Termolecular initiation

The fact that Iq depends on the nature of the carrier gas indicates that the chemiluminescent reactions take place in three body processes. The whole range of phenomena may then be explained by postulating an initial termolecular combination to an excited state of CO2, followed at a later stage by the emission of radiation or by collisional quenching to form CO2 in the ground electronic state, viz. 

F. R. Mayo (private communication) has found evidence that thermal polymerization of styrene may actually be of a higher order than second, i.e., about five-halves order. This would suggest a termolecular initiation step. Generation of a pair of monoradicals in this manner, i.e., from three monomer molecules, would be acceptable from the standpoint of energy considerations. 

The stereochemistry of addition depends on the details of the mechanism. The addition can proceed through an ion pair intermediate formed by an initial protonation step. Most alkenes, however, react via a complex that involves the alkene, hydrogen halide, and a third species that delivers the nucleophilic halide. This termolecular mechanism is generally pictured as a nucleophilic attack on an alkene-hydrogen halide complex. This mechanism bypasses a discrete carbocation and exhibits a preference for anti addition. 

Like other metal reactions studied previously in our laboratory, H2 elimination is initiated by insertion into one of the C-H bonds forming HMC3H5. The reaction rate constant for Y + cyclopropane was found to be very small at room temperature, 0.7 x 10 12 cm3 s 1, and it was suggested that the reaction most likely involved termolecular stabilization of C-H or C-C insertion complexes, rather than molecular elimination.22 By analogy with other systems studied, the dynamically most favorable route to H2 loss in this case is likely via H atom migration to the Y-H moiety, with concerted... 

A recent discovery that RNA will act as a self-catalyst, called a ribozyme, leads to a simple three-step model for self-replication - this might include a surface. In the model (Figure 8.18), the template molecule T is self-complementary and is able to act as an autocatalyst. In the first step, it reversibly binds with its constituents A and B, forming the termolecular complex M. The termolecular complex undergoes irreversible polymerisation and becomes the duplex molecule D. Reversible dissociation of D gives two template molecules T, which can initiate new replication. The model preserves the order of the moieties on the template (the direction of the arrow) and the backbone, which may be on the surface... 

There is also some evidence that the principal process initiating the stable chains in the region CD is the direct termolecular reaction... 

The reaction involving the three molecules in a single termolecular event is improbable, so the reaction via a pre-equilibrium is a reasonable initial hypothesis. Two possibilities are given in Scheme 1.3 where Xand K are equilibrium constants, and k and k are mechanistic second-order rate constants of elementary steps (see below). 

Interaction of RNA III with RNA II is initiated by the formation of a so-called kissing complex between single-stranded loops of both molecules, followed by propagation of the in-termolecular RNA helix (41). The kissing interaction involves base pairing between loops 111 and IV from RNA III with loops... 

Rebek and his co-workers have shown that replication - autocatalysis based on molecular recognition - best accommodates the facts observed in the reaction of 42 with 43, and that under the published conditions 44 is responsible for the autocatalysis. The results indicated template-catalyzed replication as the source of autocatalysis, where recognition surfaces and functional groups interact to form a productive termolecular complex. The mechanism demands that catalysis would be absent with esters that lack hydrogen-bonding sites. One complication of this system is that the initial product of this bimolecular preassociative mechanism is postulated to be a cw-amide, which isomerized to the frani-amide, the active form of template. This appears to be one major background reaction for product formation (Scheme 14). 

The phase-space model has been applied to triple collisions by F. T. Smith (1969) in a detailed study of termolecular reaction rates. He classified 3-body entry or exit channels into two classes, of pure and indirect triple collisions, and introduced kinematic variables appropriate to each class. These variables were then used to develop a statistical theory of break-up cross-sections. A recent contribution (Rebick and Levine, 1973) has dealt with collision induced dissociation (C1D) along similar lines. Two mechanisms were distinguished in the process A + BC->A + B + C. Direct CID, where the three particles are unbound in the final state, and indirect CID, where two of the particles emerge in a quasi-bound state. Furthermore, a distinction was made in indirect CID, depending on whether the quasi-bound pair is the initial BC or not. Enumeration of the product (three-body) states was made in terms of quantum numbers appropriate to three free bodies (see e.g. Delves and Phillips, 1969) the vibrational quantum number of a product... 

Employing the quantum mechanical density matrix formalism it is possible to take into account the whole reaction pathway of Fig. 21.12 where both coherent and incoherent reaction pathways are present in the case of a PHIP experiment and convert the PHIP experiment into a diagnostic tool for all stages of a hydrogenation reaction. Such an analysis was performed in Ref [62]. In these calculations the initial condition was that at the start of the reaction all molecules are in the P-H2 state of site C, i.e., a pure singlet spin state, represented as a circle in Fig. 21.12, which shows the different possible reaction pathways. Moreover, all in-termolecular exchange reactions were treated as one-sided reactions, i.e. the rates of the back reactions were set to zero. 

The compensation effect is observed log k0 = 8.6 + 0.352 a at 180°C. A termolecular initiation process was suggested for the oxidation of a-naphthol in rc-decane [218], but under conditions when a-naphthol was consumed by reaction with peroxy radicals as well as by reaction with 02. A bimolecular mechanism for this reaction in benzene and cyclohexane was established recently with the same rate coefficients for both solvents [217]. 

