Proton dissociation scheme

Thus, we understand that, in the FSGO model, for some critical distance, the single Gaussian will jump from its symmetric position at the middle of the H-H bond to a dissymetric one represented below. Thus, the FSGO dissociation scheme corresponds to one electron pair on one of the proton (H )and no electron on the second proton (H+) H2 H- + H+... 

The proton dissociation constants, of two series of 3,7-bis(arylazo)-2,6-diphenyl-1 //-irnidazo[l,2-7]pyrazoles, in the ground state and the excited state were determined by the spectrophotometric method and utilizing the Forster energy cycle, respectively. These constants were correlated by the Hammett equation and the results of such correlations with spectral data indicated that both series of compounds exist in solution almost exclusively in the l//-bis-(arylazo) tautomeric form A <2002T2875> (Scheme 3). 

If only the acid/base proton dissociation of complex MOH is considered, Scheme 4 is simplified to include only steps (a) and (b) therein. According to this model, (I) is the primary deprotonation pathway in acidic medium, while (III) is of importance in basic media. On the other hand, direct proton transfer (II) can occur around neutral pH values. 

Protons are in general indispensable for the dismutation of superoxide (Eq. (4)). Also in the case of its dismutation catalyzed by a metal center, two protons are needed for the dissociation of the product (H2O2) from the metal center (Scheme 9). Therefore, a complex which can accept two protons upon reduction and release them upon oxidation is an excellent candidate for SOD activity. The studies on proton-coupled electron transfer in Fe- and Mn-SODs 48), demonstrated that the active site of MnSOD consists of more than one proton acceptor (Scheme 10). Since the assignment of species involved in proton transfer is extremely difficult in the case of enzymatic systems, relevant investigations on adequate model complexes could be of vast importance. H2dapsox coordinates to Fe(II) in its neutral form, whereas in the case of Fe(III) it coordinates in the dapsox form. Thus, oxidation and reduction of its iron complex is a proton-coupled electron transfer process 46), which as an energetically favorable... 

The majority of the molecular-scale information concerning the effects of structure and local chemistry on proton dissociation and separation in PEM fragments alluded to previously " were initially determined using HE theory and split valence local basis sets. Refinements to the equilibrium configurations were made using both Mailer-Plesset (MP) perturbation schemes and hybrid density functional theory (described below). 

The system dihydrophenazine-phenazine shows a combination of redox and proton dissociation equlibria in aqueous solution summarised in Scheme 6.8. Phe-... 

O refers to the uncharged phenolic group. Dissociation of the amino proton only produces an uncharged form MOH, represented hy OO, while dissociation of the phenolic proton gives a zwitterion +HMO , represented hy The completely dissociated form MO is represented as 0-. The entire dissociation scheme is given in Scheme 3.4. 

The rate of proton dissociation can be obtained, either by steady-state or time-resolved measurements. The reaction describing the proton dissociation from the excited molecule is summarized in Scheme I... 

To illustrate, Figure 1 shows the change in ion intensity for the reaction of 98% O-18-labeled methanol with isopropenyl acetate. The appearance of mlz 75 means that the oxygen from the alcohol is incorporated in the product ion—the precursor ion being the proto nated ester mJz 117. Yet, an abundance of the unlabeled ion mJz 73 exists that is clearly formed at a faster rate by a pathway that is different from that producing the O-18-labeled ion, mlz 75. The key to both processes is a step involving proton transfer to the vinylic carbon of the ester mlz 73 arises from dissociation of the C-protonated ester (Scheme III), and mlz 78 arises from the condensation of the C-protonated ester with methanol (Scheme IV). 

The hydride [MoH(CO)3Cp] forms a monocation which rapidly yields [ Mo(CO)3Cp 2] by proton dissociation (39). Analogous alkyl cations are intermediates in the oxidatively induced carbonyl insertion reactions of [MR(CO)3Cp] (377). Thus, [M(CH2-p-F-C6H4)(CO)3Cp](M = MoorW), LiCl, and Ce(IV) in methanol give methyl-p-fluorophenyl acetate (Scheme 23) the mechanism is discussed more fully in Section VII,E. 

Proton transfer is a fundamental process in both chemistry [1-3] and biology [4]. In particular, proton dissociation namely, proton transfer to solvent, from aromatic dye molecules in their excited electronic state [5] can be easily studied by virtue of their strong fluorescence signal [6]. The older fluorescence measurements did not possess time resolution It was only possible to obtain steady-state quantum yields under conditions of constant illumination [6]. The conventional interpretation of the experimental data assumed a chemical kinetic scheme, such as [3]... 

Time resolved measurements of ultrafast proton dissociation in the excited state are challenging these classical views [8-13]. Our picosecond fluorescence experiments have shown that the kinetic scheme, eq 1, does not accurately describe the kinetic data. Firstly, the R OH fluorescence decays asymptotically as a power law, rather than exponentially [10]. The area under the non-exponential tail contributes to the observed quantum yield. In addition, there is a marked salt effect on the elementary dissociation process ... 

The reactivity of the complex [(OC)4FeCOOH] in the gaseous phase confirms the earlier postulated mechanism of its decomposition. According to this mechanism, the first dissociation of a proton occurs [scheme (13.244)]. Therefore, the presence of bases, which promote dissociation of the proton, should facilitate the conversion of carbon monoxide. 

