Proton donor-acceptor pairs

It should be emphasized here that electron donor-acceptor combinations are not the only ones giving rise to mesophase induction or variation. Similar phase behavior is also known for binary systems composed of suitable proton donor-acceptor pairs due to hydrogen bridging [19, 20], or for combining components of complementary molecular shapes as, for example, in the so-called filled smectics [21]. In... 

The generalization was based on the introduction of the concept of donor-acceptor pairs into the theory of acids and bases this is a fundamental concept in the general interpretation of chemical reactivity. In the same way as a redox reaction depends on the exchange of electrons between the two species forming the redox system, reactions in an acid-base system also depend on the exchange of a chemically simple species—hydrogen cations, i.e. protons. Such a reaction is thus termed proto lytic. This approach leads to the following definitions ... 

Acids are proton donors (electron-pair acceptors). 

The protons are dissociated away in contact with the water in the internal channels. Center. A covalent bonding of proton donor-acceptor molecules and a sufficiently dense stacking leads to a solvent free proton transport. Bottom. In the soggy sand electrolytes anions are absorbed at the surfaces of the insulating matrix (e.g., SiOJ. The respective cations (e.g., Li+) are free while far away from the matrix essentially associated in form of ions pairs if the solvent is a weak dielectric. 

The photoinduction ion flux derives from the similarity of vesicle systems to the proton flux in halobacterium halobium cell envelopes in the bacteriorhodopsin photocycle [126]. Liposome permeability to glucose can similarly be induced by photoexdtation in vesicles containing polyacetylene or thiophene as ion mediators [127]. As in planar bilayers, the surface charge [128] of the vesicle and the chain length of the component surfactant [129] influence assodation between the donor-acceptor pairs, and hence the distance of separation of components inside and outside the vesicle walls. 

Based on chemical considerations alone, ribozymes should be able to catalyze many different types of reactions. Ribozymes can maintain defined secondary and tertiary structures, just as protein enzymes do. Ribozymes can interact with substrates specifically via hydrogen bond networks, just as protein enzymes do. Finally, ribozymes have available to them a chemistry that, while more limited than of proteins, is substantial. RNA contains proton donors and acceptors with pK, values that cluster at 4 and 9.71 The critical lack of a good donor/acceptor with a pKt near 7 can be rectified by any of several simple expedients, such as modification of guanosine to 7-methylguanosine, protonation of triple base-paired cysto-sine,72 or inclusion of a proton donor/acceptor in an environment with a different polarity than water (in this respect, it is interesting to note that Dahm and Uhlenbeck have found that the cleavage reaction catalyzed by the hammerhead ribozyme is dependent on some dissociable proton with a pKa of 8.0).73... 

Further insight into the nature of proton-coupled electron transfer can be obtained from donor-acceptor pairs featuring an asymmetric hydrogen bonding in-... 

The system 2-naphthylamine/triethylamine forms proton donor/acceptor interaction, which was investigated in the excited state by measuring time-resolved fluorescence spectra. While the similar 2-naphthol/triethylamine system affords the ion pair interaction, via the hydrogen bond complex, the 2-naphthylamine/triethylamine system presents hydrogen bond interaction which shows a low-temperature absorption with 7max = 370 nm, and Amax = 370 nm in the fluorescence spectrum148. 

A still more general theory of acids and bases than the proton donor-acceptor theory was introduced by G. N. Lewis. He called a base anything that has available an unshared pair of electrons such as NHg, H... 

A hydrogen bond describes the attractive force that arises between the proton donor covalent pair A-H, with its significant bond dipole, and the noncovalently bound nearest-neighbor electronegative hydrogen acceptor atom B (Structure 2). 

Experimental work by Janda et al. [108] has indicated that HCl acts as proton donor when paired with HF. With regard to ab initio studies, Hobza et al. [106, 109] found from calculations with fairly small basis sets that HF HCl is likely to be somewhat more stable than HCl HF, i.e. HCl acts as proton donor. More recent work using doubly polarized sets [103] indicates the difference in energy between these two geometries is quite small, perhaps 0.25 kJ/mol. The angular features of the complexes are reminiscent of the (HF)2 or (HC1)2 homodimers above. In either case, the hydrogen of the proton donor remains within 7° or 8° of the F Cl axis. The orientation of the proton acceptor molecule seems to be fairly independent of the donor. That is, HCl is nearly perpendicular to the F Cl axis (fi = 93°) while this angle is 120° for HF. 

Note that it has been well-known for a long time that correlation energy should favour structures with the proton in a central position of the main atoms. Utilizing this experience (cf.Ref.l9), a potential wall between MC and IP derived from SCF calculations (cf.Refs.5-7) for very strong proton donor-acceptor systems is expected to be flattened by the inclusion of electron correlation, or changed to a SM potential. Having an SCF SM potential, which may represent a molecular complex or an ion pair, the correlation will broaden the corresponding minimum towards the middle of the bond. 

Using very strong proton donor-acceptor systems, we observe a tendency to change the 6—membered transition structures into ion pairs already... 

To reliably describe PT reactions in the gas and condensed phases, the usual parameterization of a force field in terms of harmonic bonded interactions is not sufficient. H-bonded systems are quite anharmonic around the bottom of the well for bond-stretching motions, and angular bending vibrations are equally affected. Furthermore, the hydrogen motion between the donor and acceptor atoms is strongly coupled to the donor-acceptor motion. These aspects need to be taken into account for a reliable model of hydrogen or proton motion between a donor-acceptor pair. 

Hydrogen bonding to the nitrogen lone pair leads to an upheld shift, the extent of which depends on the proton-donor ability of the solvent, and the acceptor ability of the base shifts of some 20 p.p.m. are commonly found. 

The great electron-pair acceptor capacity (Lewis acidity) of SbFs has been utilized in the production of extremely strong proton donors (Brpnsted acids, p. 48). Thus the acidity of anhydrous HF is substantially increased in the presence of SbF ... 

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