Hydration number neutron diffraction

Although X-ray and neutron diffraction and scattering methods give only approximate estimates of hydration numbers they can provide precise measures of ion-water distances in solution. In calcium chloride and bromide solutions of various concentrations, Ca-0 distances of between 2.40 and 2.44 A have been reported (167,168,171,172) Ca-0 — 2.26A was claimed in an early X-ray investigation of molar calcium nitrate solution (167,186). EXAFS and LAXS studies showed a broad and asymmetric distribution of Ca-0 distances centered on a mean value of 2.46 A (174). 

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... 

X-ray and neutron diffraction methods and EXAFS spectroscopy are very useful in getting structural information of solvated ions. These methods, combined with molecular dynamics and Monte Carlo simulations, have been used extensively to study the structures of hydrated ions in water. Detailed results can be found in the review by Ohtaki and Radnai [17]. The structural study of solvated ions in lion-aqueous solvents has not been as extensive, partly because the low solubility of electrolytes in 11011-aqueous solvents limits the use of X-ray and neutron diffraction methods that need electrolyte of -1 M. However, this situation has been improved by EXAFS (applicable at -0.1 M), at least for ions of the elements with large atomic numbers, and the amount of data on ion-coordinating atom distances and solvation numbers for ions in non-aqueous solvents are growing [15 a, 18]. For example, according to the X-ray diffraction method, the lithium ion in for-mamide (FA) has, on average, 5.4 FA molecules as nearest neighbors with an... 

Aqueous solutions of nickel(II) salts in the absence of strong coordinating species are usually green because of the [Ni(H20)ft]2+ cation.1447 The number of coordinated water molecules in the inner-shell complex is now well ascertained in the temperature range -30 to 30 °C by means of electronic and both 170 and LHNMR spectra.1448-1451 The most recent value of the coordination number is 5.85 0.2.145 Neutron diffraction studies of NiCl2 in D20 solution led to estimates of the Ni—O bond distance within the [Ni(D20)6]2+ cation in the range 195-220 pm.1452 A second hydration sphere of about 15 water molecules has also been proposed. 

Tables 2.5a,b provide a comprehensive list of guest molecules forming simple si and sll clathrate hydrates. The type of structure formed and the measured lattice parameter, a, obtained from x-ray or neutron diffraction are listed. Unless indicated by a reference number, the cell dimension is the 0°C value given by von Stackelberg and Jahns (1954). Where no x-ray data exists, assignment of structure I or II is based on composition studies and/or the size of the guest molecule. Tables 2.5a,b also indicate the year the hydrate former was first reported, the temperature (°C) for the stable hydrate structure at 1 atm, and the temperatures (°C) and pressures (atm) of the invariant points (Qi and Q2). Both cyclopropane and trimethylene oxide can form si or sll hydrates. Much of the contents of these tables have been extracted from the excellent review article by Davidson (1973), with updated information from more recent sources (as indicated in the tables).

Prof. J. E. Enderby has informed me that the calcium ion hydrate which is [Ca(H20)6]2+ in strong solution may well be of higher coordination number, 8-10, in dilute solutions as determined by neutron diffraction studies... 

The hydration number of the ion has been measured by a number of nonspec-troscopic methods and a value of 5 1 represents the range of results. The difference method of neutron diffraction gives 6 with reasonable consistency. The lifetime is 3 X 10 s, so the value is clearly a hydration number (i.e., it is much greater than 10 s, so water travels with the ion). 

Hydration Numbers for Some Alkali Metal and Halide Ions Obtained from MD Calculations and X-Ray and Neutron-Diffraction Experiments ... 

X-ray and neutron diffraction measurements on polyion hydration give the number of water molecules involved per repeat group in the structures. About one water molecule per repeat group is the result for polymethyl methacrylate. The results of hydration for a variety of proteins are given in Table 2.31. 

Not only does neutron diffraction allow one to determine ionic size and hydration numbers in solution but it can also be used to assess changes in hydration with concentration. In the case of Li" ", the hydration number is 6 in dilute solutions but it drops to values below 4 in very concentrated solutions. Similar conclusions have been reached regarding divalent cations such as Ca for which the ion-solvent interactions are mainly electrostatic in nature. For this system the hydration number decreases from 10 in 1 M CaCl2 to 6 in a 4.5 M solution of the same salt. 

This is probably the most powerfiil spectroscopic technique, and with X-ray and neutron diffraction is now the technique of choice. A shift in the proton resonance frequency and the intensity of the signal teUs how many water molecules are responsible. Proton relaxation shifts have proved to be a major advance, and are progressively being applied to solutions containing complex ions. For simple ions they suggest six water molecules around a cation are fairly typical. In favourable cases individual hydration numbers are obtained using this technique. In this respect they are superior to the more traditional methods which on the whole only measure overall hydration numbers and require some arbitrary way of splitting these into cation and anion contributions. Diffraction studies also furnish individual hydration numbers. 

---