Protonation bimolecular reaction

It is possible to detemiine the equilibrium constant, K, for the bimolecular reaction involving gas-phase ions and neutral molecules in the ion source of a mass spectrometer [18]. These measurements have generally focused on tln-ee properties, proton affinity (or gas-phase basicity) [19, 20], gas-phase acidity [H] and solvation enthalpies (and free energies) [22, 23] ... 

In an earlier section, measurements were described in which the equilibrium constant, K, for bimolecular reactions involving gas-phase ions and neutral molecules were detennined. Another method for detemiining the proton or other affinity of a molecule is the bracketing method [ ]. The principle of this approach is quite straightforward. Let us again take the case of a proton affinity detemiination as an example. In a 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... 

Bimolecular reactions with paramagnetic species, heavy atoms, some molecules, compounds, or quantum dots refer to the first group (1). The second group (2) includes electron transfer reactions, exciplex and excimer formations, and proton transfer. To the last group (3), we ascribe the reactions, in which quenching of fluorescence occurs due to radiative and nonradiative transfer of excitation energy from the fluorescent donor to another particle - energy acceptor. 

Reaction (I) is a bimolecular reaction involving a protonic acid on the olefin. The proton attacks the monomer and adds to a carbon atom that has the maximum electron density. 

Intramolecular general base catalysed reactions (Section II, Tables E-G) present less difficulty. A classification similar to that of Table I is used, but since the electrophilic centre of interest is always a proton substantial differences between different general bases are not expected. This section (unlike Section I, which contains exclusively unimolecular reactions) contains mostly bimolecular reactions (e.g. the hydrolysis of aspirin [4]). Where these are hydrolysis reactions, calculation of the EM still involves comparison of a first order with a second order rate constant, because the order with respect to solvent is not measurable. The intermolecular processes involved are in fact termolecular reactions (e.g. [5]), and in those cases where solvent is not involved directly in the reaction, as in the general base catalysed aminolysis of esters, the calculation of the EM requires the comparison of second and third order rate constants. 

The advent of methods for determining proton affinities by studying bimolecular reactions in the gas phase has provided a wide range of interesting thermochemical data. 

These bimolecular reactions have provided accurate proton affinities (PAs) for many amines165,166. In addition, cation affinities are accessible, usually by combining the enthalpy of formation (AH[) of cationic species derived from PA measurements with similar data for the bare cation. Thus, the knowledge that the PA of CH3NH2 is 896166 kJmol-1 sets A//f(CH3NH3 + ) = 611 kJmor1. Since A//f(CH3+) = 1092 kJmol-1 and A//f(NH3) = —46 kJmol-1 9, the methyl cation affinity of NH3 may be deduced to be 1092 — 46 — 611 = 435 kJ mol-1. 

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. 

Pathway b is the specific base catalyzed (HO -catalyzed) hydrolysis. This bimolecular 5n2 reaction leads to the alcohol and nitrate. A peculiar pathway is carbonyl elimination (Fig. 9.1,c). This bimolecular reaction is catalyzed by strong bases and produces a dismutation of the two moieties, the organic group being oxidized to a carbonyl compound and nitrate being reduced to nitrite. Note that proton-catalyzed hydrolysis does not appear in Fig. 9.1 since this mechanism either does not occur or is negligible. 

A common simplification arises when the bimolecular step in (1.153) equilibrates rapidly compared with the unimolecular step (it may, for example, be a proton-base reaction). This means that the change in concentrations of A, B, and C due to the first process in (1.153) will have occurred before D even starts to change. The relaxation time t, associated with it will therefore be the same as if it were a separated equilibrium ... 

The distribution of isomers formed as a result of this reaction tends to be higher in OX at the expense of PX, so catalysis through this route is less desirable from an industrial perspective. Comparisons of the monomolecular versus bimolecular reaction have been made, providing insight into the properties of zeolitic catalysts that favor one route over the other [64, 65]. Mechanistic aspects in MOR and TON structure zeolites have been evaluated using ah initio calculations, which suggest that the initiation step involves a defect site rather than an acidic proton [66]. It is... 

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

The formation of ether is a nucleophilic bimolecular reaction (S 2) involving the attack of alcohol molecule on a protonated alcohol, as indicated below ... 

The first step is a bimolecular reaction leading to the formation of a hydrogen bond the second step is the breaking of the hydrogen bond such that the protonated species H B+ is formed the third step is the dissociation reaction to form the products. In aqueous solutions, the bimolecular reaction proceeds much faster than would be predicted from gas phase kinetic studies, and this underscores the complexity of proton transfer in solvents with extensive hydrogen-bonding networks capable of creating parallel pathways for the first step. In their au-... 

The hydroxymethylphosphonium ion first formed changes into mono-hydroxymethylphosphine by releasing a proton. This phosphine reacts further in the same way as phosphine itself until finally the quarternary phosphonium ion is formed. For a bimolecular reaction mechanism, the first stage must be assumed to be the formation of a carbonium ion from the aldehyde molecule and a proton. This ion then reacts with phosphine. 

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 results presented below do indeed show that peroxynitrite is produced in high yields in 1 ps. The prompt formation and subsequent protonation of peroxynitrite offers a unique opportunity for studying bimolecular reactions under conditions where secondary reaction can safely be ruled out. In this work we measure the bimolecular reaction kinetics of ONOCT+ FT1" —y HONOO as a function of [it] and find it to be in excellent agreement with seminal theoretical work by Smoluchowsky. 

The nearly constant peroxynitrite concentration observed in neutral solution changes dramatically when [Ft] of the solution is increased. Fig. 2 compares the transient absorption of aqueous nitrate at [Ft ] = 10-7 M and [Ft] = 0.140 M. The peroxynitrite concentration drops rapidly as protonation leads to the formation of peronitrous acid (peroxynitrous acids absorbs relatively weakly around 240 nm and is not observable in Fig. 2.). In Fig. 3 the peroxynitrite concentration is represented by the transient absorption at 310 nm as a function of [it]. As expected the formation of peroxynitrous acid increases with the concentration of protons. The protonation of peroxynitrite can be viewed as a prototypical diffusion limited bimolecular reaction and thus constitutes an excellent test bed for diffusion models. 

Thus it is possible to study the hydrolysis reactions of esters under conditions where the substrate is completely protonated. The properties of the protonated ester, however, are more conveniently examined using more strongly acidic media, in the absence of water, where bimolecular reactions are reduced to insignificance. At sufficiently low temperatures under these conditions the rates of exchange of the added protons are slow, and the detailed structures of protonated carboxylic acids and esters can be investigated, particularly by proton nmr techniques. 

Dissociation Proton transfer Bimolecular reactions Organometallic reactions Isomerization reactions Abstract reactions Elimination reactions... 

Coherent dissociation Geminate recombination Dephasing Proton transfer Electron transfer Vibrational relaxation 8arrierless reactions Bimolecular reactions Ionic reactions Solvation dynamics Friction dynamics Polarization (kerr)... 

Schechter 55) proposed that the catalytic effect of hydroxyl groups on the epoxide-amine addition reaction involved a termolecular activated complex formed in the concerted reaction of amine, epoxide and hydroxyl. Smith 57) suggested a modified mechanism in which the same activated complex is formed in a bimolecular reaction between an adduct formed from epoxide (E) and the proton donor (HX), and the amine ... 

A third type of bimolecular reaction, summarized by equation (24), is that of excited state proton transfer where Q is now a proton acceptor. This type of process was proposed in the case of the OH" and CO32- quenching of emission from [RhCl(NH3)5]2+.48... 

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