Separated pair

If the substance is found to be far too soluble in one solvent and much too insoluble in another solvent to allow of satisfactory recrystallisation, mixed solvents or solvent pairs may frequently be used with excellent results. The two solvents must, of course, be completely miscible. Recrystallisation from mixed solvents is carried out near the boiling point of the solvent. The compound is dissolved in the solvent in which it is very soluble, and the hot solvent, in which the substance is only sparingly soluble, is added cautiously until a slight turbidity is produced. The turbidity is then just cleared by the addition of a small quantity of the first solvent and the mixture is allowed to cool to room temperature crystals will separate. Pairs of liquids which may be used include alcohol and water alcohol and benzene benzene and petroleum ether acetone and petroleum ether glacial acetic acid and water. 

Polymer alloys are generally named polymer blends within the polymer community. In a recent overview of such blends, Robeson (1994) points out that the primary reason for the surge of academic and industrial interest in polymer blends is directly related to their potential for meeting end-use requirements . He points out that, in general, miscible polymer pairs confer better properties, mechanical ones in particular, than do phase-separated pairs. For instance, the first commercial... 

The lack of solvent separated pairs raises the question whether some alternative mode of solvation should be considered. The ester group of the penultimate unit of the polymer, or the one preceeding it, could act as a solvating agent. The idea of intramolecular solvation was proposed by several workers in the field 37) and it is supported by the results of nmr studies of polymethyl methacrylate formed under various experimental conditions 38). Hypothetical structures such as those depicted below were proposed 39 h... 

The tetrone slowly decomposes dining storage at ambient temperature with generation of carbon dioxide. Two separate pairs of incidents involving bursting of bottles [1], and of pressure generation [2,3], were reported. It also explodes at temperatures above 170°C [4],... 

In the past, except for the low-temperature range, the uncertainties of noise thermometry were not comparable to those of the gas thermometry due to the non-ideal performance of detection electronics. Up to now, the most successful technique is the switched input digital correlator proposed by Brixy et al. in 1992 [89], In this method, the noise voltage is fed via two separate pairs of leads to two identical amplifiers whose output signals are multiplied together, squared and time averaged (see Fig. 9.10). 

Even though we have an exception, we can still complete the Lewis structure. We need to draw a bond from each of the fluorine atoms to the central xenon. This gives us 4 bonds and uses 8 electrons. Each fluorine atom needs to complete its octet. The bond accounts for 2 electrons, so we need 6 more electrons (3 pairs) for each. Therefore, we add 3 separate pairs to each of the fluorine atoms. Six electrons per fluorine times 4 fluorine atoms accounts for 24 electrons. Our Lewis structure now contains 8 + 24 = 32 electrons. The number of available electrons (A) is 36, so we still need to add 36 - 32 = 4 electrons. These 4 electrons will give us 2 pairs. The xenon atom will get these pairs and become an exception to the octet rule. The actual placement of the pairs is not important as long as it is obvious that they are with the central atoms and not one of the fluorine atoms. The final Lewis structure is ... 

The measurement of A vs concentration provides no evidence as to the nature of the ion pairs which form, i.e. whether they are contact or solvent separated species. Also, the mobility of the ion pairs does not influence the results. Contact ion pairs are likely to be more mobile than those separated by solvent since the latter include a section of at least one polymer chain. However, it is possible to envisage mechanisms, involving concerted motion of the cation and anion of a solvent separated pair, which would allow the effective movement of the neutral pair. This is also true for contact vs solvent separated triples. Measurements to be discussed below, involving the dc polarisation of cells, are capable of distinguishing between mobile and immobile pairs. 

Fig. 2.6 Ion pairs. A two-dimensional representation of (a) a solvent-separated pair of ions, each still retaining its intact shell of solvating solvent molecules (b) a solventsharing ion pair, which has lost some of the solvent between the partners, so that one layer of solvent shared between them separates them (c) a contact ion pair, the cation and anion being contiguous.

D. M. Silver, E. L. Mehler, and K. Ruedenberg, Electron correlation and separated pair approximation in diatomic molecules. 1. Theory. J. Chem. Phys. 52(3), 1174-1180 (1970). 

In fact, the A0+-value of (SrB)+ being 12.1 and that of 26.1 may be taken to Indicate a double solvent separated Ion pair structure for the Sr-salt and a mixed contact-solvent separated pair structure for the BaB2 The double solvated... 

The inherent electronic nature of semiconductor metal oxides can direcdy interact with molecular excited states in a manner not energetically possible with insulators. More specifically, an excited sensitizer, S, may transfer an electron to the semiconductor forming a charge separated pair [Eq. (1)] ... 

The approach to the critical point, from above or below, is accompanied by spectacular changes in optical, thermal, and mechanical properties. These include critical opalescence (a bright milky shimmering flash, as incident light refracts through intense density fluctuations) and infinite values of heat capacity, thermal expansion coefficient aP, isothermal compressibility /3r, and other properties. Truly, such a confused state of matter finds itself at a critical juncture as it transforms spontaneously from a uniform and isotropic form to a symmetry-broken (nonuniform and anisotropically separated) pair of distinct phases as (Tc, Pc) is approached from above. Similarly, as (Tc, Pc) is approached from below along the L + G coexistence line, the densities and other phase properties are forced to become identical, erasing what appears to be a fundamental physical distinction between liquid and gas at all lower temperatures and pressures. 

When the reagents are located in the form of isolated pairs, so that it is possible to neglect the reactions between the particles from different pairs, the total kinetics of an electron tunneling reaction can be obtained by averaging the kinetics for the separate pairs. Let the distribution of pairs of donors and acceptors over the distance between them, R, at the initial instant of time (t = 0) be described by the function f(jR) normalized accord-... 

Bidentate-binuclear complex A Stem inner-sphere adsorption complex that consists one adsorbed atom bonding to two separate metal oxides. As discussed in Chapter 2, an arsenate may adsorb onto two separate pairs of iron and oxygen atoms. 

The probability function /(t) gives the unity probability when the pair is within the interval —a/2integrating Equation 6.135 between the limits x = al2 with t 0. The diffusion coefficient D is the sum of the individual diffusion coefficients 1), knTI(mr)Vj (Einstein equation), where rt is the radius of the component i of the radical-ion pair. Integration of Equation 6.135 between the limits — °° < x < all and all < x < oo gives the probability of a diffusively separated pair (Equation 6.136). 

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