Enthalpy of association

Weber G 1995 van t Hoff revisited enthalpy of association of protein subunits J. Rhys. Chem. 99 1052-9... 

This discussion has emphasized the idea that the interaction of the cyclo-amyloses with organic substrates is more favorable than the interaction of the individual molecules with water. In the sense that the driving force for the inclusion process appears as a favorable enthalpy of association, this may be thought of as an atypical hydrophobic interaction. 

Table 2.1 Bond dissociation enthalpies and standard enthalpies of formation of silanes, and enthalpies of associated silyl radicals (kJ/mol) ...

If all of these polymeric organolithiums are converted into unassociated, TMEDA -coordinated species at an R value of ca. 1.0, then the data in Fig. 7 can be used to estimate the difference in enthalpies of association for these aggregates. At an R value of 1.0, the cumulative heat in calories can be associated with the process shown in Eq. (21), where... 

TABLE 15. Measured Si—H bond dissociation energies for silanes (kJ mol 1) and derived enthalpies of associated radicals (A// /k.l mol 1)... 

In the description and explanation of real experimentally testable situations, the lock-and-key model claims a success story however, in many cases - and among them are the majority of the more interesting biological examples -it fails to explain host-guest binding affinity and selectivity with reasonably ambitious satisfaction. For instance, anion binding in water very frequently shows endothermic rather than exothermic enthalpies of association, an observation that is incompatible with the naive complementarity model. 

The low frequency bridge bending vibration vp is not degenerate in this case and has two components of slighly different frequency. This leads to two series of hot bands which overlap and, especially at the high frequency side of v1( smear out the structure. In Fig. 10 taken from Thomas 79) the first six hot band origins are marked. The interpretation of the fine structure leads to 145 and 170 cm-1 for the two components of vp while v can be estimated to be 180 cm-1 and the enthalpy of association AH° = —26 kJ mol-1. 

Similarly, the change in enthalpy of association A Ha°ssoc, of the system can be defined in terms of the internal energy E such that... 

ITC has been used to determine the equilibrium constant of supramolecular polymers formed either from complementary monomers [201] or from self-complementary monomers [92,164], In the former case, the A - A monomer solution is injected into the B - B monomer solution. The exchanged heat measured is proportional to the number of hydrogen bonds formed and thus directly related to the equilibrium constant, hi the latter case, the self-complementary monomer is simply diluted into pure solvent. This time, the exchanged heat measured is proportional to the munber of hydrogen bonds broken and related to the self-equilibrium constant. ITC is a very powerful technique because equilibrium constants as high as 10 Lmol are accessible [202]. Moreover, a single experiment yields the equihbrium constant together with the molar enthalpy of association (Affassoc)-... 

Mattoo (28) which differ by less than 1%. The enthalpy of association has not been published but it has been estimated by assuming that the difference between it and FeSOlf is equal to the difference between A1(804)2 and AISO4. Although the reliability of this estimation cannot easily be determined, it certainly is better than assuming AH = 0. 

A further example of the influence of steric interference in ether adducts of BF3 was provided by a comparison of the stabilities of (C2H5)20-BF3 and (CH2)40-BF3. Although the atomic compositions of these molecules differed only by two protons, the enthalpy of association of the latter molecule exceeded that of the former by more than 40%. It seems clear that the increase in stability of the tetrahydrofuran complex must be attributed to the ring structure of the donor which locks the interfering ethyl groups out of the way of the fluorine atoms, thus eliminating, or at least reducing, steric interference (3, 17). 

For metal ions that form complexes of predominantly covalent character, valid generalizations can be made only within closely related groups. For the divalent ions of the first transition-metal series, Irving and Williams S found the order of stability to be Mn " < Fe" "S < Co< Ni" " < Cu" " > Zn " for many ligands. AndereggS pointed out correlations involving entropy and enthalpy of association with atomic number for various complexes of transition metals. 

In studying vapor association, Lambert (1184), Vines (2119, 192, 193), and Foz Gazulla (687, 686) have been most active. The last author, working with Schafer (1804 see also C.A. 37, 4943, 5294), derived an equation relating conductivity to the heat of dimerization and the dimerization constant. The final equation has three limitations it is rather complicated, it is limited to dimerization, and it is based on a modified perfect gas law. Vines (2119) gives examples of this method and calculates —AH for methanol as 4.2 kcal/mole, in agreement with other values discussed in Chapter 7. A rather high value for the enthalpy of association of HF gas (6.8 kcal/mole) was found by another method based on heat conductivity (694). 

The enthalpy of association of the hydrated structure is l+.l kcal/mole (23.) the process is exothermic in nature. This energy is thus released when the change in the structural configuration from 1-to-l+ water molecules takes place. Therefore, in the particular case of the samples exhibiting a time dependent behavior, the formation of such structures could be verified to a certain extent. 

Microwave spectroscopy of gaseous mixtures of HF with MeCN " or has confirmed that hydrogen-bonded species are generated this technique promises to yield useful structural information for such complexes. Thus the N— F distance in MeCN-HF has been estimated to be 2.741 and the O—F distance in H2O-HF is 2.68 A. The i.r. spectrum of the latter complex in the vapour phase has been reported for the first time. The enthalpy of association was estimated to be -26kJmor at 315 K. The i.r. spectra of mixtures of HCl and DCl or DBr cooled to 166 K have been shown to give additional lines in the HCl rotation—vibration band attributable to HC1,DC1 and HCl,DBr molecules. ... 

Titration calorimetry was used to determine values of log,g K, A //° and AjS°, reported for the formation of NiS04(aq) for 25°C and 7 = 0. The data analysis is suspect because of a possible correlation between the value for the calculated equilibrium constant and the enthalpy of association. A good reassessment was reported by Powell [73POW], who pointed out that if the data of Izatt et al. were used with the association constant at 298.15 K set equal to the value from Nair and Nancollas [59NAI/NAN], then an enthalpy of ion-pair formation of 5.7 to 5.9 kJ-mol was obtained, not the reported 1.7kJ mol [691ZA/EAT]. 

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