Dissociated species

Following a similar procedure, we locate and verify the transition structure connecting cis hydroxycarbene and the two dissociated species. Here is the transition structure and the two structures at the end of the reaction path computed by the IRC calculation ... 

In general the compounds have properties intermediate between those of the parent halogens, though a combination of aggressive chemical reactivity and/or thermal instability militates against the determination of physical properties such as mp, bp, etc., in some instances. However, even for such highly dissociated species as BrCl, precise molecular (as distinct from bulk) properties can be determined by spectroscopic techniques. Table 17.12 summarizes some of the more important physical properties of the... 

For a strong electrolyte such as HC1 we assume that all of the electrolyte is dissociated to form the ionic species. In this case, it is appropriate to consider the chemical potential of the HC1 as the sum of the chemical potentials of the dissociated species. That isdd... 

Chiral lactones were also obtained by cyclocarbonylation of chiral acetylenic alcohols with Pd and thiourea (H2NCSNH2) (Scheme 32). No loss in chirality was observed, but large amounts of Pd and thiourea were used (10 mol %) since the catalyst deactivates by forming metal particles. The catalytic precursor (Pdl2 > PdCl2) and the ratio of thiourea to Pd were very important, thiourea being necessary for this reaction. The active species was supposed to be [Pd(thiourea)3l]I, which forms in situ from [Pd(thiourea)4]l2 and [Pd(thiourea)2]l2. It had to be a partially dissociated species since [Pd(thiourea)4](Bp4)2 was inactive [121]. 

They assumed that the mobilities of the dissociated species LL and K, wf and respectively, are independent of composition. They have applied this model to (Li, Cs)Cl using three experimental values, which are M[(0), uj 1), and M i( X 2) = 2 (X 2) at the Chemla crossing point. Since the calculated isotherms deviate considerably from the experimental ones, they suppose that the dependence of and K2 on the mixing ratio must be assumed. 

The dynamic dissociation model resembles the association (or dissociation) model in that electrically conducting species are assumed to he nonassociated species, and it differs from the association model in that in the dynamic dissociation model the dissociation process itself is the electrically conducting process, while in the association model, the amount of the dissociated species is constant according to the chemical equilibrium. 

Just as in the case of (16), an equation of the form (20) applies to any other association-dissociation reaction in which one of the dissociated species is mobile, the other fixed. When the two species are distinct but both mobile, as for hydrogen combining with, say, an interstitial silicon, a similar line of reasoning, whose details we omit, leads to equations of the same form as (16) and (20) but with D+ replaced by the sum of the diffusion coefficients of the two species. When the two mobile species are the same, as for the reaction H° + H° 5H2, it turns out that nA and n+ should each be replaced by the monatomic density n, D+ by the monatomic diffusion coefficient, and 4ir by 8tt in (16) but not in (20). 

DB-1) Formation of a dative bond leads to a change of formal charge at each atom (A B+), so the calculated atomic charges 0A(dat) and gB(dat) for a dative-bonded pair deviate markedly from those (( A(sep) and 0B(sep)) in the dissociated species by approximately equal and opposite amounts for A and B,... 

The ion-pair concept leads to two types of activity coefficients y t for free plus dissociated species and y. These y s are related by 1... 

An indication has been obtained that the opening of the salicylate hydrogen bond may become partially rate limiting in proton transfer (33) from substituted salicylate ions to hydroxide ions and buffer species in 50% (v/v) MejSO-HjO (Hibbert and Spiers, 1989a). Temperature-jump measurements of the equilibration between the salicylate ion and its dissociated species lead to curved plots of against buffer concentration and against hydroxide-ion concentration. Analysis of the results in terms of the mechanism in (33) gave the approximate values ki = 5x 10 s" and A, = 3 X 10 s ... 

Such a chemical approach which links ionic conductivity with thermodynamic characteristics of the dissociating species was initially proposed by Ravaine and Souquet (1977). Since it simply extends to glasses the theory of electrolytic dissociation proposed a century ago by Arrhenius for liquid ionic solutions, this approach is currently called the weak electrolyte theory. The weak electrolyte approach allows, for a glass in which the ionic conductivity is mainly dominated by an MY salt, a simple relationship between the cationic conductivity a+, the electrical mobility u+ of the charge carrier, the dissociation constant and the thermodynamic activity of the salt with a partial molar free energy AG y with respect to an arbitrary reference state ... 

