Symmetric Anion Research Articles

The evolution of the monoelectron dihydrogen bond H⋯e⋯H in the symmetric and asymmetric cluster anions (FH)n{e}(HF)m

Using the aug-cc-pVDZ basis set supplemented with diffuse bond functions (BF), the evolution of the monoelectron dihydrogen bond H⋯e⋯H in the symmetric (n=m=2–5) and asymmetric (n,m:3,1;3,2;4,2;4,3) cluster anions (FH)n{e}(HF)m have been studied. When n=m=3, 4, 5, and n, m: 3, 2 the H⋯e⋯H bond evolves into the FH–e–HF monoelectron bimolecular bond. While in some asymmetric cluster anions (n,m:4,2;4,3) the FH–e–HF bond changes to H–e–HF bond, and some bond rearrangements occur in the larger subunit (HF)4 at the same time. This evolution shows a size effect of the subunits on the structure and the bond for the molecular cluster anions.

The cohesive energetics of solid cesium chloride

The cohesive energetics of the sixfold and eightfold coordinated phases of solid cubic cesium chloride are studied using a nonempirical fully ionic description. The Oxford Dirac–Fock program is used to compute the ion wave functions and rearrangement energies needed to convert a free anion into the compressed yet still spherically symmetric anions optimally adapted to crystal. The Relativistic Integrals Program (RIP) is used to compute uncorrelated two-body interionic interaction potentials which take full account of relativity and are exact for given ion wave functions. These potentials and the further contributions arising from electron correlation are reported. The OEMFS model for describing the environment of the anion in-crystal yields lattice energies and equilibrium interionic separations in good agreement with experiment and correctly predicts the eight-fold coordinated structure to lie lowest in energy. The “optimal” anion–anion short-range potentials, computed by using at each geometry the wave function optimal for that geometry, are compared with the “frozen” potential in which the same anion wave function is used to compute the entire potential. The “frozen” anion–anion potential in solid sodium chloride is reported. The basic idea in the ab initio perturbed ion (AIPI) method, that the wave functions of anions in-crystal remain spherically symmetric yet as strongly orthogonal as possible to those of neighboring ions, cannot be justified. It differs from the standard view in the RIP computations that an ion wave function only remains spherically symmetric whilst overlapping with those of neighboring ions, such overlaps generating the short-range interionic repulsions. The RIP predictions of crystal cohesive energetics are preferable to those of the AIPI method. The correlation energy used in AIPI calculations is theoretically objectionable for crystals and is numerically poor. For CsCl, the anion wave function, rearrangement energy, and closest cation–anion repulsion optimal for the 8:8 structure at each geometry are all essentially identical to their counterparts in the 6:6 phase. The different coordination numbers in the two phases render unequal the effective cation–anion interactions given as the sum of the true cation–anion potentials and a fraction of the rearrangement energy. This inequality removes RIP computations from that class of theories which fail to predict the correct energy ordering of the phases through using the same effective potential in both structures.

Synthesis and structure of [Fc(RO)PS2]? complexes

Reaction of Fc(S)PS2P(S)Fc with NaOR [R = Me, iPr] gives the non symmetric phosphonodithioato anions [Fc(RO)PS2]− which can be complexed to a range of metals. The square planar NiL2 complexes adopt two distinct configurations, ‘trans’ with the ferrocenyl groups above and below the square plane and ‘cis’ with the ferrocenyl groups on the same side of the square plane. There is good evidence that the isomerisation occurs via a cis-ML2(solv)2 intermediate. The zinc and cadmium complexes adopt dimeric M2L4 structures whilst the PbL2 complex has a distorted pyramidal structure and forms infinite chains via Pb⋯S interactions. All new compounds have been characterised spectroscopically and seven demonstrative X-ray structures are reported.

Application of the chiral acyl anion equivalent, trans-1,3-dithiane 1,3-dioxide, to an asymmetric synthesis of (R)-salbutamol.

An enantioselective synthesis of (R)-salbutamol has been carried out using the chiral, C2 symmetric acyl anion equivalent, (1R,3R)-1,3-dithiane 1,3-dioxide, which undergoes addition to an aromatic aldehyde with very high stereocontrol at 0 degrees C. Pummerer reaction and work-up with lithium ethanethiolate generated the alpha-hydroxy thiolester in high yield and further transformations led to the target compound with high enantiomeric excess.

