Dissociative step

The dynamics of fast processes such as electron and energy transfers and vibrational and electronic deexcitations can be probed by using short-pulsed lasers. The experimental developments that have made possible the direct probing of molecular dissociation steps and other ultrafast processes in real time (in the femtosecond time range) have, in a few cases, been extended to the study of surface phenomena. For instance, two-photon photoemission has been used to study the dynamics of electrons at interfaces [ ]. Vibrational relaxation times have also been measured for a number of modes such as the 0-Fl stretching m silica and the C-0 stretching in carbon monoxide adsorbed on transition metals [ ]. Pump-probe laser experiments such as these are difficult, but the field is still in its infancy, and much is expected in this direction m the near fiitiire. 

Monoprotic weak acids, such as acetic acid, have only a single acidic proton and a single acid dissociation constant. Some acids, such as phosphoric acid, can donate more than one proton and are called polyprotic weak acids. Polyprotic acids are described by a series of acid dissociation steps, each characterized by it own acid dissociation constant. Phosphoric acid, for example, has three acid dissociation reactions and acid dissociation constants. 

Substrate reduction is accompHshed by a series of sequential associations and dissociations of the two proteias, and duting each cycle, two molecules of MgATP are hydroly2ed and a single electron is transferred from the Fe proteia to the MoFe proteia (11,133), with the dissociation step being rate-limiting at about 6 (H)- Although the kinetics of aU. the partial reactions have been measured, Httie is known about the physical details of the... 

These equiUbrium constants K, K, K, and are known (10). The piC values for the four dissociation steps as well as the proportions of the species present in aqueous solution as a function of pH are shown in Eigure 3. The reaction of Na2EDTA and, represented by equation 5 and noting that... 

Because a neutral molecule is eliminate4 rather than an anion, there is no electrostatic attraction (ion pairing) between the products of the dissociation step. As a result, the carbocations generated by diazonium-ion decomposition frequently exhibit somewhat different behavior from those generated from halides or sulfonates under solvolytic conditions. ... 

In benzene solution one of the triphenylphosphine ligands in (131) is replaced by a solvent molecule to give intermediate (132). The latter can add a mole of deuterium leading to (133) or can equilibrate with (134) in the presence of an olefin. There is some evidence, however, that in the presence of alcohol and oxygen the dissociation step (131 -> 132) is inhibited and the displacement of the triphenylphosphine by the solvent in forming (133) occurs only in the presence of hydrogen (or deuterium). ... 

First, let us consider the formation of ions from covalently bound species, i.e., the heterolytic cleavage of the covalent (or partially covalent) bond. Charge separation under the influence of the solvent generates an ion pair in a process called ionization this ion pair may then separate into free ions in a dissociation step (Eq. 8-18). 

As the pre-equilibria in Schemes 5-10 and 5-11 are not identical and their equilibrium constants are therefore likely to be different from one another, the rate constants k - and are not intrinsic rate constants of the corresponding slow dissociation steps, but are dependent in addition on the constants of these pre-equilibria. 

In reviewing reported values of E for calcite decompositions, Beruto and Searcy [121] find that most are close to the dissociation enthalpy. They suggest, as a possible explanation, that if product gas removal is not rapid and complete, readsorption of C02 on CaO may establish dissociation equilibria within the pores and channels of the layer of residual phase. The rate of gas diffusion across this barrier is modified accordingly and is not characteristic of the dissociation step at the interface. 

In drafting a catalytic cycle as in Eqs. (132)-(135) we naturally have to ensure that the reaction steps are thermodynamically and stoichiometrically consistent. For instance, the number of sites consumed in the adsorption and dissociation steps must be equal to the number of sites liberated in the formation and desorption steps, to fulfill the criterion that a catalyst is unaltered by the catalytic cycle. 

The simplest overall interpretation of these data is in terms of a rate-determining dissociation. Entropies of activation are positive and the solvent-dependence for a better leaving group (Cl) is less marked than for a worse one (Br) in the case of reaction (38) . For the dimeric carbonyls, [M(CO)4X]2, bridge-breaking, essentially the same dissociation, could result in a rapid pre-equilibrium. If this were followed by a second dissociative step, then the kinetics could be first-order (as for Mn), while a rate-determining entry of L could produce second-order kinetics (as for Re). 