From this relationship we may expect to be proportional to the salt thermodynamic characteristics, if u+ and K -,ss have constant values at constant temperature and pressure in a given glassy system. The square root dependency of ionic conductivity on Umy has been experimentally verified over several orders of magnitude. The dissociating species is either a network modifier or a doping salt. Potentiometric (Ravaine and Souquet, 1977) or calorimetric (Reggiani, Malugani and Bernard, 1978)... 

We must be careful to distinguish between bond enthalpy and the dissociation enthalpy of a given bond. The latter is a definite quantity that refers to the enthalpy absorbed when a given bond of some specific compound is broken. However, bond enthalpy is an average value of the dissociation enthalpies of a given bond in a series of different dissociating species. 

Any substance that stabihzes the concentration of one of its dissociated species, most commonly a proton (or more properly a hydronium ion). The free concentration of a metal ion may also be buffered by use of a metal ion-chelator complex, such as Ca EGTA or KMgEDTA. 

Field ionization mass spectroscopy of chemically reactive molecular gases found often dissociated species and associated species of these... 

This value seems reasonable considering the attack of the alkoxide anion on electrodeficient carbonyl group. Considering the great tendency of association of alkoxides, the kp value should not be ascribed to an dissociated species. 

Another PES topology for molecular dissociation occurs when an intermediate molecularly chemisorbed state lies parallel to the surface between the physisorption well and the dissociated species as shown in Figure 3.2(b). This molecular state is usually described in terms of a diabatic correlation to a state formed by some charge transfer from the surface to the molecule [16]. In this case, there can be two activation barriers, V] for entry into the molecular chemisorption state of depth Wx and barrier V2 for dissociation of the molecularly chemisorbed state. This PES topology is relevant to the dissociation of some it bonded molecules such as 02 on metals, although this is often an oversimplification since distinct molecularly adsorbed states may exist at different sites on the surface [17]. In some cases, V < 0 so that no separate physisorbed state exists [18]. If multiple molecular chemisorption... 

Hence, when considering the whole epilimnion, the direct photolysis half-life of 4-NP is about 10 times longer as compared to the half-life at the surface. Note that in contrast to the near-surface situation, because of the very different screening factors, the reaction of the dissociated species is about four times more important in determining the overall direct photolysis rate of 4NP in the well-mixed epilimnion. 

Shown below are some data concerning light absorption in the uv/vis range by pentachlorophenol (PCP). Absorption values for the neutral species are given in ethanol, those for the dissociated species at pH 10 in aqueous solution. 

We consider a situation where the interaction potential between two associating/dissociating species is described sufficiently well by the simple charge-dipole potential... 

The sorption of a weak electrolyte by a charged polymer membrane is another case where Nernst + Langmuir-like dual mode sorption, involving the undissociated and dissociated species respectively, may be expected. The concentration of each species in solution follows, of course, from the dissociation constant of the electrolyte. The sorption isotherms of acetic acid and its fluoroderivatives have been analysed in this manner, and the concentration dependence of the diffusion coefficient of acetic acid interpreted resonably successfully, using Nylon 6 as the polymer substrate 87). In this case the major contribution to the overall diffusion coefficient is that of the Nernst species consequently DT2 could not be determined with any precision. By contrast, in the case of HC1, which was also investigated 87 no Nernst sorption or diffusion component could be discerned down to pH = 2 and the overall diffusion coefficient obeyed the relation D = DT2/( 1 — >1D), which is the limiting form of Eq. (25) when p — 00. 

The form in which chemical analyses of sea water are given records the history of our thought concerning the nature of salt solutions. Early analytical data were reported in terms of individual salts NaCl, CaSO/i, and so forth. After development of the concept of complete dissociation of strong electrolytes, chemical analyses of sea water were given in terms of individual ions Na+, Ca++, Cl-, and so forth, or in terms of known undissociated and partly dissociated species, e.g., HC03 , In recent years there has been an attempt to determine the thermodynamically stable dissolved species in sea water and to evaluate the relative distribution of these species at specified conditions. Table 1 lists the principal dissolved species in sea water deduced from a model of sea water that assumes the dissolved constituents are in homogeneous equilibrium, and (or) in equilibrium, or nearly so, with solid phases. 

Boudart et al.20 studied nitrogen adsorption and ammonia decomposition on polycrystalline molybdenum and reported that the surface was predominantly covered with adsorbed N and some dissociated species such as NH2. N2 formation was thought to proceed through the recombination of adsorbed nitrogen on the surface above 645 K.22. Haddix et al.23 have reported that decomposition of NH3 adsorbed on y-Mo2N produces... 

The validity of the concept that only unassociated organolithiums are reactive in initiation is predicated on an assumption that the associated and dissociated species are in rapid equilibrium with one another. This, of course, would result in the... 

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