Mobile proton

In amine-imine systems (75 76) the mobile proton can in principle be located at either... 

Most organic reactions are done in solution, and it is therefore important to recognize some of the ways in which solvent can affect the course and rates of reactions. Some of the more common solvents can be roughly classified as in Table 4.10 on the basis of their structure and dielectric constant. There are important differences between protic solvents—solvents fliat contain relatively mobile protons such as those bonded to oxygen, nitrogen, or sulfur—and aprotic solvents, in which all hydrogens are bound to carbon. Similarly, polar solvents, those fliat have high dielectric constants, have effects on reaction rates that are different from those of nonpolar solvent media. 

Interconversion between two tautomeric structures can occur via discrete cationic or anionic intermediates (scheme 24, where T is an anion capable of reacting with a proton at a minimum of two distinct sites). Alternatively, interconversion can occur by simultaneous loss and gain of different protons (scheme 25, w here T has the same definition as in scheme 24). These mechanisms are well established for acyclic compounds, but they have been much less thoroughly investigated for heteroaromatic systems. The rate of interconversion of two tautomers is greatest when both of the alternative atoms to which the mobile proton can be attached arc hetero atoms, and isolation of the separate isomers is usually impossible in this case. If one of the alternative atoms involved in the tautomerization is carbon, the rate of interconversion is somewhat slower, but still fast. When both of the atoms in question are carbon, however, interconversion is... 

The existence of strongly polar water molecules and mobile protons also makes H2O an excellent and almost universal solvent for ionic... 

In a typical study of conductivity. Cook (1982) used a cell consisting of two platinum disc electrodes, 12 mm in diameter and 1-5 mm apart. The setting AB cement was examined in this cell which had been calibrated using a standard solution of0 02 M potassium chloride. Plots were recorded of spedfic conductance against time for each of the setting cements. For zinc polycarboxylate there was found to be a rapid drop in spedfic conductance about 10 minutes after the start of mixing. This behaviour was consistent with the replacement of relatively mobile protons by significantly less mobile zinc ions in the polycarboxylate chain. Con-... 

Intramolecular hydrogen bonding, steric hindrance, and location of the mobile proton on a carbon atom (Mcarbon acids ) can all act to decrease somewhat the reaction rates (5). 

Dongre A.R., Jones J.L., Somogyi A., and Wysocki G.H. (1996), Influence of peptide composition, gas-phase basicity, and chemical modification on fragmentation efficiency evidence for the mobile proton model, J. Am. Soc. Mass Spectrom. 118, 8465-9374. 

Kuck, D. Petersen, A. Fastabend, U. Mobile Protons in Large Gaseous Alkylben-zenium Ions. The 21-Proton Equilibration in Protonated Tetrabenzylmethane and Related "Proton Dances . Int. J. Mass Spectrom. 199 , 179/180, 129-146. 

The total electro-osmotic coefficient = Whydr + mo includes a contribution of hydrodynamic coupling (Whydr) and a molecular contribution related to the diffusion of mobile protonated complexes—namely, H3O. The relative importance, n ydr and depends on the prevailing mode of proton transport in pores. If structural diffusion of protons prevails (see Section 6.7.1), is expected to be small and Whydr- If/ ori the other hand, proton mobility is mainly due to the diffusion of protonated water clusters via the so-called "vehicle mechanism," a significant molecular contribution to n can be expected. The value of is thus closely tied to the relative contributions to proton mobility of structural diffusion and vehicle mechanism. ... 

This section intends to provide the reader with some examples of how the high relaxivity challenge has been tackled for Gd(III) complexes so far. For the sake of clarity, this section has been divided in three sub-sections in relation to the specific relaxation parameter considered for relaxivity enhancement, namely the hydration state of the metal centre, the tumbling rate of the CA, and the exchange rate of the mobile protons coordinated to the paramagnetic center. 

Fig. 11. Schematic representation showing the network of mobile protons close to a negatively charged Gd(III) complex bound to a protein.

The saturation transfer is induced by the selective irradiation of the mobile protons of the probe with a proper radiofrequency field. When different pools of exchangeable protons are available, this modality enables the set-up of ratiometric methods in which a responsive PARACEST agent provides a MRI response no longer dependent on its absolute concentration (119-120). 

Chemical Nature of Species Associated uith Mobile Protons in Coals A Study by Field Ionization Mass Spectrometry... 

Proton n.m.r. investigations of coals swollen in deuterated pyridine showed that the free induction decay /FID/ consists of Gaussian and Lorentzian components related to two populations of protons which have widely different degrees of rotational mobility (1-5). The Gaussian component of FID has been unanimously ascribed to the macromolecular part of the coal matter that is supposed to have very limited rotational mobility. These publications as well as the ensuing debates (6-9) however, reflected the controversy regarding the nature of the Lorentzian /mobile/ protons in coals. 

A more realistic approach is a separation of a mixture of free molecules and structural units linked to the network by single bonds, after these bonds have been cleaved by heat treatment of the coal. An insight into the composition of such mixture can provide information on the nature of species that are likely to be associated with the mobile protons. It seems that pyrolysis-field ionization mass spectrometry can be used in order to attain this goal. 

Proton n.m.r. measurements of Zeeman relaxation for the same coal showed (Jurkiewicz,A., Colorado State University, personal communication, 1988) that mobile protons represent 32% of all protons. Assuming that the content of hydrogen in the mobile and immobile phases of the coal is approximately the same, the relaxation measurements indicate that the coal mobile phase content is app. 32 wt % of organic material. 

Morrison CA, Siddick MM, Camp PJ, Wilson CC (2005) Toward understanding mobile proton behavior from first principles calculation the short hydrogen bond in crystalline urea-phosphoric acid. J Am Chem Soc 127 4042 048... 

In amine-imine systems (75 76) the mobile proton can in principle be located at either of the two basic nitrogen sites in the anion (77). Since the canonical form with aromatic (benzenoid) structure is polar in the imine (76b) and non-polar in the amine (75a), the amine structure should be favoured, particularly in non-polar solvents. This seems generally to be the case, although solvent and medium effects do not appear to have been investigated the results available at the present time refer mainly to aqueous and other polar solvents. 

As in the case of cytosine, several NMR and NQR studies were performed in search of the predominating tautomeric structures of uracil and thymine and their nucleotides and nucleosides. Investigation of PMR spectra of these compounds in nonaqueous solvents, such as dimethyl sulfoxide, localized the mobile protons in a number of 5- and 6-substituted uracils.59,61,328 These and similar studies63,85,329,330 indicated that dilactam structure 32 predominates in uracil compounds in aqueous and nonaqueous solutions as well as in the solid state. Proton and N-15 magnetic resonance spectra of several pyrimidines85 confirmed the diketo structure usually ascribed to uracil. 

In Sections II and IV we have described the application of NMR and NQR spectroscopy to the determination of the localization of mobile protons of the pyrimidine bases and for the prediction of the tautomeric structures of these molecules. Here, we shall describe the application of these spectroscopies to the elucidation of the electronic structures of the most stable tautomeric forms of the bases. 

---