Double protonation

Alternatively, proton double quantum (DQ) NMR, based on a combined DQ excitation and a reconversion block of the pulse sequence, has been utilized to gain direct access to residual DCCs for cross-linked systems.69,83-89 For this purpose, double-quantum buildup curves are obtained with use of a well-defined double-quantum Hamiltonian along with a specific normalization approach. Residual interactions are directly proportional to a dynamic order parameter Sb of the polymer backbone,87... 

It is difficult to see why the spectra of the ions from diphenylethylene and triphenylethylene should resemble that of the ion from anthracene if the ions have the non-classical protonated double bond structure. 

The H NMR spectram (CDClj, 500 MHz) of 12 showed two singlets (8 0.83 and 8 0.95), each integrating for three protons due to the C-18 and C-19 methyl protons. Three 3H doublets at 8 0.78 (J= 6.5 Hz), 8 0.79 (J= 6.5 Hz) and 8 0.85 (J = 7.0 Hz) were due to the secondary C-26, C-27 and C-21 methyl protons, respechvely. The C-3 methine proton resonated as a one-proton double doublet at 8 3.63 (JJ= 10.5 Hz and J2= 3.5 Hz) and its downfield chemical shift value was indicative of the presence of a geminal hydroxyl funchonality. A one-proton mulhplet at 8 5.21 was ascribed to the C-6 olefinic proton. The C-28 exocyclic methylene protons appeared as two broad singlets at 8 5.40 and 5.58. The C-NMR spectram (CDCl, 125 MHz) showed the resonance of all 28 carbon atoms. The combination of H and C-NMR data suggested that compound 12 has a sterol like structure as most of the H and C-NMR chemical shift values of 12 were similar to those of sterols reported in the literature [19, 20]. The H and C-NMR chemical shift values were assigned with the aid of COSY-45 , HSQC and HMBC spectral data. Compound 12 was found to have modest inhibitory activity against C. xerosis and S. aureus with minimal inhibitory concentration values of 82.35 and 146 pg/ml, respectively. 

In the presence of excess ammonia, the surface complex coexists with its ammonia adduct ](=SiO)2Ta(=NH)(NH2)(NH3)]. NMR studies on the fuUy N-labeled samples have led to unambiguous discrimination between imido, amido and amino resonances of the surface complex and its NH3 adduct, and [=Si- NH2] through the combination of soHd-state magic-angle spinning(MAS), heteronuclear correlation (HETCOR), 2D proton double-quantum (DQ), singlequantum (SQ) correlation, and 2D proton triple quantum (TQ) single quantum... 

The HREIMS of 2,7-dibromocarbazole (388) was identical to that of 3,6-dibromocarbazole (386). Differences were identified in the chemical shifts and splitting pattern of the H-NMR spectrum. Thus, a deshielded, two-proton doublet at 7.87 with ortho-coupling (/=8.3 Hz), assignable to the C-4 and C-5 protons, indicated the location of two bromine atoms at the C-2 and C-7 positions of the carbazole nucleus. This was also evident from the shielded ortho- and meto-coupled (/=8.3, 1.5 Hz) two-proton double doublet at S 7.36, assignable to the C-3 and C-6 protons. Further, a mutually meto-coupled (/= . 5 Hz) two-proton doublet at S 7.57, assignable to the C-1 and C-9 protons, supported the location of the two bromine atoms ortho to these carbons. Based on these spectral analyses and the HREIMS identical to that of... 

Also in the 1930s, detailed studies about the thermodynamic stability of adducts of silver(I) with olefins were carried out by Howard Lucas and coworkers, who determined the equilibrium constants between the hydrated Ag+ ion and the corresponding cationic olefin silver(I) complex in dilute aqueous solutions of silver nitrate [25]. In the context of this work, Saul Winstein and Lucas made an initial attempt to describe the interaction between Ag+ and an olefin by quantum mechanics [26]. Assisted by Linus Pauling, they explained the existence of olefin silver(I) compounds in terms of resonance stabilization between the mesomeric forms shown in Fig. 7.4. Following this idea, Kenneth Pitzer proposed a side-on coordination of Ag+ to the olefin in 1945 and explained the stability of the corresponding 1 1 adducts as due to an argentated double bond , in analogy to his concept of the protonated double bond [27]. He postulated that the unoccupied s-orbital of silver(l) allowed the formation of a bond with the olefin, similar to the s-orbital of the proton. 

