Bimolecular Proton Transfer Reactions

The borazine cation formed by irradiation of borazine reacts with a second molecule in a bimolecular proton transfer reaction ... 

The origin of the sequence corresponding to protonated methanol peaks is a rapid intracluster proton transfer reaction following ionization of the neutral clusters. This reaction has a well-known bimolecular counterpart that proceeds at near collision rate 104... 

The pK of tyrosine explains the absence of measurable excited-state proton transfer in water. The pK is the negative logarithm of the ratio of the deprotonation and the bimolecular reprotonation rates. Since reprotonation is diffusion-controlled, this rate will be the same for tyrosine and 2-naphthol. The difference of nearly two in their respective pK values means that the excited-state deprotonation rate of tyrosine is nearly two orders of magnitude slower than that of 2-naphthol.(26) This means that the rate of excited-state proton transfer by tyrosine to water is on the order of 105s 1. With a fluorescence lifetime near 3 ns for tyrosine, the combined rates for radiative and nonradiative processes approach 109s-1. Thus, the proton transfer reaction is too slow to compete effectively with the other deactivation pathways. 

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). 

In eqn. (8), the acid catalyzed reactions of HS and S2 are formulated as first-order decompositions of H2S and HS- (uncatalyzed). Consequently, a distinction between bimolecular proton transfer to a substrate and unimolecular decomposition of the conjugate acid of the substrate is not possible solely on the basis of the experimental rate equation. For both mechanisms, that represented by eqn. (7) as well as that represented by eqn. (8), the same equation is obeyed for the dependence of k on the hydrogen ion concentration, viz. 

The mechanism discussed above for the deprotonation of alkylaromatic radical cations, involving a bimolecular reaction between the radical cation and the base (B), leading to a carbon centered neutral radical and the conjugated acid of the base (BH" ") as described in Scheme 28, has been recently questioned by Parker who provided evidence for an alternative mechanism in proton-transfer reactions between methylanthracene radical cations and pyridine bases [154] this involved reversible covalent adduct formation between the radical cation and the base followed by elimination of BH+ (Scheme 36). 

The interfacial proton transfer reactions responsible for both the B2 and the B2 component are bimolecular reactions. According to the law of mass action, the relaxation of the photosignal is dependent on three factors ... 

The novel phospha-alkyne system (303) is formed in the reaction of lithium bis(trimethylsilyl)phosphide with 0,0 -diethyldithiocarbamate. C-chlorophosphaethyne, ClCsp, has been generated by the pyrolysis of trichloromethyldichlorophosphine over granulated zinc at 550°C, and characterised by infrared spectroscopy. A bimolecular proton transfer mechanism has been suggested for the base-promoted isomerism of alkynyl- and alkenyl-phosphines to the phospha-alkynes (304). The potential of phospha-alkynes as novel building blocks in heterocyclic chemistry has been reviewed. New examples of phospha-alkyne-derived... 

Cross section measurements of the proton transfer reaction of PHJ with Ca atoms have been carried out for ion kinetic energies E between 1 to 6 eV. The cross section drop sharply with increasing E, indicating exothermic behavior. The bimolecular rate constant is k= 13.9x10" ° at E=1 eV and 7.3x10" ° cm molecule" S" at E = 2 eV [54]. 

A theoretical study using the density functional theory (DFT) method has been performed to rationalize the mechanism of the reactions between 1,3-dialkynes and hydroxylamine or hydrazine for the formation of 3,5-disubstituted isoxazoles or pyrazoles, respectively The computational results support a bimolecular proton transfer as the rate-determining step providing valuable clues for the use of Bronsted acid/base catalysts to promote the cyclization reaction (14OBC7503). 

The presence of two metal centres opens up the possibility to smdy electronic coupling, and for multielectron photoredox processes to take place, whilst relatively long lifetimes of the excited states (see below) makes possible bimolecular reactions in solution. Accordingly, quadruply-bonded di-Re complexes have been reported to engage in bimolecular electron-transfer reactions, whereas the di-Mo and di-W complexes participate in oxidative addition and two-electron redox reactions. For example, UV irradiation of phosphate-supported M2 dinuclear complexes under acidic conditions leads to one- or two-electron oxidation of the metal-metal core accompanied by production of H2 gas by reduction of two protons. 

Scheme 15.9 is once again similar to Scheme 15.6 except that the protonation back reaction is bimolecular. Thus, the two-state formalism is applicable with some changes concerning the determination of the fourth unknown. Because the lifetime of N in the absence of reaction, tn = 1 / fn> cannot be reliably measured with N even at very low pH values, it has to be obtained with a parent compound, with which the proton transfer reaction does not occur (in this case, 2-methoxy-naphthalene). However, the implicit assumption of the procedure, that the lifetime measured with the methoxylated compound would be equal to tn, may be dangerous with the strongly hydrogen bonding solvent water (the most common solvent for proton transfer). 