Typical ion/molecule reactions between anions and neutral molecules [7, 9,152-155] can be classified as displacement (Scheme 2.1, Eq. (2.5)), proton transfer (Scheme 2.1, Eq. (2.6)), charge exchange (Scheme 2.1, Eq. (2.7)) and association (Scheme 2.1, Eq. (2.8)). Among these, the displacement reaction has been studied extensively in the gas phase [156,157], and the prototypical example is an anionic Sn2 reaction studied by Brauman [156]. In addition, interactions between a neutral molecule and an electron involving electron capture [158] and dissociative electron capture [159], are also important types of ion/molecule reactions in the gas phase. A molecule M vhth a positive electron affinity can form a stable anion M by capturing a thermal electron. In the case of dissociative electron capture, capture of an electron by a compound MX leads to a repulsive state of MX, which dissociates to form M and X vhth excess internal and/or translational energy. 

In order to introduce three chelating chains on the lower rim of calix[4]arenes, two general methodologies have been developed for the selective synthesis of calix[4]arene monoalkyl ethers [22]. The synthesis of trihydroxamate siderophores was achieved through the sequence of reactions reported in Scheme 1. The convergence of the three chelating chains was established by and NMR [23]. The pKa values of compound 10 were evaluated in ethanol/water 9 1 the first proton dissociation constant... 

The increase in the value of the acidity constant can be predicted somewhat more quantitatively by interpreting the hybridisation of boron s orbitals. In the case of unbound boronic acid, boron has an sp trigonal planar geometry with an empty p orbital perpendicular to the plane of the molecule. It is with this unoccupied p orbital on boron that the nucleophilic oxygen lone pair on the approaching water molecule will mix. As the boron-oxygen interaction strengthens, concomitant proton dissociation occurs. By definition, the ease with which this proton is dissociated determines the value of the acidity constant (Scheme 10). 

A similar behavior is observed from competitive dissociations of protonated monoamides of maleic and fumaric acids which lead to the formation of [MH H2O] and [MH NH3] +, respectively. They are accompanied by the presence of NH, although the loss of water corresponds to the base peak from the Z stereochemistry but is of lower abundance from the E isomer. From fumarate monomethyl ester or monoamide, the major pathway for protonated molecule dissociation corresponds to the loss of XH as methanol or ammonia, respectively, which suggests that the modified carboxylic group is the preferred protonation site (Scheme 17.8). Consequently, the favorable loss of water from the Z isomers (not only for maleic acid, but also for the monoester and monoamide derivatives) indicates that the water loss rate constant, via 1,6-H" transfer, is much larger than that occurring from the E isomer which involves either 1,3-H" transfer (a symmetry forbidden process) or a multistep proton migration which is characterized by higher transition state level(s) (Scheme 17.8). 

The reversibility of aromatic diazotization in methanol may indicate that the intermediate corresponding to the diazohydroxide (3.9 in Scheme 3-36), i. e., the (Z)-or (is)-diazomethyl ether (Ar — N2 — OCH3), may be the cause of the reversibility. In contrast to the diazohydroxide this compound cannot be stabilized by deprotonation. It can be protonated and then dissociates into a diazonium ion and a methanol molecule. This reaction is relatively slow (Masoud and Ishak, 1988) and therefore the reverse reaction of the diazomethyl ether to the amine may be competitive. Similarly the reversibility of heteroaromatic amine diazotizations with a ring nitrogen in the a-position may be due to the stabilization of the intermediate (Z)-diazohydroxide, hydrogen-bonded to that ring nitrogen (Butler, 1975). However, this explanation is not yet supported by experimental data. 

A similar case is the catalysis of Gomberg-Bachmann arylations by A,A-diphenyl-hydroxylamine, which was discovered by Cooper and Perkins (1969). As Scheme 8-46 shows, the covalent adduct cation 8.62 first loses a proton. This facilitates the homolytic dissociation, as a stable radical, A/,A-diphenylnitroxide (8.63), is formed. This... 

The first step was found to be a fast pre-equilibrium (Scheme 12-8). The dependence of the measured azo coupling rate constants on the acidity function and the effect of electron-withdrawing substituents in the benzenediazo methyl ether resulting in reduced rate constants are consistent with a mechanism in which the slow step is a first-order dissociation of the protonated diazo ether to give the diazonium ion (Scheme 12-9). The azo coupling proper (Scheme 12-10) is faster than the dissociation, since the overall rate constant is found to be independent of the naphthol con-... 

As a third and final example of a chain reaction, we shall consider a net reaction that produces sulfate and hydrogen phosphate ions.7 The scheme is more intricate than the earlier ones. It starts with the homolytic dissociation of S2Ojj- as one of two parallel initiation steps, and utilizes SO -, HO, and HPO - as intermediates. The scheme suggested is shown here, and one can easily allow for the products that are identical save for protonation ... 

The equilibrium ratios of enolates for several ketone-enolate systems are also shown in Scheme 1.1. Equilibrium among the various enolates of a ketone can be established by the presence of an excess of ketone, which permits reversible proton transfer. Equilibration is also favored by the presence of dissociating additives such as HMPA. The composition of the equilibrium enolate mixture is usually more closely balanced than for kinetically controlled conditions. In general, the more highly substituted enolate is the preferred isomer, but if the alkyl groups are sufficiently branched as to interfere with solvation, there can be exceptions. This factor, along with CH3/CH3 steric repulsion, presumably accounts for the stability of the less-substituted enolate from 3-methyl-2-butanone (Entry 3). 

---