(NEt(4))(2)[Fe(CN)(2)(CO)('S(3)')]: an iron thiolate complex modeling the [Fe(CN)(2)(CO)(S-Cys)(2)] site of [NiFe] hydrogenase centers.

In the search for complexes modeling the [Fe(CN)(2)(CO)(cysteinate)(2)] cores of the active centers of [NiFe] hydrogenases, the complex (NEt(4))(2)[Fe(CN)(2)(CO)('S(3)')] (4) was found ('S(3)'(2-)=bis(2-mercaptophenyl)sulfide(2-)). Starting complex for the synthesis of 4 was [Fe(CO)(2)('S(3)')](2) (1). Complex 1 formed from [Fe(CO)(3)(PhCH=CHCOMe)] and neutral 'S(3)'-H(2). Reactions of 1 with PCy(3) or DPPE (1,2-bis(diphenylphosphino)ethane) yielded diastereoselectively [Fe(CO)(2)(PCy(3))('S(3)')] (2) and [Fe(CO)(dppe)('S(3)')] (3). The diastereoselective formation of 2 and 3 is rationalized by the trans influence of the 'S(3)'(2-) thiolate and thioether S atoms which act as pi donors and pi acceptors, respectively. The trans influence of the 'S(3)'(2-) sulfur donors also rationalizes the diastereoselective formation of the C(1) symmetrical anion of 4, when 1 is treated with four equivalents of NEt(4)CN. The molecular structures of 1, 3 x 0.5 C(7)H(8), and (AsPh(4))(2)[Fe(CN)(2)(CO)('S(3)')] x acetone (4 a x C(3)H(6)O) were determined by X-ray structure analyses. Complex 4 is the first complex that models the unusual 2:1 cyano/carbonyl and dithiolate coordination of the [NiFe] hydrogenase iron site. Complex 4 can be reversibly oxidized electrochemically; chemical oxidation of 4 by [Fe(Cp)(2)PF(6)], however, led to loss of the CO ligand and yielded only products, which could not be characterized. When dissolved in solvents of increasing proton activity (from CH(3)CN to buffered H(2)O), complex 4 exhibits drastic nu(CO) blue shifts of up to 44 cm(-1), and relatively small nu(CN) red shifts of approximately 10 cm(-1). The nu(CO) frequency of 4 in H(2)O (1973 cm(-1)) is higher than that of any hydrogenase state (1952 cm(-1)). In addition, the nu(CO) frequency shift of 4 in various solvents is larger than that of [NiFe] hydrogenase in its most reduced or oxidized state. These results demonstrate that complexes modeling properly the nu(CO) frequencies of [NiFe] hydrogenase probably need a [Ni(thiolate)(2)] unit. The results also demonstrate that the nu(CO) frequency of [Fe(CN)(2)(CO)(thiolate)(2)] complexes is more significantly shifted by changing the solvent than the nu(CO) frequency of [NiFe] hydrogenases by coupled-proton and electron-transfer reactions. The "iron-wheel" complex [Fe(6)[Fe('S(3)')(2)](6)] (6) resulting as a minor by-product from the recrystallization of 2 in boiling toluene could be characterized by X-ray structure analysis.

Zinc and lithium hydrogen-β-glutamate: large-pore network layer structures

The symmetrical anion [β-GluH]− of β-glutamic acid [β-GluH2] has been introduced into crystal engineering of metal complexes for the first time. Neutralization of aqueous solutions of β-glutamic acid with zinc oxide or lithium hydroxide affords crystalline samples of zinc bis(hydrogen-β-glutamate) trihydrate and lithium hydrogen-β-glutamate hydrate, respectively. [Zn(β-GluH)2(H2O)3] has a layer structure with corrugated sheets featuring a square-grid pattern of the tetra-coordinated zinc dications and bridging [O2C–CH2CH(NH3)CH2–CO2]− anions (in their gauche-conformation). The uniform meshes of the network are based on puckered 32-membered rings. Neighbouring undulated layers are mutually shifted into a space-filling indentation which also generates a network of hydrogen bonds involving the ammonio and carboxylate groups and the intercalated water molecules. [Li(β-GluH)(H2O)] has a less corrugated layer structure based on a parallelogram-grid of lithium cations which are connected by three bridging [β-GluH]− anions (in their anti-conformation). The water molecule is part of the tetrahedral coordination sphere of the metal atom. The layers represent a mixed-mesh network with pores based on 8- and 32-membered rings. Neighbouring layers are mutually shifted to allow for extensive hydrogen bonding.