The mechanism of the oxidation of 2-mercapto-5-methyl-l,3,4-thiadiazole (McMT) to its disulfide dimer and its subsequent reduction was examined with a combined approach employing experimental data and digital simulation. To elucidate the influence of proton transfers on these redox processes, special attention was paid to the influence of various bases and proton donors on both the oxidation and reduction reactions. In particular, McMT oxidation is facilitated by a rapid bimolecular proton transfer from McMT to weak bases such as pyridine that produces McMT , the thiol-ate form, which is then oxidized. There is no such facilitation in the presence of the sterically hindered base 2,6-di-r-butylpyridine, suggesting that the facilitation occurs through the formation of a discrete hydrogen-bonded complex/ ... 

Not all ionization methods rely on unimolecular conditions as strictly as El does. Chemical ionization (Cl, Chap. 7), for example, makes use of reactive collisions between ions generated from a reactant gas and the neutral analyte to achieve its ionization by some bimolecular process such as proton transfer. The question which reactant ion can protonate a given analyte can be answered from gas phase basicity (GB) or proton affinity (PA) data. Proton transfer, and thus the relative proton affinities of the reactants, also play an important role in many ion-neutral conplex-mediated reactions (Chap. 6.12). In the last decade, proton transfer reaction (PTR) MS has emerged as a tool for analyzing volatile organic compounds (VOCs) in air. Therefore, PTR-MS is interesting for analytical work concerning environmental issues and in occupational health and safety (Chap. 7.3). 

This is of particular interest for reactions along a RP without a SP, i.e. "reactions on attractive PES", e.g. potentials of unimolecular bond fission processes and the reverse bimolecular recombinations, ion molecule reactions or a large number of proton transfer reactions in... 

In contrast to the bimolecular single proton transfer reactions, large barriers may be calculated when studying double proton transfer reactions, for instance in 4- or 6-membered rings. Here, the analysis of the experimental data in gaseous or inert media... 

Rate constants were determined for a number of bimolecular photoreactions of hydroxy- and aminoaromatic compounds and heteroaromatic compounds with carboxylate anions and organic acids, respectively, in various micelles. Such bimolecular photoreactions were found to improve the efficiency of forward proton transfer reactions and to reduce reverse recombination of the primary products ( up to 2 orders of magnitude ) comparatively to aqueous solutions. 

Proton transfer reactions are often depicted as simple bimolecular processes in which a proton is transferred directly from the acid to the base. However, with the development of proton magnetic resonance techniques for measuring the rates of fast reactions, it has become clear that many proton transfer reactions in hydroxylic solvents actually proceed with participation of one or two solvent molecules. The solvent molecule then acts as a bifunctional catalyst, that is, both as a proton acceptor and a proton donor. Thus, termolecular proton transfer from an acid HA to a base B with participation of a water molecule is shown in equation 1. 

Proton transfer with solvent participation always competes with direct bimolecular proton transfer, at least in principle, and comparison of the two rates is of theoretical interest. Unfortunately, data are available only in rare instances. The most complete series is that for the methylamines in water second-order rate constants for reaction with (equation 3) and without (equation 4) water participation are compared in Table 3 (p. Ill)... 

Thus, when bimolecular proton transfer is relatively slow, we surmise that the displacement of water molecules from the solvation shells requires an activation energy and is the rate-determining step for reaction. The fact that proton transfer with solvent participation remains fast when k2 becomes small, suggests that the proton-transfer step is inherently fast and the fact that 2 is small also for the reaction of (CH3)3N with NH4 suggests that steric hindrance is not the primary factor [32]. 

There is other evidence that the exchange of water molecules between the site in A W B and bulk solvent is slow compared to both proton transfer within the complex and the separation of A from B. Regarding the former, the fact that so many proton-transfer reactions in which AG° is negative are diffusion-controlled proves that the proton-transfer step is fast compared to the dissociation of the complex. Regarding the latter, if the departure of a water molecule from A W B were fast compared to the dissociation of the complex, one would expect to find more examples of rapid direct bimolecular proton transfer without solvent participation. 

For acid dissociation in water, the two mechanisms are readily distinguished by means of HOD—D2O solvent isotope effects if the reverse reaction (rate constant or kL ) is diffusion controlled. For bimolecular proton transfer according to equation 18 the resulting lyonium ion is HOD2, while for proton transfer with water participation equation 19 it is DsO. ... 

Two main topics are usually distinguished the influence of the catalyst in proton-transfer reactions showing general acid or general base catalysis (the Bronsted equation) and the nucleophilicity of reagents in bimolecular reactions. These topics, however, are not totally unconnected. 

In aqueous solution is of the order oflO M" s for molecules and slightly more or less for ions of opposite or the same charge. This value constitutes the limit for rate coefficients of bimolecular processes, which are then diffusion-controlled and have an activation energy of 3 to 5 kcal mole Many proton-transfer reactions are of this type. 

Proton transfer reactions occur, as a rule, vay promptly. Special methods were developed to measure rates of these reactions meAods of temperature jump, pressure drop, electric field pulse, and dielectric absoption ultrasonic method several electrochemical methods, and method of absorption line broadening of protons and in NMR spectra (see Chapter 8). These m ods allow the measurement of rate constants of bimolecular reacticms in the 10 - lO" l/(mol-s) interval. Results of measurements by different methods s netimes diverge dramatically. For example, ftx the reaction of H30 and CH3COO" the rate constant values obtained by different electrochemical methods lie in an interval of (l-9)-10 l/(mol-s) (H20,298 K). 

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