Enthalpies of complex formation

Fig. 4. Plot of enthalpies of complex formation in the gas phase of CpNi complexes (Cp = cyclopentadienyl) vs those for the corresponding Mn complexes. The ligands are segregated into correlations for soft (O) (N and S donors) and hard (oxygen donors) ( ). Energies are in kcal mol-1. Redrawn after Ref. (14). 

Such equilibria are known to consist of a number of consecutive complex equilibria. The formation constant / represents the free enthalpy of complex formation in the gas phase. This quantity can not be determined by experiment. 

Using equation (4), Torocheshnikov et al. (50) have calculated the values of <5C and K for Me3SnCl and Et3SnCl in acetone, acetonitrile, and dioxan. The molar enthalpy of complex formation, AH, can be obtained from a In K vs. l/T plot (Fig. 5) using the Van t Hoff relationship ... 

For review, see also MacDiarmid (269). S(29Si) and 5(31P), Chemical shift in ppm AH, enthalpy of complex formation inkcal/mol A5, entropy of complex formation in cal/mol degree Py, pyridine NMI, [Pg.283]

For the quantitative evaluation of the inductive effect of R3 and R4, the pKBH+ values of the conjugate acid of the enaminone, or the enthalpy of complex formation, AHBFi, of boron trifluoride with the carbonyl oxygen for different NR3R4 groups were proposed. The variation of <5H(2) was considered to be a measure of the steric interaction. The following correlations were found ... 

Table 2-4. Molar enthalpies of complex formation between boron trifluoride and several non-HBD solvents, determined in dichloromethane at 25 °C, according to Eq. (2-lOa) [211, 212].

Analysis of enthalpies of complex formation between functional monomer and template... 

Results showed that ILs formed a complex with this surface, and the enthalpies of complex formation were seen to correlate with the tribological properties of the ILs. 

Cvetanovic and co-workers 29> first showed that the variation in complex stability was primarily determined by a variation in the enthalpies of complex formation, while the entropies of complexing varied very little. The constancy of AS is comparable to that found by Traynham and Olechowski for complexes... 

A Ho(PDC)3(o-phen) complex has been obtained and studied by Xie et al. [184] from the reaction of hydrated holmium chloride, ammonium pyrrolidinedithiocarbamate (APDC) and 1,10-phenanthroline (o-phen-H20) in absolute ethanol. The enthalpy of complex formation from a solution of the reagents and the molar heat capacity of fre complex were determined by using a heat conduction microcalorimeter. The enthalpy of complex formation from the reaction of the reagents in the solid phase, was calculated on the basis of an appropriate thermochemical cycle and other auxiliary thermodynamic data. The thermodynamics of formation of the complex were investigated by the reaction in solution at the temperature range of 292.15 - 301.15 K. The constant-volume combustion energy of the complex, Act/, was also determined. 

K, and using the van t Hoff s equation, the enthalpy of complex formation was calculated to be -AH = 16.7 4.3 kJ/mol. It was found, moreover, that the absorption intensity in ultraviolet at wavenumber of v = 35 000 cm is the greatest at equimolar concentration of the substances tested. Therefore in the complex one n-butyro-nitrile molecule corresponds to one n-dodecene-1 molecule (Fig. 13). 

The enthalpy of complex formation was determined by measuring the equilibrium constants at different temperatures. Since thecomplex band (3540 cm ) showed more marked intensity changes with temperature than did the free OH band (3620cm" in Fig. 5.7), the former band was used to determine AH. To do this it is necessary to determine the molar absorptivity of the complex band by the following procedure. The concentration of the complex is given (in terms of the absorbance measurements referred to in the preceding paragraph) as... 

Plot of Opj vs. in 7 hydroxylic solvents enthalpies of complex formation determined. 

By performing calorimetry during complex formation it has been found that the enthalpy of complex formation of systems that form glass-like phases has an opposite sign to the enthalpy of systems that form liquid-like phases, i.e., the formation of glass-like phases is exothermic and the formation of liquid-like phases is endothermic. The free energy (AfG), enthalpy (Af//) and entropy (Af5) of polyelectrolyte complex formation and how they vary as a function of the ionic strength will be discussed. 

Avidin is a basic glycoprotein (Table 11.5). Its amino acid sequence has been determined. Noteworthy is the finding that 15 positions (12% of the total sequence. Table 11.7) are identical with those of lysozyme. Avidin is a tetramer consisting of four identical subunits, each of which binds one mole of biotin. The dissociation constant of the avidin-biotin complex at pH 5.0 is k i/ki = 1.3 x 10 mol/1, i. e., it is extremely low. The free energy and free enthalpy of complex formation are AG = — 85kJ/mole and AH = —90kJ/mole, respectively. Avidin, in its form in egg white, is practically free of biotin, and presumably fulfills an antibacterial role. Of interest is the occurrence of a related biotin-binding protein (streptavidin) in Streptomyces spp., which has antibiotic properties. 

Table 2 Selected Enthalpies of Complex Formation with Boron Trifluoride in Dichioromethane at 298 K. For a more complete list, see reference 19.

The relationship between Af/ and the enthalpy of complex formation, A//f, was clearly demonstrated in a study of 17 Ni(II) amine complexes. It was assumed that A ff could be expressed as the sum of enthalpy contributions firom individual Ni-N bonds plus a correction for steric strain, equation (8). The enthalpy contributions in aqueous solution were —20.1 kJ mol per primary Ni-N bond and —27.2 kJ mol per secondary Ni-N bond. For each complex Af/ was calculated as the difference in steric energy between the product, i.e., the Ni(II)-amine complex and the reactants, i.e., the amine ligand(s) and the Ni(II) aquo complex. Results of this analysis are summarized in Table 3. 

CHaHgCl>(31) from a crystal structure determination, the Q-Ca bond was shown to be somewhat shorter [1.44(3)A] than a normal C-C-bond. In other studies, C and i Hg n.m.r. spectra of RCOCHR HgBr and (RCOCHR )aHg e.g., R = mesityl or Bu R =Me) were recorded. The <5CH and /(C-H) values were found to be higher than expected for purely sp carbons. Association of Hg(CHaCOEt)a (32) and Hg(CHaC02Me)a (33) in benzene solutions involves weak mercury-carbonyl oxygen interactions, with enthalpies of complex formation of —3.2 0.1 and -2.2 0.1 kcal mol for (32) and (33), respectively. ... 

Complex formation of strong PEs is mostly dependent on the entropy. The enthalpy changes of nearly all processes and complexes of strong PEs are 2-15 times smaller than the changes in entropy. For details of the entropy see Sect 3.4 [63, 65]. The enthalpy of complex formation can be calculated with the formalism of Overbeek (Eq. 40-41) [62], or with the method of Muthukumar (43-47) [19,59]. Another method is to determine the enthalpy of complex formation by isothermal calorimetric titrations or other calorimetric methods, turbidity titrations or molecular dynamics simulations [19, 63, 65]. Recent molecular dynamics simulations [19] and experimental measurements showed that the enthalpy of PE complex formation decreases with increasing ionic strength, but this value is always below the entropy [23, 62, 63, 65]—see following section for detail. 

Maria, P.C. and Gal, J.F. (1985) A Lewis basicity scale for nonprotogenic solvents enthalpies of complex formation with boron trifluoride in dichloromethane. J. Phys. Chem., 89,1296—1304. 

Gal, J.F., Elegant, L. and Azzaro, M. (1974) Basicity of the carbonyl group. II. Effect of substituents on the enthalpy of complex formation between substituted aromatic carbonyl compounds and boron trifluoride. Bull. Soc. Chim. Fr., 411-414. 

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