We present a proton-selective method to determine 17 O-1 H distances in organic, biological and biomimetic materials by fast magic-angle-spinning (MAS) solid-state NMR spectroscopy. This method allows the determination of internuclear distances between specific ( 17 O, 1 H) spin pairs selectively. In this way medium-range 17 O‚‚‚ 1 H distances across hydrogen bonds can be estimated in the presence of short-range 17 O-1 H contacts sharing the same 17 O site. The method employs the newly developed symmetry-based radiofrequency (rf) pulse sequence SR4 1 2 applied to the protons to achieve heteronuclear dipolar recoupling, while simultaneously decoupling the homonuclear proton dipolar interactions. Fast MAS (50 kHz) and high static magnetic fields (18.8 T) achieve the required proton spectral resolution.Hydrogen bonding is an essential component of the structure and functioning of bio(mimetic) materials. 1 17 O solid-state NMR has started to develop into a powerful tool to study hydrogen bonding in nucleic acids, amino acids, peptides, and proteins, 2 as it gives direct access to one of the hydrogen-bonding partners in X-H‚‚‚O (X ) O, N, C) hydrogen bonds. So far, mainly the indirect correlation of 17 O chemical shifts and quadrupolar couplings has been used for this purpose. However, the most direct and accurate method to characterize these types of hydrogen bonds is to measure O‚‚‚H distances across the hydrogen bonds by determining 17 O-1 H heteronuclear dipolar couplings.Important examples of moderate hydrogen bonds 1 in biological systems are N-H‚‚‚OdC hydrogen bonds in -sheets and R-helices and O-H‚‚‚O hydrogen bonds in polysaccharides. O‚‚‚H distances d across moderate hydrogen bonds are in the medium-range order of 150-220 pm, hence significantly longer than O-H interbond distances (98-99 pm). O-H groups can act simultaneously as proton donors and acceptors, for example, in the H‚‚‚O-H‚‚‚O type hydrogen bonds in polysaccharides such as cellulose. 3 In the crystal structure of L-tyrosine‚HCl the O η -H η group acts as donor in the O η -H η ‚‚‚Cl -hydrogen bond, and simultaneously the O η acts as acceptor in the intermolecular O′′ -H′′‚‚‚O η hydrogen bond 4 (shown in Figure 1).The determination of dipolar couplings under MAS conditions requires the application of recoupling rf pulse sequences in order to suspend the MAS averaging of these couplings over a defined time interval. The rotational echo double resonance (REDOR) sequence 6 applied to the 1 H possesses the properties ii-iv; however, under fast MAS and the usual rf field limitations, substantial recoupling of the 1 H homonuclear dipolar interactions occurs. The symmetrybased R18 2 5 heteronuclear recoupling sequence fulfills the criteria i and iv and has been used to determine 17 O-1 H interbond distances. 7 Recently, we presented supercycled symmetry-based RN n V pulse sequences achieving heteronuclear longitudinal twospin-order (I z S z ) recoupling that possess all properties i-iv. 8 These sequences were used to determine the short-range O η -H η in...

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Microcrystalline TiO(2) with an anatase crystal structure is used as an anode material for lithium rechargeable batteries, and also as a material for electrochromic and solar-cell devices. When intercalated with lithium, as required for battery applications, TiO(2) anatase undergoes spontaneous phase separation into lithium-poor (Li(0.01)TiO(2)) and lithium-rich (Li(0.6)TiO(2)) domains on a scale of several tens of nanometres. During discharge, batteries need to maintain a constant electrical potential between their electrodes over a range of lithium concentrations. The two-phase equilibrium system in the electrodes provides such a plateau in potential, as only the relative phase fractions vary on charging (or discharging) of the lithium. Just as the equilibrium between a liquid and a vapour is maintained by a continuous exchange of particles between the two phases, a similar exchange is required to maintain equilibrium in the solid state. But the time and length scales over which this exchange takes place are unclear. Here we report the direct observation by solid-state nuclear magnetic resonance of the continuous lithium-ion exchange between the intermixed crystallographic phases of lithium-intercalated TiO(2). We find that, at room temperature, the continuous flux of lithium ions across the phase boundaries is as high as 1.2 x 10(20) s(-1) m(-2).

Using 27 Al MAS and 27 Al MQ MAS NMR, dealumination of zeolite H-Beta has been observed at specific T-positions in the framework. 27 Al MQ MAS NMR is able to resolve aluminum in the T1 and T2 positions from the other (T3 -T9) positions in the framework. A quantitative analysis of the 27 Al MQ MAS NMR spectra shows that aluminum atoms in positions T1 and T2 resist dealumination and do not adopt an octahedral oxygen coordination. Moreover, it is shown that a heat treatment of 450 °C of NH 4 -Beta gives a single type of fairly symmetric framework octahedral aluminum, which can be reconverted to framework tetrahedral aluminum by ammonia treatment. A more severe heat treatment (550 °C under steam) causes some of the tetrahedral framework aluminum atoms on positions T3-T9 to convert to at least two different types of octahedral aluminum that are connected to the framework. These sites are proposed to be different consecutive steps in the process of framework dealumination of zeolite Beta.