Proton clusters, hydrogen-bonded

Figure 5. Fragmented cluster model of adsorbed water showing hydrogen-bonded proton clusters in a porous glass pore...

Figure 9-1 Possible structures for the hydroiiium ion. (a) The species H9O4 has been observed in the solid state and may be an important contributor in aqueous solution, (b) The species (H20)2iH exhibits a dodecahedral caged stracture in mixed water-trimethylamine cluster ions. The hydroniiim ion (not shown) is encased in the hydrogen-bonded cage with 10 non-hydrogen-bonded protons protaiding from its surface. S. Wei, Z. Shi, and A. W. Castleman, Jr., J. Chem. Phys, 1991, 94, 3268. Structure reproduced by courtesy of the American Institute of Physics.

The increase of the A-value on protonation is the measure of the increase in size of the protonated substituent relative to the unprotonated one. This extra bulk is due to the additional proton itself and also to the additional solvent molecules attached to the positive charge needed to stabilize it The increase in protic solvent is due to hydrogen bonding, which clusters solvent molecules around the polar group [63, 64]. Since the C-O bond is shorter than the C-C bond, steric repulsimis in the tetrahydropyran system with the axially oriented 2-amino group are greater than in the cyclohexane system with the axially oriented amino group, and should be corrected to 2-2.5 kcal/mol for aprotic solvents and 2.4—2.9 kcal/mol for protic solvents. 

Forming a hydrogen bond between (i )-homocitrate and His 442 could effectively release electron density into the cluster. Studies on structurally defined N2 complexes have shown that the binding of N2 to a metal site and its ability to be protonated are favored by electron-rich sites. Thus, it is postulated that the electron-richness of the... 

Water forms hydrogen-bonded clusters with itself and with other proton donors or acceptors. Hydrogen bonds account for the surface tension, viscosity, liquid state at room temperature, and solvent power of water. 

Other studies conducted on mixed protonated clusters of ammonia bound with TMA showed that the ion intensity distributions of (NH3)n(TMA)mH+191 display local maxima at (n,m) = (1,4), (2,3), (2,6), (3,2), and (3,8). Observation that the maximum ion intensity occurs at (n,m) = (1,4), (2,3), and (3,2) indicates that a solvation shell is formed around the NHJ ion with four ligands of any combination of ammonia and TMA molecules. In the situation where the maximum of the ion intensity occurs at (n,m) = (2,6) and (3,8), the experimental results suggest that another solvation shell forms which contains the core ions [H3N-H-NH3]+ (with six available hydrogen-bonding sites) and [H3N-H(NH2)H-NH3]+ (with eight available hydrogen-bonding sites). The observed metastable unimolecular decomposition processes support the above solvation model. 

A. Fujii, S. Enomoto, M. Miyazaki, and N. Mikami, Morphology of protonated methanol clusters An infrared spectroscopic study of hydrogen bond networks of H+(CH30H)n (n 4 15). 

Hoffman and collaborators (Doan et al. 1994) also performed ENDOR experiments on the oxidized [3Fe-4S] cluster in D. gigas hydrogenase. The authors detected resonances from strongly coupled protons which were assigned to the [3-CH2 of the cysteines and exchangeable protons that are probably involved in three different hydrogen bonds to the sulfurs of the cluster. Based on these data a model for the binding of the cluster to the protein was developed. 

Variations of R with A suggest a two-step hydration process solvation and formation of disconnected water clusters centered on polar head groups, followed by the formation of a continuous hydrogen-bond network. At low A, Ri depends logarithmically on co, suggesting bidimensional diffusion of protons in the interfacial region between polymer and water. 

The situation for hydrated Nafion in the acid form, or as containing aqueous acids or strong bases, is more complex because protons and defect protons (i.e., OH ions), migrate according to a somewhat different mechanism. Proton transfer in either case occurs throughout and between clusters of hydrogen bonded water molecules to a degree that depends on the relative water content. 

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