Proton acids, survey

All of the azines are weak bases, in which the natures and positions of substituents alter the ease and orientation of N-protonation. This review does not aim to provide a comprehensive survey of basic characteristics within the series, but merely highlights the reactivities of particular ring nitrogen atoms with proton acids, since these reactivities are frequently related to trends observed in N-alkylation, N-acylation, and N-oxidation. 

From this survey we found that several proton acids catalyze the reduction of ketimines using Hantzsch dihydropyridine 2 as the hydride donor. With regard to the different acids tested diphenyl phosphate... 

Data on the protonation sites for a variety of azaindolizines have been summarized in the previous survey (76AHCS1, p. 536).Tlie measured magnetic circular dichroism spectra for 14 azaindolizines confirmed with some corrections the protonation sites of polyazaheterocycles previously reported and also established the conjugated acid structures of additional compounds (85JOC302).Tlie data are summarized in Scheme 1. 

This survey focuses on recent developments in catalysts for phosphoric acid fuel cells (PAFC), proton-exchange membrane fuel cells (PEMFC), and the direct methanol fuel cell (DMFC). In PAFC, operating at 160-220°C, orthophosphoric acid is used as the electrolyte, the anode catalyst is Pt and the cathode can be a bimetallic system like Pt/Cr/Co. For this purpose, a bimetallic colloidal precursor of the composition Pt50Co30Cr20 (size 3.8 nm) was prepared by the co-reduction of the corresponding metal salts [184-186], From XRD analysis, the bimetallic particles were found alloyed in an ordered fct-structure. The elecbocatalytic performance in a standard half-cell was compared with an industrial standard catalyst (bimetallic crystallites of 5.7 nm size) manufactured by co-precipitation and subsequent annealing to 900°C. The advantage of the bimetallic colloid catalysts lies in its improved durability, which is essential for PAFC applicabons. After 22 h it was found that the potential had decayed by less than 10 mV [187],... 

The proton resonances of the common amino acids, which were recently surveyed by McDonald and Phillips (78), and by Roberts and Jardetzky (94), are all in the spectral region from DSS to —10 ppm. The spectrum... 

Whereas the subject of basicity of C=0 groups has already been covered by Palm and coworkers1, in this chapter we summarize more recent results and significant developments in the field of nitrones, nitriles and thiocarbonyls. This chapter is not exhaustive in scope, but rather consists of surveys of the most recent two decades of work in this still developing area. As indicated by the title of this contribution, we emphasize the more physical aspects such as acidity, basicity and proton affinity less attention is paid to synthesis, structure and bond theory which can be found in other specialized chapters of this book. 

The present volume covers Muscle and Molecular Motors . The first few chapters describe the ultrastructures of striated muscles and of various muscle filaments (myosin, actin, titin), they discuss the regulation of muscle contractile activity, and they explore the mechanism of force production and movement. The book then sets out to survey other kinds of motor systems microtubules and their interactions with both microtubule associated proteins (MAPs) and the motor proteins kinesin and dynein, the major sperm protein in nematodes, the rotary ATPases driven by or driving proton gradients, and the action of motor enzymes, polymerases, on nucleic acids. The aim throughout is to explore different molecular mechanisms of motor action in order to identify common themes. 

Since both cocrystals and salts are multicomponent crystalline forms, it is clear that the distinction between the two depends on the degree of proton transfer between the donor and the acceptor. In this view, salts would be characterized by effectively complete proton transfer, while cocrystals would exhibit proton sharing with little or no transfer [17,18]. In a survey study of over 80 salts and cocrystals prepared by the interaction of carboxylic acids and N-heterocyclic compounds, it was reported that structure prediction and targeted synthesis appeared to be more difficult for salts than for cocrystals [19]. 

George Olah published a perspective in which he provided an overview of the research of his group over five decades.1 Christopher Reed surveyed the chemistry of carborane acids, the strongest pure Brpnsted acids known, with a conjugate base that engages in little chemistry.2 These acids are employed to prepare highly acidic carbocations, including protonated benzene, protonated C6o, tertiary carbocations, vinyl cations, and silylium ions. The synthesis and properties of unusual... 

There is an interesting structural similarity between (66) and the other subjects of this section -(67) [149], (68) [150], (69) [151] and (70) [152,153] - in that they are all composed of 4-aminophenyl groups substituted at a meso position of a linearly fused tricyclic aromatic frame. The similarity continues even in their experimental behaviour in that protonation causes significant fluorescence enhancements in all cases. The structural elaboration of the aniline unit in (70) allows similar switching on with calcium ions. The case (70) is taken up further in Sect. 7. Additionally, (69) with its very electron deficient acridinium acceptor unit shows extensive perturbation of absorption spectra by several cations. Again, the acidity constants are experimentally identical in both the ground and excited singlet states. We close this final section of the survey... 

The three-center bonds represent — 70% of the total number of hydrogen bonds in the crystal structures surveyed (Thble 2.3). This is a significantly higher proportion than in the other biological molecules, and was attributed to proton deficiency, which occurs because the amino acids form zwitterionic crystal structures where the predominant hydrogen bonding is between the -NH3 and the... 

Proteins contain, on average, more acceptor than donor sites [596]. Similar proton deficiency in the amino acid zwitterion crystal structures results in the formation of three-center hydrogen bonds rather than in unsatisfied acceptor sites [74] (Part I A, Chap. 2.6). There is less flexibility in the orientation of hydrogen-bond donor and acceptor groups in proteins which would lead to a relatively larger number of unsatisfied acceptor sites. Some of the more unsymmetrical three-center bonds which might have been missed in the survey [596] because of the 3.5 A X - A cut-off limit will also contribute to reduce the number of unsatisfied acceptors in side-chains. 

This survey focuses on recent catalyst developments in phosphoric acid fuel cells (PAFC), proton exchange membrane fuel cells (PEMFC), and the previously mentioned direct methanol fuel cell (DMFC). A PAFC operating at 160-220 °C uses orthophosphoric acid as the electrolyte the anode catalyst is Pt and the cathode can... 

Scheme 7.21). A survey of different chiral proton sources showed that it was necessary to use a P-amino alcohol rather than an alcohol, an amine, or an acid as chiral inductor. 

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