Proton hydration number

The complete hydration shell of the proton consists of both the central FI O unit and fiirther associated water molecules mass spectrometric evidence would suggest that a total of four water molecules fomr the actual FIgOj unit, givmg a hydration number of four for the proton. Of course, the measurement of this number by... 

There has been considerable discussion about the extent of hydration of the proton and the hydroxide ion in aqueous solution. There is little doubt that this is variable (as for many other ions) and the hydration number derived depends both on the precise definition adopted for this quantity and on the experimental method used to determine it. H30" has definitely been detected by vibration spectroscopy, and by O nmr spectroscopy on a solution of HF/SbFs/Ha O in SO2 a quartet was observed at —15° which collapsed to a singlet on proton decoupling, 7( 0- H) 106 Hz. In crystalline hydrates there are a growing number of well-characterized hydrates of the series H3O+, H5O2+, H7O3+, H9O4+ and H13O6+, i.e. [H(0H2) ]+ n = 1-4, Thus... 

The sequential reactions 4.1 and 4.2 represent the self-dissociation of water as the exchange of a proton between water molecules, where hydration of the proton according to reaction 4.2 is the driving force for its separation (reaction 4.1) although the proton hydration is not limited to one H20 (hydration number 1), nor is the occurrence of unhydrated OH ion realistic, the overall reaction 4.3 is generally written as the simplest form to show the principle of proton acidity. 

It is clear from the above equations that numerous parameters (proton exchange rate, kcx = l/rm rotational correlation time, tr electronic relaxation times, 1 /rlj2e Gd proton distance, rGdH hydration number, q) all influence the inner-sphere proton relaxivity. Simulated proton relaxivity curves, like that in Figure 3, are often used to visualize better the effect of the... 

A primary hydration number of 6 for Fe + in aqueous (or D2O) solution has been indicated by neutron diffraction with isotopic substitution (NDIS), XRD, 16,1017 EXAFS, and for Fe " " by NDIS and EXAFS. Fe—O bond distances in aqueous solution have been determined, since 1984, for Fe(H20)/+ by EXAFS and neutron diffraction, for ternary Fe " "-aqua-anion species by XRD (in sulfate and in chloride media, and in bromide media ), for Fe(H20)g by neutron diffraction, and for ternary Fe -aqua-anion species. The NDIS studies hint at the second solvation shell in D2O solution high energy-resolution incoherent quasi-elastic neutron scattering (IQENS) can give some idea of the half-lives of water-protons in the secondary hydration shell of ions such as Fe aq. This is believed to be less than 5 X I0 s, whereas t>5x10 s for the binding time of protons in the primary hydration shell. X-Ray absorption spectroscopy (XAS—EXAFS and XANES) has been used... 

JHNMR spectra of zirconyl perchlorate in water-acetone mixtures at —70°C indicate that ZrIV has an average hydration number of —four.151 This is in accord with [Zr4(0H)8(H20)i6]8+ if it is assumed that only the bound water molecules are observed the lack of an NMR signal for the hydroxy protons could be due to rapid proton exchange. 

All Gd(III) chelates approved for contrast agent application have one inner sphere water molecule. The inner sphere proton relaxivity is linearly proportional to the hydration number q (Eq. (5)), thus a higher q would result in increased relaxivities. However, ligands that leave space for more than one water molecule in the first coordination sphere form complexes of reduced stability,... 

Information on the hydration state of the Gd(III) chelate in solution is indispensable for the analysis of its proton relaxivity Several methods exist to determine q, though they are mostly applicable for other lanthanides than Gd(III). In the case of Eu(III) and Tb(III) complexes, the difference of the luminescence lifetimes measured in D20 and H20 can be related to the hydration number [15, 16]. For Dy(III) chelates, the lanthanide induced 170 chemical shift of the bulk water is proportional to the hydration number [17]. Different hydration states of the same chelate may also coexist in solution giving rise to a hydration equilibrium. Such an equilibrium can be assessed by UV-Vis measurements on the Eu(III) complex [18-20]. These techniques have been recently discussed [21]. 

The NMR spectrum of an aqueous A1(C104)3 solution in [Dgjacetone shows nicely the two different signals of bulk water and hydration water in the AP inner shell, even at room temperature [245]. The addition of acetone slows down the proton exchange rate. A primary hydration number of six for Al has been obtained in this way [245]. 

As we have seen above, a large number of parameters (proton exchange rate, kex = 1/Tm5 rotational correlation time, r, electronic relaxation times, 1/Ti 2e> Gd - proton distance, hydration number, q) influence the inner sphere proton relaxivity. If the proton exchange is very slow (Ti , t ), it will be the only limiting factor (Eq. (5)). If it is fast (t Ti ,), proton relaxivity will be determined by the relaxation rate of the coordinated protons,, which also depends on the rate of proton exchange, as well as on rotation and electronic relaxation. The optimal relationship is ... 

In Section 2.15, methods for obtaining the propaties of individual ions (their hydration numbers, heats, and entropies) have been considered. Starting with a general method—extrapolation to eliminate the effect of a partner on the value of the (easily obtainable) corresponding electrolyte property—two special cases were dealt with how one obtains the individual values of the heat of hydration of the proton and then its entropy. 

The first detection of stratospheric positive ions by rocket borne mass spectrometer (Arnold et al., 1977) showed that above 45 km the most abundant ions were proton hydrates (H+- (H20)n) while below that altitude non-proton hydrates (NPH) of masses 29 2, 42 2, 60 2, and 80 2 were dominant. Arijs et al. (1978) also observed ions with a mass number of 96 2. 

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