Foreword by the Guest Editor of this Issue
Many of the early NMR experiments were carried out on solids, but when the potential of high-resolution solution-state studies for the determination of chemical structure and molecular dynamics began to be fully appreciated, interest in solid-state NMR dwindled (at least as a proportion of the total NMR effort). In fact NMR of solids remained almost entirely within physics rather than chemistry for many years. The principal reason for this situation was, of course, that, in the absence of special techniques, NMR spectra of solids are generally broad and relatively featureless because of the existence of strong anisotropic interactions (particularly dipolar spin-spin coupling and shielding anisotropy). In the solution state these interactions are averaged by molecular tumbling, thus allowing the observation of relatively small indirect spin-spin couplings and differences in isotropic chemical shifts. However, it was shown in the 1950s that magic-angle spinning (MAS) of a solid sample at a suitable rate could, under favourable circumstances, significantly narrow NMR signals, this procedure being particularly developed by Professor Raymond Andrew. However, it was not until MAS was combined with high-power proton decoupling and crosspolarization (CP) by Schaefer and Stejskal in 1976 to produce acceptable high-resolution spectra of dilute spins such as carbon-13 in solids that chemists started to appreciate the potential of solid-state NMR. Many laboratorieq now practice CP MAS NMR using commercial instrumentation. Techniques for obtaining high-resolution spectra of abundant spins (such as protons) in solids have, however, been slower to win acceptance and general usage, largely because the pulse-sequences involved (which may be combined with MAS) are very demanding on the electronics.
High-resolution NMR spectroscopy has now been applied to a wide range of solids, and an attempt has been made in this Special Issue to give examples from both organic and inorganic chemistry. A range of NMRactive nuclei are involved, including 'H, 13C, 27Al, 29Si, 31P and 9sMo. The topics of the articles range over NMR techniques, chemical structure determinations, molecular dynamics, chemical reactions (catalysis), and morphology investigations.
However, although broadline NMR (including relaxation studies) is now relatively unfashionable, it can still provide valuable information. Indeed, the data obtained can be crucial in interpreting high-resolution, dilute-spin spectra, as well as being useful in their own right. It can properly be argued that NMR is a multi-faceted technique and that, for its full potential to be realized, it is necessary to bring together the results from many different experiments (particularly for heterogeneous systems). Therefore, included in this Special Issue are a couple of articles which describe broadline or nonselective relaxation experiments.
It is also important for the chemistry community to appreciate that solidstate NMR can be applied to a wide variety of materials. In this Special Issue, different articles are concerned with crystalline compounds, polymer blends, disordered solids, adsorbates, glasses and matrix-isolated materials.
In spite of the variety of techniques and material covered in this Issue, many areas of solid-state NMR are not mentioned. To do so would require at least a complete year's volume of Magnetic Resonance in Chemistry! Nevertheless it is hoped that readers will find something of the flavour and excitement of modern solid-state NMR herein. Obviously I am greatly indebted to the authors who accepted my invitation to write articles for this issue (and who sometimes had to put up with my insistence on precise notation and use of English!). The Special Issue is essentially the sum of its parts! Thanks are also due to the referees and, especially, Dr Pam Lewis, who was always willing to put herself out to expedite the Issue.