Pair formation

The formation of ion pairs may occur either by direct dissociation or by predissociation [processes (3) and (4) of Section 3. ]. The behavior of the cross section for such processes is the same as that for dissociation or predissociation into neutral fragments a smooth continuum indicates direct dissociation, while predissociation is characterized by a band structure broadened according to the lifetime of the state. The most intense ion-pair formation processes usually exhibit the structure characteristic of predissociation. The occurrence of intense ion-pair formation requires a favorable set of potential curves (or surfaces) and is not readily predictable. The mere fact that a molecule contains an atom or radical of high electron affinity gives no assurance that the negative ion will be formed in detectable amounts by photon absorption. 

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It is important to evaluate the surface distortion associated with the assymetric field at the surface, a difficult task often simplified by assuming that distortion is limited to the direction normal to the plane [64, 6S]. Benson and co-workers [6S] calculated displacements for the first five planes in the (100) face of sodium chloride and found the distortion correction to of about 100 ergs/cm or about half of itself The displacements show a tendency toward ion pair formation, suggesting that lateral displacements to produce ion doublets should be considered [66] however, other calculations yielded much smaller displacements [67]. 

Carbonera D, DiValentin M, Corva]a C, Agostini G, Giacometti G, Liddell P A, Kuciauskas D, Moore A L, Moore T A and Gust D 1998 EPR investigation of photoinduced radical pair formation and decay to a triplet state in a carotene-porphyrin-fullerene triad J. Am. Chem. Soc. 120 4398-405... 

In summary, doubly exeited CSFs are often employed to permit polarized orbital pair formation and henee to allow for eleetron eorrelations. Singly exeited CSFs are ineluded to permit orbital relaxation (i.e., orbital reoptimization) to oeeur. 

The equation does not take into account such pertubation factors as steric effects, solvent effects, and ion-pair formation. These factors, however, may be neglected when experiments are carried out in the same solvent at the same temperature and concentration for an homogeneous set of substrates. So, for a given ambident nucleophile the rate ratio kj/kj will depend on A and B, which vary with (a) the attacked electrophilic center, (b) the solvent, and (c) the counterpart cationic species of the anion. The important point in this kind of study is to change only one parameter at a time. This simple rule has not always been followed, and little systematic work has been done in this field (12) stiH widely open after the discovery of the role played by single electron transfer mechanism in ambident reactivity (1689). 

Ion-pair formation. An ionization process in which a positive fragment ion and a negative fragment ion are the only products. 

Since the solvent molecules, the polymer segments, and the lattice sites are all assumed to be equal in volume, reaction (8.A) impUes constant volume conditions. Under these conditions, AU is needed and what we have called Aw might be better viewed as the contribution to the internal energy of a pairwise interaction AUp jj., where the subscript reminds us that this is the contribution of a single pair formation by reaction A. 

The mode of action has been a subject for research for a number of years. While it was originally thought that maleic hydrazide replaced uracil in the RNA sequence, it has been deterrnined that the molecule may be a pyrimidine or purine analogue and therefore base-pair formation is possible with uracil and thymine and there exists the probabiHty of base-pair formation with adenine however, if maleic hydrazide occurs in an in vivo system as the diketo species, then there remains the possibiHty of base-pairing with guanine (50). Whatever the mechanism, it is apparent that the inhibitory effects are the result of a shutdown of the de novo synthesis of protein. 

The Co nucleus decays with a half-life of 5.27 years by /5 emission to the levels in Ni. These levels then deexcite to the ground state of Ni by the emission of one or more y-rays. The spins and parities of these levels are known from a variety of measurements and require that the two strong y-rays of 1173 and 1332 keV both have E2 character, although the 1173 y could contain some admixture of M3. However, from the theoretical lifetime shown ia Table 7, the E2 contribution is expected to have a much shorter half-life and therefore also to dominate ia this decay. Although the emission probabilities of the strong 1173- and 1332-keV y-rays are so nearly equal that the difference cannot be determined by a direct measurement, from measurements of other parameters of the decay it can be determined that the 1332 is the stronger. Specifically, measurements of the continuous electron spectmm from the j3 -decay have shown that there is a branch of 0.12% to the 1332-keV level. When this, the weak y-rays, the internal conversion, and the internal-pair formation are all taken iato account, the relative emission probabilities of the two strong y-rays can be determined very accurately, as shown ia Table 8. 