So the initial rate of intramolecular reaction within the termolecular S - T - S complex is ... 

Part of the difficulty in interpretation may arise from the fact that in-termolecular interactions between Py-PEG-Py will also contribute to the observed Id/Im- This point was explicitly examined in the initial work on the macromolecular complexation (32, 41). In this work, it was assumed that the intermolecular contribution was simply that resulting when the intramolecular portion was subtracted from the Iq/Im value of the fully labeled PEG sample, but as Figure 2 shows, normalization to the initial solution value of I q/I before addition of the poly(carboxylic acid) did not provide much simplification. With reference to the data for the Py-PEG-Py(9200) mixed with the 40% neutralized PMAA(9500) for which no complexation occurs, we see that there are dramatic overshoots at low poly(carboxylic acid) content and plateau values at high acid content that lie either above or below the neutralized data. 

Here, in spite of the chain sequence of steps, the reaction is apparent first-order. However, it can be seen that the apparent first-order rate constant is a combination of the rate constants of the individual elementary steps. A comparison of this example with the contents of Table 1.3 shows that the Rice-Herzfeld mechanism corresponds in this case to Two Active Centers with Second-Order Cross-Termination Chain. The apparent first-order behavior here is a consequence of the particular kinetics of the initiation and termination steps. It is not difficult to show that various combinations of unimolecular or bimolecular initiation with bimolecular or even termolecular termination can result in apparent orders that range from 0 to 2 (M.F.R. Mulcahy, Gas Kinetics, John Wiley, New York, 1973, pp. 87-92). 

When no initiators such as peroxides are initially added to a hydrocarbon mixture, an induction period is observed before oxygen consumption takes place. There are several hypotheses on what might happen during this induction-initiation stage (i) the direct reaction of triplet oxygen with singlet closed-shell molecules (a thermodynamically disfavored reaction), (ii) the R-H homolysis (a reaction that is very slow due to typical C-H bond dissociation energies of 80-110 kcal/mol), and (iii) the termolecular reaction... 

The suggested mechanism represents a fairly simple chain reaction, wherein the chains are initiated by thermal dissociation of bromine molecules into atoms. The chain-propagating steps involve the conversion of the reactant H2 into HBr by reaction with Br atoms and, since H atoms are produced in the process, the conversion of the reactant Br2 into HBr by reaction with H atoms. The destruction of HBr by H atoms in step 4, which is characterized by 4, accounts for the observed inhibition by the product of the reaction. Finally, the chains are terminated by the termolecular association of Br atoms to form Br2 molecules. 

X 10 and 8.9 x 10 for Nd and Yb , respectively. Both the above-described in-termolecular mechanism, as well as an intramolecular pathway in the ternary complex with aad which forms in solution, are responsible for the observation of NIR luminescence in these systems. Addition of water to the toluene solutions quenches the NIR luminescence, while it enhances the visible CL emission of the corresponding solution of Eu and Tb (Voloshin et al., 2000c). Neodymium and ytterbium tris(benzoyltrifluoroacetonates) display the same CL as tta complexes, although for Yb its intensity is about 2.5 times lower than for the tta chelate. On the other hand, almost no CL is detected for acetylacetonate complexes (Voloshin et al., 2000a). Thermal or photochemical decomposition of aad also triggers CL from [Pr(dpm)3] and Pr(fod)3], both in the visible (from the Pi, Po, and levels) and in the NIR at 850 nm ( D2 p2 transition) and 1100 nm ( D2 -> p4 transition). The excited chelate [Pr(fod)3] also initiates decomposition of aad by branched quantum chain reaction (Kazakov et al., 1998). 

If equations (4.2.25) and (4.2.26) are substituted for equations (4.2.11) and (4.2.15), respectively, in the mechanism described above, the net effect is to replace by [M] and 5 by [M] everywhere that they appear. Since these quantities appear as a ratio in the final rate expression, the third-body concentration will drop out and i/ 5 becomes identical to k /k y The necessity for the use of the third-body concentration thus is not obvious in kinetic studies of the thermal reaction. However, from studies of photochemical reaction between hydrogen and bromine, there is strong evidence that the termination reaction is termolecular. This fact and others based on studies of various atomic recombination reactions imply that the correct initiation and termination reactions are not reactions (4.2.11) and (4.2.15) but reactions (4.2.25) and (4.2.26) (29). 

The distinctive features of Warneck s photoionization technique are the pressure range covered (up to 0.2 Torr) and the direct measurement of ion residence times. The capability of working at high pressures makes possible the study of reactions with low rates, even termolecular association reactions. The residence time may be varied considerably and well-defined ion temperatures and drift velocities established at the higher pressures. The direct measurement of residence time eliminates certain errors which can occur in the calculation of this quantity—e.g., the electric field may be affected to an unknown extent by surface charges, space charge, contact potentials, and electric field penetration. The rate constant is directly determined from measured values of the ion residence time and of the initial and final concentrations of reactants or of products or of both. 

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