Anion shielding of electrostatic repulsions in transthyretin modulates stability and amyloidosis: insight into the chaotrope unfolding dichotomy.

The balance between stabilizing forces and the localized electrostatic repulsions destabilizing the transthyretin (TTR) tetramer is tunable via anion shielding. The two symmetrical anion interaction sites in TTR are comprised of residues Lys15 and Lys15' from opposing subunits on the periphery of the two thyroxine binding sites. These epsilon-ammonium groups repel one another and destabilize the tetramer, unless an appropriate anion is present, which stabilizes the tetramer. Chaotrope denaturation of TTR exhibits unusual behavior in that urea appears to be a stronger denaturant than GdmCl (guanidinium chloride), even though GdmCl is typically twice as powerful as a denaturant. The shift in the midpoint of the urea denaturation curve to higher concentrations as well as the increase in the mole fraction of tetramer that is highly resistant to denaturation with increasing KCl concentration provides strong evidence that anion shielding stabilizes the TTR tetramer. A consequence of tetramer stabilization is folding hysteresis, because the high GdmCl concentrations required to denature the anion-stabilized tetramer do not allow refolding of the unfolded monomers. The formation of amyloid fibrils by TTR requires that its normal tetrameric structure dissociate to alternatively folded monomers, a process mediated by acidification (pH 5-4). This process is inhibited by Cl(-) ions in a concentration-dependent fashion. Chloride ion may not be the relevant physiological TTR stability modulator, but it is the main focus of these studies explaining the hysteresis observed in the denaturation and refolding studies with GdmCl.

The electronic structure and stabilities of the symmetric gas phase anion clusters of hydrogen fluoride

Ab initio UHF, MP2 and MP4 calculations were used to study the geometries, the electronic structure and the thermochemical stabilities of the gas phase anionic clusters of hydrogen fluoride, (HF) n −, for n=2–6. Basis sets up to the aug-cc-pVDZ set were employed. These were augmented with a 6-term even tempered (sp) cavity set. The thermochemical stabilities of these complexes against vertical ionization as well as dissociation to various products were determined. These clusters involve a single shell of two to six HF molecules complexed in a symmetrical head-to-head orientation. They show higher thermochemical stabilities than similar clusters of water and ammonia and have stabilities similar to those of the neutral hydrogen bonded clusters of hydrogen fluoride. As with water, the smallest stable anionic cluster of hydrogen fluoride has two monomers which is considerably smaller than those observed for ammonia. The integrated electron densities as well as the natural bond orbital analysis reveals that a significant portion of the excess electron density is localized in the cavity region interior to the hydrogen atoms of the inner shell. The stability of the clusters increased with the number of monomers. Cavity orbitals had a nominal effect on the properties of the complexes and effected the prediction of the geometrical features of the complexes more than their stability.

Stable enols of carboxylic esters. The poly(methoxycarbonyl)cyclopentadiene systems

The structures of the poly(methoxycarbonyl)cyclopentadienes C(5)H(6)(-)(n)()(CO(2)Me)(n)(), n = 5 (Cp-5), n = 4 (Cp-4), n = 3 (1, 2,4-; Cp-3) and n = 2 (1,2-; 1,2-Cp-2-I) were investigated. The X-ray diffractions of Cp-5 (known), Cp-4, and Cp-3 showed an enol of ester structure in the solid state. The enolic hydrogen forms a symmetrical hydrogen bond to a neighboring ester carbonyl, so that the vicinal "enolic" CO(2)Me groups in the 1, 2-C(=CO(2)Me)-C(CO(2)Me)(4) moiety are identical. The relevant X-ray parameters for the three enols are similar. The CP-MAS spectra of Cp-3-Cp-5 generally resemble their (13)C NMR spectra in CDCl(3) except for some differences of mostly <1 ppm. The (1)H, (13)C, and (17)O NMR spectra of Cp-3-Cp-5 in CDCl(3) are consistent with those of the hydrogen bonded enols. Most characteristic are the (1)H and (17)O signals of the OH groups at 19.7-20.1 and 221-225 ppm, respectively. Proton addition to sodium 1, 2-bis(methoxycarbonyl)cyclopentadienide gave a mixture of four 1, 2-bis(methoxycarbonyl)cyclopentadienes. The isomer (1,2-Cp-2-I) formed in 10-20% displays delta(O(1)H) at 19.3 ppm and is the enol analogue of Cp-5 whereas its main isomer (30-55%) (1,2-Cp-2-IV) has the ester structure. In CD(3)CN and DMSO-d(6) only one signal was observed at room temperature for each type of H, C, or O of Cp-5, suggesting a complete ionization to the symmetrical anion of Cp-5. In contrast, Cp-4 and Cp-3 in CD(3)CN at room temperature display OH signals in both (1)H and (17)O NMR spectra, and Cp-5 shows a broad OH signal in the (1)H spectrum at 240 K. The enol of ester structure is the main species, although exchange with the corresponding anion is possible. On standing in DMF-d(7) at room temperature, new signals are observed for Cp-3 and Cp-4. On raising the temperature in Cl(2)CDCDCl(2), Cp-3-Cp-5 show line broadening and appearance of new signals. These were ascribed to rearrangment and decomposition processes.

