Enthalpy of desolvation

Step 1 in Fig. 2 corresponds to the measured enthalpy of the reaction in a polar solvent, step 4 the enthalpy of forming the adduct in the gas phase, steps 2 and 3 the enthalpies of desolvating the acid and base respectively, and step 5 the enthalpy of solvating the adduct. The enthalpy measured, AHi, is the sum of many contributions, i.e.,... 

The DSC thermograms of a number of lactose monohydrate samples were determined, and it was ultimately found that the dehydration endotherm was characterized by an enthalpy of desolvation equal to 150.5 J/g.48 This value for the pure monohydrate phase can then be used in conjunction with the measured desolvation enthalpies of actual samples to determine either the anhydrate phase content in a monohydrate sample or the monohydrate phase content in an anhydrate sample. To illustrate the utility of DSC analysis for such determinations, a series of calibration samples were prepared at levels approximately 90% or 10% in the monohydrate phase, using anhydrous /1-lactose as a diluent in bulk a-monohydrate.48 These were subjected to a standard DSC evaluation, and the results of this analysis are shown in Table 2. The results indicate that the DSC method was able to provide very good estimations of the phase composition. It is concluded that the magnitude of the error associated with the use of DSC analysis would be acceptable for quantitative analysis of mixtures of lactose monohydrate and anhydrate. 

Actual weighed composition of sample, expressed as monohydrate (%) Anticipated DSC enthalpy of desolvation for sample (J/g) Experimentally measured enthalpy of desolvation (J/g) Calculated monohydrate content, based on an enthalpy of desolvation of 150.5 j/g (%)... 

The possibility of an entropy-enthalpy relationship for the reaction was examined and found to give a correlation coefficient of only 0.727 which was however improved to 0.971 if only the external contributions to these parameters were used, i.e. these contributions arising from solvent interactions only. If compounds with substituents ortho to the amino group were excluded, this further improved to 0.996 and is likely therefore to be real [cf. the comments on p. 9). It was argued that the different amounts of desolvation of the aromatic on going to the transition state would depend upon the substituent, and that the resultant greater freedom for solvent molecules would mean decreased interaction energy or increased enthalpy so that the linear relationship follows. 

The DS increase as a function of increasing the chain-length of the acyl group may be also attributed to hydrophobic interactions between the cellu-losic surface, whose hydrophobic character increases as a function of increasing DS, and the acylating species. This cooperative interaction. Fig. 10, may contribute to the activation enthalpy, as a result of desolvation of the entering species. Since association between the chains attached to cellulose and those... 

Enthalpies of reaction for nucleophilic substitution of ethyl iodide by a series of 27 nucleophiles in acetonitrile have been determined.147 Various empirical correlations were developed. Partial desolvation accompanying activation has been identified as the major contributor to activation thermodynamic parameters, while the propensity of the reacting central atom in the nucleophilic anion plays a crucial role in determining reaction thermodynamic parameters. 

The proposed mechanism is in accordance with the observed solvent dependence of the reaction. Whereas the dipolar sulfoxide is expected to be more strongly solvated with an increase in solvent polarity, the less dipolar sulfenate should be relatively insensitive to such a solvent change. Stabilization of the sulfoxide, relative to the less dipolar activated complex (which should be similar to the sulfenate intermediate), increases the enthalpy of activation, AH. This is refiected in the necessity of breaking increasingly strong solute-solvent interactions. On the other hand, because desolvation on activation is expected to increase the degrees of freedom in the system, a more positive AS is expected to work in the opposite direction and effect a compensating increase in k with... 

The solute is often an ion that is solvated by a coordination shell of solvent atoms (or other ions). At the crystal surface, desolvation of one or two of the solvent molecules in the coordination shell must occur before the solute can (1) adsorb, (2) attach to a step, and (3) integrate into the crystal at a kink. The difftision of solvent molecules (or other coordination ions) away from the crystal surface may limit the diffusion of solute toward the crystal surface and thus limit the growth rate. The solute does not become a part of the crystal until the enthalpy of crystallization has been liberated and desolvation is complete. Figure... 

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