Proton transfer, hydrogen bonds aqueous systems

The effect of the solvent is usually modelled either by the use of the Onsager s self consistent reaction field (SCRF) [20] or by the polarizable continuum method (PCM) [21]. With regard to the relative stability of cytosine tautomers in aqueous solution, these methods provided results [14,15] which, in spite of some discrepancies, are in reasonable agreement with experimental data [3]. However, continuum-based methods do not explicitly take into consideration the local solvent-solute interaction which is instead important in the description of the proton transfer mechanism in hydrogen-bonded systems. A reasonable approach to the problem was recently proposed [22,23] in which the molecule of interest and few solvent molecules are treated as a supermolecule acting as solute, while the bulk of the solvent is represented as a polarizable dielectric. 

The purpose of this review has been to illustrate and document the kinds of information about non-aqueous solvent systems which have been obtained by vibrational spectroscopy. We have seen that these include insight into intermolecular forces and structure of the pure solvents, the nature of the solvation shell around ions and their solvation numbers, the identification of ion pairs and complexes, measurement of mass law constants and their dependence on the polarity of the solvent, the detection and characterisation of the hydrogen bond and measurement of acid and base strengths. Little kinetic data have so far been obtained by Raman spectroscopy but recent progress in the study of ultra-fast proton transfer and the detection of associated ions of type [Br , (Bra)] during the bromination of acetic acid presage considerable advance in this area in the future. ... 

We can safely conclude that, for the proton transfer reactions to carbon discussed here, there are no solvent bridges and the ratedetermining step is the direct proton transfer from the acid to the carbon base. This is because there are no hydrogen bonds between the receiving carbon and the aqueous solvent. These systems therefore contrast with the oxygen and nitrogen bases discussed by Grunwald and Eustace (Chapter 4). 

Addition of 0- to double bonds and to aromatic systems was found to be quite slow. Simic et al. (1973) found that O- reacts with unsaturated aliphatic alcohols, especially by H-atom abstraction. As compared to O, HO reacts more rapidly (by two to three times) with the same compounds. In the case of 1,4-benzoquinone, the reaction with O consists of the hydrogen double abstraction and leads to the 2,3-dehydrobenzoquinone anion-radical (Davico et al. 1999, references therein). Christensen et al. (1973) found that 0- reacts with toluene in aqueous solution to form benzyl radical through an H-atom transfer process from the methyl group. Generally, the O anion-radical is a very strong H-atom abstractor, which can withdraw a proton even from organic dianions (Vieira et al. 1997). 

Cis dihydrohumulinic acid (136, Fig. 66) has a melting point of 98°C. It is obtained from the CCD band with K 1.23 after 100 transfers in the two-phase system ether aqueous buffer pH 7.65. (+) Trans dihydrohumulinic acid (137, Fig. 66) melts at 126°C and has a K value of 1.0 in the same two-phase CCD system. The spectroscopic characteristics, particularly the absence of the H NMR resonances for methyl and methine protons on a carbon-carbon double bond, prove that the double bond of the humulinic acids has been hydrogenated. 

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