Proton transfer unimolecular

As for the acetyl phosphate monoanion, a metaphosphate mechanism has also been proposed 78) for the carbamoyl phosphate monoanion 119. Once again, an intramolecular proton transfer to the carbonyl group is feasible. The dianion likewise decomposes in a unimolecular reaction but not with spontaneous formation of POf as does the acetyl phosphate dianion, but to HPOj and cyanic acid. Support for this mechanism comes from isotopic labeling proof of C—O bond cleavage and from the formation of carbamoyl azide in the presence of azide ions. 

This result means that at any rate for bulk isobutene at -78 °C the DP is governed by the unimolecular proton transfer to the monomer. 

When this probability is equal to 1 (uniform concentration), the reaction is of pseudo-first order. This is the case, for example, in photoinduced proton transfer in aqueous solutions from an excited acid M (=AH ) (see Section 4.5) M is always within the encounter distance with a water molecule acting as a proton acceptor, and thus proton transfer occurs effectively according to a unimolecular process. This is also the case of photoinduced electron transfer in aniline or its derivatives as solvents an excited acceptor is always in the vicinity of an aniline molecule as an electron donor. In both cases, the excited-state reaction occurs under non-diffusive conditions and is of pseudo-first order. 

Not all ionization methods rely on such strictly unimolecular conditions 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. Furthermore, proton transfer, and thus the relative proton affinities of the reactants, play an important role in many ion-neutral complex-mediated reactions (Chap. 6.12). 

Recently, some attempts were nndertaken to uncover the intimate mechanism of cation-radical deprotonation. Thns, the reaction of the 9-methyl-lO-phenylanthracene cation-radical with 2,6-Intidine (a base) was stndied (Ln et al. 2001). The reaction proceeds through two steps that involve the intermediary formation of a cation-radical/base complex before unimolecular proton transfer and separation of prodncts. Based on the value of the kinetic isotope effect observed, it was concluded that extensive proton tnnneling is involved in the proton-transfer reaction. The assumed structure of the intermediate complex involves n bonding between the unshared electron pair on nitrogen of the Intidine base with the electron-deficient n system of the cation-radical. Nonclassical cation-radicals wonld also be interesting reactants for snch a reaction. The cation-radical of the nonclassical natnre are known see Ikeda et al. (2005) and references cited therein. 

Another important family of elimination reactions has as the common mechanistic feature cyclic transition states in which an intramolecular proton transfer accompanies elimination to form a new carbon-carbon double bond. Scheme 6.16 depicts examples of the most important of these reaction types. These reactions are thermally activated unimolecular reactions that normally do not involve acidic or basic catalysts. There is, however, a wide variation in the temperature at which elimination proceeds at a convenient rate. The cyclic transition states dictate that elimination occurs with syn stereochemistry. At least in a formal sense, all the reactions can proceed by a concerted mechanism. The reactions, as a group, are referred to as thermal syn eliminations. 

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). 

Trajectory calculations for proton transfer and ionization in water cluster,112 116 isomerization,117 and various types of unimolecular reactions6,118 128 have been carried out, and the analyses on time course of the reaction, product ratio, and product energy distribution were reported. 

Activation energies for unimolecular 1,3-hydrogen shifts connecting ketones and enols are prohibitive, so that thermodynamically unstable enols can survive indefinitely in the gas phase or in dry, aprotic solvents. Ketones are weak carbon acids and oxygen bases enols are oxygen acids and carbon bases. In aqueous solution, keto-enol tautomerization proceeds by proton transfer involving solvent water. In the absence of buffers, three reaction pathways compete, as shown in Scheme 2. 

In the present section some applications of the previously outlined theoretical framework, recently reported in literature [24,25], will be illustrated. In particular we will focus our attention on two benchmark reactions of computational-theoretical chemistry, namely the carbon monoxide (CO) binding-unbinding reaction in myoglobin (Mb) and the unimolecular tautomeric proton transfer in aqueous malonaldehyde. 

Steps 1,3, and 5 cannot be slow as they are just proton transfers between oxygen atoms (Chapter 13). That leaves only steps 2 and 4 as possible rate-determining steps. The bimolecular addition of the weak nucleophile water to the low concentration of protonated ester (step 2) is the most attractive candidate, as step 4—the unimolecular loss of ethanol and re-formation of the carbonyl group—should be fast. What p value would be expected for the reaction if step 2 were the rate-determining step It would be made up of two parts. There would be an equilibrium p value for the protonation and a reaction p value for the addition of water. Step 1 involves electrons flowing out of the molecule and step 2 involves electrons flowing in so the p values for these two steps would have opposite charges. We know that the p value for step 2 would be about +2.5 and a value of about -2.5 for the equilibrium protonation is reasonable. This is indeed the explanation step 2 is the rate-deter-... 

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. 

If one of the ions formed by proton-transfer from acid to base is resonance-stabilised, the hydrogen-bonded complex first formed may convert by a unimolecular process involving the motion of a proton along the hydrogen bond to form a hydrogen-bonded ion-pair complex. The complete reaction scheme is thus... 

X 10 sec . For reaction (2) a two-step mechanism with proton transfer through hydrogen bonding has been preferred, although the unimolecular alternative has not been excluded. 

I have also studied unimolecular reactions. There are also atom transfer reactions and proton transfer reactions. 

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