coupling dissociation product (Fig. 1.3) [11]. In F -h H2reaction, Feshbach resonance results forward scattering, producing HF (v = 2), see Chap. 3 of this thesis. 

Many possible sources can be imagined for the fluctuations including (1) metastable excited states, such as photoionization with transient electron trapping, (2) metastable complex formation, such as association/dissociation with a nearby molecule, (3) isomerization, including small changes in the nuclear coordinates that result in a spectral shift, (4) changes in the molecular configuration of the environment and a concomitant spectral shift, and (5) reorientation of the optically active molecule. 

Similarly, one has equation (45) for the average specific rate constant k(E,J) for unimolecular decay of metastable complexes ... 

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