Proton delocalization

C.A. Chatzidimitriou-Dreismann, E.J. Brandas, Proton Delocalization and Thermally Activated Quantum Correlations in Water Complex Scaling and New Experimental Results, Ber. Bunsenges. Phys. Chem. 95 (1991) 263. 

The extraordinary proton delocalizing effect of Pi encountered in Pt hydride clusters is another factor which may also be related to the quite unique efficiency of this metal in proton exchange. 

The Arg-82 side chain moves away from the now protonated Asp-85 (11), toward the Glu-194/Gln-204 pair, and a proton delocalized in the aqueous network is released to the surface. The rise of the M-state occurs with multi-exponential kinetics that originates partly from the additional step of proton re-equilibration within the Schiff base-Asp-85 pair as the proton is released to the bulk. At this stage, the pKa of Asp-85 will have risen considerably as the result of its long-range coupling (5) to the proton release site through Arg-82, by about five units, and if the Schiff base is to be reprotonated, it must be by a proton donor other than Asp-85. In the next steps, a suitable proton donor will be generated on the cytoplasmic side. 

The most significant result which comes out of this work is that Mg(OH)2, Ca(OH)2 and Al(OH)3 may be protonic semiconductors that can be described by highly localized protons occupying a valence band, VB, and the presence of a conduction band, CB, providing for proton delocalization. Between VB and CB lie intermediate levels, corresponding to the vibrationally excited states of the O-H oscillator, separated by Si 0.4 eV. The gap values are 2.0-2.1 eV for Mg(OH)2,2.2 eV for the more basic Ca(OH)2 and 1.6-1.7 eV for the more acidic Al(OH)3. 

Water-molecule dimers are proposed to be the main, low-energy, component of liquid water. The molecular distances under interaction of two polarized dimers are presented in Fig.la and in Supplementary Tablel. This molecular structure is consistent with measurements of X-ray and neutron scattering [17], and with recent reports of X-ray absorption and emission spectroscopy, revealing proton delocalization [18], strong electron sharing [19] and double, rather than tetrahedral. 

An interesting case are the a,/i-unsaturated ketones, which form carbanions, in which the negative charge is delocalized in a 5-centre-6-electron system. Alkylation, however, only occurs at the central, most nucleophilic position. This regioselectivity has been utilized by Woodward (R.B. Woodward, 1957 B.F. Mundy, 1972) in the synthesis of 4-dialkylated steroids. This reaction has been carried out at high temperature in a protic solvent. Therefore it yields the product, which is formed from the most stable anion (thermodynamic control). In conjugated enones a proton adjacent to the carbonyl group, however, is removed much faster than a y-proton. If the same alkylation, therefore, is carried out in an aprotic solvent, which does not catalyze tautomerizations, and if the temperature is kept low, the steroid is mono- or dimethylated at C-2 in comparable yield (L. Nedelec, 1974). 

The first step protonation of the double bond of the enol is analogous to the pro tonation of the double bond of an alkene It takes place more readily however because the carbocation formed m this step is stabilized by resonance involving delocalization of a lone pair of oxygen... 

Notice too that the carbonyl oxygen of the carboxylic acid is protonated m the first step and not the hydroxyl oxygen The species formed by protonation of the car bonyl oxygen is more stable because it is stabilized by electron delocalization The pos itive charge is shared equally by both oxygens... 

Electron delocalization in carbonyl protonated benzoic acid... 

A key feature of the first stage is the site at which the starting ester is protonated Protonation of the carbonyl oxygen as shown m step 1 of Figure 20 4 gives a cation that IS stabilized by electron delocalization The alternative site of protonation the alkoxy oxygen gives rise to a much less stable cation... 

Protonation of imidazole yields an ion that is stabilized by the electron delocalization represented in the resonance structures shown... 

NMR data for 4-methyloxazole have been compared with those of 4-methylthiazole the data clearly show that the ring protons in each are shielded. In a comprehensive study of a range of oxazoles. Brown and Ghosh also reported NMR data but based a discussion of resonance stabilization on pK and UV spectral data (69JCS(B)270). The weak basicity of oxazole (pX a 0.8) relative to 1-methylimidazole (pK 7.44) and thiazole (pK 2.44) demonstrates that delocalization of the oxygen lone pair, which would have a base-strengthening effect on the nitrogen atom, is not extensive. It must be concluded that not only the experimental measurement but also the very definition of aromaticity in the azole series is as yet poorly quantified. Nevertheless, its importance in the interpretation of reactivity is enormous. 

The results obtained for 1-phenylpyrazole (32) and its conjugate acid (34) are consistent with those of Minkin. The bond order between thd two rings decreases by protonation (from 0.341 to 0.241) and this is in agreement with the expected effect of the N-2 positive charge on the delocalization of the lone pair on N-1 over the phenyl ring. 

TT-electron delocalization over the O—C—N system. No such delocalization is possible in the N-protonated form. 

The EPR spectrum of the ethyl radical presented in Fig. 12.2b is readily interpreted, and the results are relevant to the distribution of unpaired electron density in the molecule. The 12-line spectrum is a triplet of quartets resulting from unequal coupling of the electron spin to the a and P protons. The two coupling constants are = 22.38 G and Op — 26.87 G and imply extensive delocalization of spin density through the a bonds Note that EPR spectra, unlike NMR and IR spectra, are displayed as the derivative of absorption rather than as absorption. 

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