Recent trends of general interest in high-resolution N M R of solids are mentioned, and some examples briefly discussed. Stress is laid on the use of the multiple-pulse technique, particularly when combined with magic-angle spinning to give CRAMPS. Two studies of proton CRAMPS are described, one concerning hydrogen bonding in carboxylic acids and the other involving phosphonic acid derivatives. The relationship between proton isotropic chemical shifts and hydrogen-bond distances is explored in some detail. A clear correlation is found.As was found by Carrington' in an earlier Special Issue of Faraday Transactions (on Reaction Dynamics and Spectroscopy), it is difficult to know how to formulate an introductory Keynote Paper, although in his case the paper was first delivered as a Faraday Lecture. To attempt a comprehensive review of the advances in solid-state NMR in recent years would clearly not be feasible in an article of reasonable length, yet to' write on a single topic would not provide anything different in kind from the ensuing articles. Consequently, we are attempting to use a mixture of these two approaches. First, we propose to highlight a few developments of major significance, with brief examples where appropriate from our own laboratory. Secondly, we will develop in a little more detail a particular area which appears to us to be underused, although it has a respectably long pedigree,' namely combined rotation and multiple-pulse spectroscopy (CRAMPS). In this approach we will inevitably be briefly mentioning work carried out by a number of our colleagues, to whom we are greatly indebted.The ensuing articles in this Special Issue show something of the wide diversity of solidstate NMR experiments and their areas of application. However, it must be said that it would have been easy to increase the number of articles by an order of magnitude and still have totally left out many important aspects. This Special Issue is thus by no means comprehensive, but perhaps gives readers a feel for the excitement and rapid pace of developments in the NMR of solids. Unfortunately, within the length of our own article it has not proved generally feasible to fully describe many of the experimental techniques discussed nor to give a suitable number of appropriate references. General Solid-state NMR DevelopmentsAlthough it is difficult to pick out the highlights of a rapidly developing area such as solid-state NMR, and doubtless any choice is subjective, the following appear to us to be some of the trends of general interest, at least as far as high-resolution spectroscopy is concerned (relaxation studies form a separate topic). 3649 3650 Perspectives in High-resolution Solid-state N M R(1) Multinuclear spin-; experiments. Most of the 24 elements having non-radioactive spin-; isotopes have now been studied by high-resolution NMR of solid samples using the magic-angle spinning (MAS) technique with or without cross-polarization (CP). Initially 13C was the favoured nucleus, followed by "Si and 31P, but the range is now much...

Abstract-Anhydrous nepheline, jadeite, and albite glasses doped with F as well as hydrous F-containing haplogranitic glasses were investigated using "F combined rotation and multiple-pulse spectroscopy; 19F + 29Si cross-polarization/magic angle spinning (MAS); and high-power r9F decoupled 29Si, 23Na, and 27A1 MAS nuclear magnetic resonance methods. Fluorine preferentially coordinates with Al to form octahedral AIF:-complexes in all glasses studied. In addition, F anions bridging two Al cations, units containing octahedral Al coordinated by both 0 and F, or tetrahedral Al-F complexes might be present. The presence of Si-F bonds cannot be entirely ruled out but appears unlikely on the basis of the 19F + 29Si CP/MAS spectra. There is no evidence for any significant coordination of F with alkalis in the glasses studied. 23Na spectra are identical for the samples and their F-free equivalents and the spectra do not change upon decoupling of 19F. The speciation of F in the hydrous and anhydrous glasses appears to be very similar. Over the range of F contents studied (up to 5 wt.%), there seems to be hardly any dependence of F speciation on the concentration of F in the samples. The spectroscopic results explain the decrease of the viscosity of silicate melts with increasing F content by removal of Al from bridging AlO,-units due to complexing with F, which causes depolymerization of the melt. The same mechanism can account for the shift of the eutectic point in the haplogranite system to more feldspar-rich compositions with increasing F content, and for the peraluminous composition of most F-rich granites. Liquid immiscibility in F-rich granitic melts might be caused by formation of (Na,K),AlF, units in the melt with little or no interaction with the silicate component.The presence of F in granitic melts might increase the solubility of high field strength cations by making nonbridging 0 atoms available which form complexes with these cations. INTRODLJCPIONFLUORINE IS ONE OF THE MOST important volatile components of natural magmas, both in mantle-derived melts (e.g., lamproites) and in highly fractionated granitic magmas. Concentrations of several wt.% are reported in high-fluorine rhyolites (CONGDON and NASH, 1988) and in ongonites ( KOVALENKO, 1973). Similar to water, the presence of F strongly reduces the viscosity of silicate melts ( DINGWELL et al., 1985;DINGWELL, 1989) and increases diffusion coefficients ( DINGWELL, 1985). Even small amounts of F in the order of 1 wt.% will therefore significantly change the transport properties, mobility, and eruption behavior of such magmas. The correlation between high concentrations of certain trace elements (Sn, Zr, Nb, Ta, U, Th, etc.) and high F contents in granites and granitic pegmatites (STEMPROK, 1982) has stimulated the investigation of phase equilibria in F-containing granitic systems. MANNING ( 198 1) has shown that F strongly lowers both solidus and liquidus temperatures in the haplogranite system and causes a shift of the eutectic point towards more feldspar-rich...

6405the function vRAB is small. The expansions of the two Bessel functions are KO(YRAB) = -In (vRAB)ym + In (2) ... K~(YRAB) = (vRAB)-' ...where ym is the Euler Mascharoni constant. Using these two expressions in eq 13, we obtain for the diffusion coefficientUsing the Arrhenius expression for the diffusion coefficient and for the diffusion-controlled rate coefficient and the definition of Y , given in eq 12, we arrive at kdc 217 Using the definition of T given in eq 2, we expand the rightmost expression in eq 22We now defineUsing these two definitions and taking the logarithm of both sides of eq 23, we arrive atFinally under the assumption that the prefactors Do and ko are similarly temperature dependent and differentiating eq 25 results in Registry No. Ru(bpy)32+, 15158-62-0; 02, 7782-44-7; silica, 7631-86-9. References and Notes(1) Photochemistry on Solid Surfaces; Anpo, M., Matsuura, T., Eds.; Elsevier, Amsterdam, 1989. Photochemistry in Constrained and Organized Media; Ramamurthy, V., Ed.; VCH: New York, 1991.(2) Rideal, E. K.

Abstract.A 298i and 27A1 magic angle spinning nuclear magnetic resonance study is reported for differently synthesized mullites. The 298i MAS NMR spectra of all samples are essentially identical. They consist of a main resonance at -86.8 ppm, a shoulder around -90 ppm and a second resonance at -94.2 ppm. The main resonance is interpreted as being due to a sillimanite-type geometry around Si and the second one is tentatively assigned to a Si environment typical for mullite. The 27A1 MAS NMR spectra of sinter-and fused-mullite measured at different Larmor frequencies revealed clearly the presence of three distinct A1 sites in mullite, i.e. of octahedral (M1), tetrahedral (M2) and distorted tetrahedral (AI*) sites.