Structure and magnetism of a phenazinium cyclopentadienide salt

Electron spin resonance (esr) study of the salt of the stable organic cation radical, N, N′-dimethylphenazinium (M 2P +) with the symmetric diamagnetic anion, penta(methoxycarbonyl)cyclopentadienide (PMC −), reveals weak antiferromagnetic interactions in this material. Analysis of the crystal structure shows dimerized stacks of the cation radical with electrostatic stabilization offered by the surrounding anion lattice. The antiferromagnetic coupling of the spins can be explained as arising from the direct exchange interaction between the radicals in the stack. The anions do not appear to have any specific influence on the magnetic property of this material.

New E-unidentate [Ph 2P(O)NP(E)Ph 2] − (E=S or Se) complexes of palladium(II) and platinum(II)

Metathesis of the monochloro metal(II) complexes [MCl(dien)]Cl (M=Pd or Pt; dien=diethylenetriamine) with K[Ph 2P(O)NP(E)Ph 2] (E=S or Se) or K[Ph 2P(O)NPPh 2] (prepared in situ from Ph 2P(O)NHPPh 2/KOBu t) in methanol gave the cationic, mononuclear compounds [M{Ph 2P(O)NP(E)Ph 2- E}(dien)] + or [M{Ph 2P(O)NPPh 2- P}(dien)] +, isolated as their hexafluorophosphate salts. Ligand substitution reactions of the dichlorometal(II) precursors [MCl 2(N∼N)] (M=Pd or Pt; N∼N=ethane-1,2-diamine, 1,2-diaminocyclohexane) with K[Ph 2P(O)NP(E)Ph 2] or K[Ph 2P(O)NPPh 2] gave the neutral bis substituted compounds [M{Ph 2P(O)NP(E)Ph 2- E} 2(N∼N)] or [M{Ph 2P(O)NPPh 2- P} 2(N∼N)] in high yields (63–89%). Reaction of the unsymmetrical palladium(II) compound [PdCl 2(ampy)] (ampy=2-aminomethylpyridine) with K[Ph 2P(O)NP(E)Ph 2] gave [Pd{Ph 2P(O)NP(E)Ph 2- E} 2(ampy)], analogous to the recently described compound [Pd{Ph 2P(O)NP(E)Ph 2- E} 2(en)], in which both [Ph 2P(O)NP(E)Ph 2] − ligands are exclusively E-monodentate bound. In contrast, transmetallation of K[Ph 2P(O)NP(E)Ph 2] with [PdCl 2(tmeda)] (tmeda= N,N,N′,N′-tetramethylethylenediamine) gave only the O,E-chelate monocationic complexes [Pd{Ph 2P(O)NP(E)Ph 2- O,E}(tmeda)]PF 6 and not [Pd{Ph 2P(O)NP(E)Ph 2- E} 2(tmeda)]. Under similar reaction conditions the symmetrical anions [Ph 2P(E)NP(E)Ph 2] − (E=S or Se) gave, without exception, the known bis homoleptic complexes [M{Ph 2P(E)NP(E)Ph 2- E,E′} 2] (M=Pd or Pt) with displacement of all chloride and amine ligands. All new compounds have been characterised by a combination of multinuclear NMR ( 1H, 31P{ 1H}, 195Pt{ 1H}), IR spectroscopy and elemental analyses. Furthermore, the X-ray crystal structures of five compounds have been determined and reveal a range of intermolecular and intramolecular hydrogen bonding contacts. There is also evidence for π-delocalisation within the E–P–N–P–O and P–N–P–O backbones of the E-ligating or O,E-chelating ligand.

