Association constants hydrogen-bonded complexes

The biological interest of hydrogen bonding in aqueous solutions is well known pro-tonated amines and phenolate ions in water are associated by hydrogen bonds. The association constant (K = 0.81 mol 1 dm3, measured in buffered aqueous solution) for the complex of the dication of ethylenediamine with the phenolate ion agrees with a weak hydrogen bonding N—H- O68. 

The linear response function [3], R(r, r ) = (hp(r)/hv(r ))N, is used to study the effect of varying v(r) at constant N. If the system is acted upon by a weak electric field, polarizability (a) may be used as a measure of the corresponding response. A minimum polarizability principle [17] may be stated as, the natural direction of evolution of any system is towards a state of minimum polarizability. Another important principle is that of maximum entropy [18] which states that, the most probable distribution is associated with the maximum value of the Shannon entropy of the information theory. Attempts have been made to provide formal proofs of these principles [19-21], The application of these concepts and related principles vis-a-vis their validity has been studied in the contexts of molecular vibrations and internal rotations [22], chemical reactions [23], hydrogen bonded complexes [24], electronic excitations [25], ion-atom collision [26], atom-field interaction [27], chaotic ionization [28], conservation of orbital symmetry [29], atomic shell structure [30], solvent effects [31], confined systems [32], electric field effects [33], and toxicity [34], In the present chapter, will restrict ourselves to mostly the work done by us. For an elegant review which showcases the contributions from active researchers in the field, see [4], Atomic units are used throughout this chapter unless otherwise specified. 

The i.r. and Raman data suggest a strongly hydrogen-bonded complex, with the association at the carbonyl oxygen of the phenazone and not at the nitrogen. Infrared studies of the complex in carbon tetrachloride solution have been used to calculate a stability constant for the complex (1 1), and cryoscopic measurements on dilute aqueous solutions show the molecular com-... 

The association constants K for hydrogen-bonded dimer formation in chloroform solution by 9-ethyladenine and by 1-cyclohexyluracil, as well as for the formation of the 1 1 hydrogen-bonded complex of the two bases, were measured by infrared methods at 4 to 58°C. At 25°C, for the mixed dimer was 30 times larger than Kaa for the 9-ethyladenine dimer and 15 times larger than for the 1-cyclo-hexyluracil dimer. From the temperature dependence of association constants, A// for association was calculated to be — 4.3 0.4kcal/mole of dimer for cyclo-hexyluracil, —4.0 0.8kcal for ethyladenine, and —6.2 + 0.6kcal for the mixed dimer. The entropy differences AS° were —11.0 + 1, —11.4 2, and —11.8 1.2 e.u., respectively. The method used for the calculations was based on work by Lord and Porro (1960). 

Scaffold 19 with two carboxyl groups intersecting at an angle of 120" forms hydrogen-bonded complexes by host-guest interaction with weak bases and exhibits large association constants Thus. 2-aminopyrimidine... 

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