It is of interest to note that the exchange between PCI5 and PCI3 also proceeds, in CCI4 media, via a dissociation step ... 

These workers have concluded that, in the exchange of FeY and FeY, a dissociation step cannot take part in the mechanism of exchange. 

The different species formed by steps (18) to (20) or (18 ) to (20 ) have been detected by in situ infrared reflectance spectroscopy, and such dissociative steps are now widely accepted even if the exact nature of the species formed during (20) or (20 ) is still a subject of discussion. Several groups proposed the species (COH)3js as the main, strongly adsorbed species on the platinum surface, even though no absorption infrared band can be definitely attributed to (COH),, . However, the formyl-like species ( CHO), , . has been formally identified, since it gives an IR absorption band ataroimd 1690cm . ... 

In most systems, K2 < K[. Otherwise (e.g. the first two dissociation steps for ethylenediaminetetraacetic acid—EDTA) the first hydrogen ion capable of dissociation is stabilized by the presence of the second one and can dissociate only when the second begins to dissociate. As a result both hydrogen ions dissociate in one step. 

Rearrangements must be involved also in radical eliminations in which the open shell species X is not present as a structural unity of M+ but has to be created prior to the eventual dissociation step. Examples of such processes are the loss of HC = O from ionized methylvinylsulfide 6- 7i) or the elimination of both OH and HC = O from the cation radical of 8S,6> (the arrows in (2) indicate the atoms of 8 which are involved in the formation of the radicals OH and HCO. Both radicals are obviously not present as structural functions in the precursor. For HCO loss from 6 no labelling work has been reported). 

There is another mechanistic variation to circumvent the direct cleavage process 11- 12. The isomerization 11- 13 does not necessarily have to involve a skeletal reorganization. For example, it can be envisaged that a specific hydrogen transfer occurs onto a suitable acceptor function Y, 24- 25 (5). In this way a reactive radical site is created which induces the dissociation step (elimination of X ) by... 

The effect of the cluster size on the energy profile was calculated by comparing the first elementary steps of the N20 decomposition reaction route considering the two clusters (Z) and (Z0H). In Figures 2i. and 2ii, the calculated energy profiles for the adsorption and dissociation steps (equations 1 and 2), occurring over [Fe (p-0)(p-OH)Fe]+ inserted in part of the zeolite framework Z and Z 0H respectively, are reported. Structures (a), (b) and (d) correspond to local minima on the PES, whereas structure (c) represents the TS for N20 dissociation. 

It is conceivable that analogous changes also take place here but probably more efficiently, because the ligand dissociation step involved only weakly bonded bridging Cl- rather than a relatively strongly coordinated phosphite ligand ... 

The tetrahedral nickel isonitrile cluster Ni4(CNR)7 (R = cyclohexyl or f-Bu) contains three bridging isonitrile ligands and a terminal one at each nickel (52) the bridging ligands are mobile on the cluster surface and a ligand dissociation step occurs (33, 390, 391). 

FIGURE 5-12 A processive clamp model for the ATPase cycle of the ABC transporter Mdllp. ATP binding (step I) on NBD domains of both monomers induces formation of the dimer (step 2). After ATP hydrolysis by the first NBD (step 3), either the P, is released first (step 4), followed by hydrolysis of the second ATP step 5) and release of the second P, step 8), or the second ATP is hydrolyzed first step 6) and then both phosphates are set free steps 7 and 8). After both ATPs are hydrolyzed to ADP and both phosphates are released, the dimeric complex dissociates step 9) and ADP step 10) is released. The hydrolysis cycle can then start again with ATP binding. (With permission from Fig. 7 of reference [34].)... 

The argument against Scheme 1 is a negative one. Its basis derives from extensive studies carried out on the ligand replacement reactions of oxorhenium complexes of the family MeReO(dithiolate)L (20,34-37). Those studies (Sections V.B and V.C) show that all such processes studied to date proceed by direct displacement reactions without a recognizable intermediate from unassisted Re-L dissociation. (Indeed, in an early work, a dissociative step was written, but that formulation has since been revised see Section V.D.)... 

A major problem encountered in studies of the cluster carbonyls is the comparatively drastic conditions required to bring about chemical reaction with substrates such as acetylene or ethylene. The difficulty lies in the reluctance of these systems to undergo ligand exchange either via (a) a CO dissociation step,... 

---