A novel symmetric silicate anion

For the first time, a simple symmetrical silicate anion containing Q 4 sites has been detected in alkaline solution by the use of high-field 29 Si NMR spectroscopy involving isotopic enrichment.

Ionic Interactions and Transport Properties in Methyl Terminated Poly(propylene glycol)(4000) Complexed with LiCF3SO3

Alternating current (ac) impedance, restricted diffusion, and vibrational spectroscopic (Raman and IR) measurements have been conducted on complexes of methyl capped poly(propylene glycol) of molecular weight 4000 and LiCF{sub 3}SO{sub 3} salt. The relative concentrations of anions in different chemical environments have been calculated from an analysis of the symmetric anion SO{sub 3} stretch (Raman) over a wide concentration range. Comparisons are made to previous studies on hydroxyl capped PPG systems, and we find that polar polymer end groups play an important role in the solvation of the salt. The relative fraction of anions interacting directly with lithium cations is considerably higher over the entire concentration range in the present study, and we infer the existence of negatively charged ionic aggregates in the solutions. We also note that the fraction of spectroscopically `free` anions increases with increasing salt concentration in the ether oxygen to alkali metal cation ratio (O:M) range 502:1 to 12:1, contrary to a decrease reported for the analogue hydroxyl terminated electrolytes. The ionic conductivity has a more pronounced concentration dependency in the methyl capped system; notably, the molar conductivity ({Lambda}) increases dramatically with increasing salt concentration passing through a relatively sharp maximum at O:M = 20:1 atmore » room temperature. 71 refs., 8 figs., 1 tab.« less

In vitro investigations of the direct effects of complex anions on thyroidal iodide uptake: Identification of novel inhibitors

Iodide uptake (IU) by thyrocytes from the plasma against chemical and electrical gradients is by a specific iodide transporter or ‘pump’. Perchlorate (ClO 4 −) and other univalent, symmetrical anions are competitive inhibitors of iodide uptake (IU), and apparent K i for individual anions can be correlated with ion size. This study uses cultured thyrocytes and a broad range of anion size, in particular a series of spherical hexafluoride ions, in order to understand more about the parameters governing the activity of competitive inhibitors of IU. 125I uptake and organification by cultured porcine thyrocytes was combined with biochemical enzyme inhibition studies on thyroid peroxidase in order to identify specific effects on IU. Known inhibitors were used in a validation phase and demonstrated that a combination of the in vitro thyrocyte 125I assay and thyroid peroxidase inhibition assay could be used to identify selective inhibitors that can be difficult to identify using thyrocytes alone. Anions of less than, or similar, volume to I − (35.0 Å 3) were weak inhibitors with potency increasing proportional to ion size up to an apparent maximum for AsF 6 − (94.45 Å 3); this correlation was strong ( r = 0.96). PF 6 −, AsF 6 − and SbF 6 − were identified as novel inhibitors of IU, showing that the size range of anions active in IU inhibition is greater than that previously identified. The biological significance in vivo of the inhibitory action of the hexafluoride ions is not known. Their potency in this study suggests that these anions may have the potential to affect thyroid function in vivo if they were available systemically.

The molar conductivity behavior in polymer electrolytes at low salt concentrations; A Raman study of poly(propylene glycol) complexed with LiCF 3SO 3

Raman scattering measurements have been carried out on poly(propylene glycol) complexed with LiCF 3SO 3 salt over a wide salt concentration range, the ether oxygen to alkali metal cation ratios (O:M) ranging from 4820: 1 to 12: 1. The relative concentrations of solvated anions, anion-cation pairs and ionic aggregates have been calculated from an analysis of the symmetric anion stretch. The degree of association is found to be almost constant in the O:M range 4820: 1–40: 1 whereafter it increases with increasing salt concentration. The results show that the dramatic increase reported for the molar conductivity in the O:M range 1000:1–40: 1 cannot be explained by the redissociation of contact ion pairs or the formation of conducting triplets. Instead the major contribution to the conductivity increase seems to be a concentration dependent enhancement of the ionic mobility. A percolation based ionic hopping process involving exchange between ions in pairs and dissolved ions, either free or coordinated to ether oxygen sites, is advanced as a possible microscopic mechanism.