AIterna.tives to y-Ray Emission. y-Ray emission results ia the deexcitation of an excited nuclear state to a lower state ia the same nucHde, ie, no change ia Z or. There are two other processes by which this transition can take place without the emission of a y-ray of this energy. These are internal conversion and internal pair formation. The internal-conversion process iavolves the transfer of the energy to an atomic electron. 

Fig. 63. Molecular arrangement in (a, c) plane of a mixed ethylene-chlorine binary crystal illustrating (a) radical pair formation, (b) single chain growth and (c) chain growth in the vicinity of product line. Molecules labelled 1-4 are ethylene (C2H4), chlorine, chloroethyl radical (C2H4CI) and anti 1,2-dichloroethane (C2H4CI2), respectively.

This alkyl migration is believed to proceed via ion-pair formation. These and many other simple 0-alkyIated cyclic hydroxamic acids are thermally stablebelow 180°. 

Detailed kinetic studies of the substitution reactions of anions with heterocyclic compounds to include, for example, the effects of solvent, added salts, and ion pair formation have not been made as yet. 

At pH 7, [13]aneN3 or [12]-[15]aneN4 accommodate only two nitrogen-bound protons and these dipositive ammonium cations are apparently unable to provide sufficient electrostatic attraction to polycarboxylate anions for ion-pair formation. In contrast, the macrocyclic spermines, pentaamines and hexaamines accommodate more than three nitrogen-bound protons at pH 7 and for these ligands 1 1 associations... 

Steadman, J., and Syage, J. A. (1991). Time-resolved studies of phenol proton transfer in clusters. 3. solvent structure and ion-pair formation. J. Phys. Chem. 95 10326-10331. 

According to Eigen and Tamm [87,88], ion-pair formation proceeds stepwise, starting from separated solvated ions which form a solvent-separated ion pair [C+SSA ]°, followed by a solvent-shared ion pair [C+SA ]° and finally a contact ion pair, [C+A ]° [Eqs. (4)-(6)]. All these species are solvated. The types of ion pair formed depend on the relative strength of the interaction of the involved species. 

UV spectra are, however, very useful for the determination of acid-base and ion pair formation equilibria, and for photochemical investigations (e. g., determination of quantum yield in photolytic dediazoniation, Tsunoda and Yamaoka, 1966 fluorescence and phosphorescence at low temperature, Sukigahara and Kikuchi, 1967a). 

The proton is not the only entity that can dissociate from a substrate or bond to it. We can enumerate other interactions, such as metal-ligand complexation, ion-pair formation, charge-transfer complex formation, etc. For the sake of brevity, we treat all of these as... 

This reaction scheme agrees with the experimental rate equation, with k = k 2ftk 2 i/k-126 and k = k 21/k-126- The rate constant at a high [SO42-] would plateau at the value of Jk 26. On the other hand, one can consider ion-pair formation ... 

An alternative view that also favors an intermediate mechanism is that of Schleyer and co-workers, who believe that the key to the problem is varying degrees of nucleophilic solvent assistance to ion-pair formation. They have proposed an Sn2 (intermediate) mechanism. ... 

Boron pair formation Boron pair formation... 

Because of the tendency to form B—B bonds, further substitution in the direction of a higher B P ratio results in boron-pair formation, even if the total boron content is as low as in Cr5PB2 (ordered Cr5B3 type structure see also 6.7.2.2). 

P6222 prism, boron-pair formation MgNi2sB2 13... 

Nickel atoms in BajNi B form distorted, puckered 3.6.3.6-kagome nets stacked in six layers perpendicular to the c axis. The densely packed framework of trigonal-Ni prisms again result in boron-pair formation, although Ba atoms are too large to be sandwiched between two Ni layers, and only four Ba can be accommodated within six Ni layers. Superconductivity is found for ( a, Sr, Ba)2pt9Bg borides with a structure related to Ba2Ni9Bfi and e o,B2 however, with respect to crystal chemistry and boron coordination, only the subcell is derived so far. 

A brief discussion of the systematics of solvent effects on the p/, pr, and values of Tables II and III is presented in the discussion section. However, it is worthy of note here that sets 7, 37, 38, 39, 40, and 41, which involve nonhydroxylic solvents, are fitted with comparable precision to that for reaction series in aqueous or mixed aqueous organic solvents. The present analysis does not support the previous assignment (7b) of ion-pair formation of benzoic acids... 

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