There is a strong similarity between this picture of the protonated double bond and the conception of the bonding of silver ions to an olefin in complexes such as AgC104.cyclo-hexene,. benzene etc., in which the silver ion is embedded in... 

One early and insightful model for diborane is the protonated double-bond model shown in Figure 1.12. Symmetrical protonation of the it bond of ethylene above and below the molecular plane leads to the structure of diborane. Replacement of each C in ethylene with [B] leads to [B2H4]2. In fact Li2B2R4, where R is a bulky substituent, has been structurally characterized and shown to possess B-B multiple-bond character. 

The molecular formula for a hydrocarbon with the butterfly structure of tetraborane is C4H6. The structure with two double bonds is 1,3-butadiene shown alongside. Is it a surprise then that the structure shown below was once proposed for B4H10, i.e., two protonated double bonds in a trans arrangement ... 

An alternative structure has been suggested by Pitzer which may be referred to as a protonated double bond and is represented schematically in IX ... 

The concepts discussed above, permit proposals to be made concerning the structure of other boron hydrides which have not been subjected to such an extensive experimental investigation as has diborane. The proposed formulae fall into two groups, that favouring the resonance of structures analogous to those represented schematically for diborane in IV to VIII (Syrkin and Dyatkina ) and that employing the concept of the protonated double bond (Pitzer ). ... 

As pointed out above, it is doubtful whether resonance alone would be able to link the two outer boron atoms to the central group, although the ready dissociation of this molecule is explained by the looseness of this bond. A more fundamental objection is the small value of the B B H and B+B H angles in the central group of atoms. The protonated double bond structure is H H... 

Figure 18.21. Proton Transport by Cytochrome C Oxidase. Four "chemical" protons are taken up from the matrix side to reduce one molecule of O2 to two molecules of H2O. Four additional "pumped" protons are transported out of the matrix and released on the cytosolic side in the course of the reaction. The pumped protons double the efficiency of free-energy storage in the form of a proton gradient for this final step in the electron-transport chain.

Figure 1. The major transmembrane photosynthetic reaction centers (RC) (top) and respiratory complexes (bottom) are composed of light (zigzag) activated chains (dark gray) of redox centers (open polygons) that create a transmembrane electric field and move protons (double arrows) to create a transmembrane proton gradient, fulfilling the requirements of Mitchell s chemiosmotic hypothesis. Diffusing substrates include ubiquinone (hexagon) and other sources of oxidants and reductants. PSI and PSII, photosystems I and II, respectively.

The preparation of thioacetals involves treatment of the carbonyl substrate with a dithiol in the presence of an acid catalyst, usually TsOH or Bp3 OEt2. Since thioacetals are quite stable toward hydrolysis, there is no special need to remove the H2O formed during the reaction. Also, since it is more difficult to equilibrate thioacetals than acetals via protonation, double bond migration in thioacetalization of enones is usually not observed. 

Another approach is to intentionally select a very small coupling optimization such that integer multiples of that coupling will be in phase and read out at the end of the experiment with maximal intensity. The author (GEM) and a co-worker reported this approach using a 1.75 Hz optimization in the acquisition of proton double quantum coherence of strychnine (1) many years ago with some degree of success. 

The solute molecule is dissolved in the liquid crystal solvent at low concentration. A variety of nematic solvents are available, some of which are nematic at room temperature. Representative high-resolution proton NMR spectra are given in Figure 1. Because the solvent order depends on composition and temperature, it is important that temperature and composition gradients at the NMR probe be minimized if the narrow line widths of a few hertz are to be obtained. The spectra of Figure 1 show the rapid increase of spectral complexity with the number of nuclei. The spectra become almost continuous and uninterpretable at about 10 spins. Simplified proton NMR spectra can be obtained by partial deuterium substitution and decoupling.6 This has been described for cyclohexane, but has not been used extensively. Proton double resonance is also a useful experimental technique for the identification of spectral lines.6... 

The method of molecular orbitals has been applied to the problem of diborane by Mullikan9 who considered both the ethane-like structure and the bridge structure XI which is really identical with the protonated double... 

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