μ3-Oxo-tris[(triisopropylphosphine)gold(I)] Tetrafluoroborate

μ 3 -Oxo-tris[(triisopropylphosphine)gold(I)] tetrafluoroborate, [Au 3 O(C 9 H 21 P) 3 ]BF 4 , crystallizes with C 3v -symmetrical cations and anions. The cations show intramolecular Au...Au bonding, but any intermolecular aggregation, as detected in other homologues, is prevented by the bulky phosphine ligands.

Tetra(N-methylimidazol)-kupfer(II)-bis(triiodid) [Cu(C4H6N2)4](I3)2

The structure of the title compound, tetrakis(1-methylimidazole-N 3 )copper(II) bis(triiodide), consists of square-planar coordinated (Cu(C 4 H 6 N 2 ) 4 ] 2+ cations and symmetric triiodide anions I 3 - , which form long contacts with the cations. In the neutral complex, four coplanar N atoms and two triiodide ions form the expected quasi-octahedral coordination around Cu

Theoretical study of the highly vibrationally excited states of FHF−: Ab initio potential energy surface and hyperspherical formulation

A three-dimensional description of vibrationally highly excited linear molecules is formulated in hyperspherical coordinates, based on a successive adiabatic reduction scheme. The method is applied to the low-lying and highly excited vibrational states of FHF−, a prototype of symmetric bihalide anions, which has attracted spectroscopic interest due to its peculiar vibrational anharmonicity. Ab initio potential energy surfaces (PESs) which cover the ground-state potential well of FHF− and/or its dissociation to the F−+HF channel have been obtained by using the coupled electron pair approach (CEPA) method. An hyperspherical calculation using the ab initio PES of the sixth-order Simons–Parr–Finlan analytical form has correctly reproduced the experimental fundamental frequencies. Specifically, the vibrationally highly excited FHF− above the dissociation threshold is proposed as a candidate for transition state spectroscopy (TSS) of unimolecular dissociation reactions without barrier.

An ab initio study of the 19F nuclear magnetic resonance chemical shielding tensor in the hydrogen difluoride ion FHF − and its variation with bond length and angle deformation

The hydrogen difluoride ion FHF has long been of interest as a prime example of strong hydrogen bonding [ 11. Structural studies of simple salts of the anion, using in the main neutron diffraction techniques [2,3] but including some solid state NMR investigations [4-61, have consistently revealed a linear symmetric structure for the anion (Dh). One exception is the p-toluidinum salt, in which the hydrogen difluoride anion is reported to be asymmetric owing to the formation of a secondary F. . H . . . N hydrogen bond between the anion and the organic cation [7]. The hydrogen-bond energy has been reported from thermochemical estimates and ab initio theoretical calculations to lie in the range 155-255 kJmol_’ [l]. A value in this range is typical of that expected for a single covalent bond energy. This suggests that it is appropriate to regard the hydrogen difluoride anion as a covalently bonded species and to use methods of calculation appropriate to this type of bonding. In this short communication we report the results of an ab initio study of the NMR chemical shielding tensor for an isolated symmetric hydrogen difluoride anion. It is well known that secondorder properties such as the chemical shielding

Solid-state 31P magic-angle spinning NMR relaxation study of silica-supported polyoxoanions of the Keggin and Dawson structures

Solid-state 31P magic-angle spinning NMR investigations performed on silica-supported phosphopolyoxoanions [PMo 11Vo 40] 4−, [PW 12O 40] 3− and [P 2W 18O 62] 6− reveal a similar trend as already observed with [PMo 12O 40] 3− — the 31P spin-lattice relaxation rate increases with decreasing concentration of the polyanions at the silica surface. However the relaxation time T 1 decreases with increasing the age of the samples, especially for the tungstic compounds. More generally T 1 depends on the history of the sample after having been supported on silica; a vacuum treatment increases T 1 whereas exposure to water vapour has an opposite effect, showing the influence of water molecules assisting the relaxation. In addition the chemical shift anisotropy for the less symmetrical anions ([PMo 11Vo 40] 4− and [P 2W 18O 62] 6−) is partially averaged after water exposure, which could be explained by a “pseudoliquid phase”, already invoked for the interpretation of the “spillover” effect at the silica surface.