Magic-Angle Spinning: a Historical Perspective

Abstract

At the time of its discovery in 1945 [1, 2], NMR was hailed as a new method for the accurate measurement of nuclear magnetic moments. However, several years later Dickinson [3], Proctor and Yu [4], and Hahn [5], found that the res-onance frequency of a nucleus depends on its chemical environment.While the discovery of the chemical shift disappointed many physicists, it enabled NMR to be a very powerful tool for the study of molecular structure.Although it still took 20 years to convince chemists that NMR was widely applicable to their problems, by the mid-1960s NMR spectrometers had penetrated most chemi-cal laboratories, thanks to good textbooks [6–11] and to the commercial avail-ability of high-resolution spectrometers. Unfortunately, the usefulness of NMR for the investigation of chemical prob-lems was strictly limited to liquid samples, so solid samples first had to be dis-solved or melted. This is because of the anisotropic nuclear interactions which strongly depend on molecular orientation, and are therefore averaged by mol-ecular motion. In liquids, the molecules reorient randomly very quickly: a wa-ter molecule requires ca. 10 –11 s for complete reorientation. Although certain solids have sufficient molecular motion for their NMR spectra to be obtainable without resorting to special techniques, in the general case of a 'true solid,' there is no such motion, and conventional NMR, instead of sharp spectral lines, yields a broad hump which conceals most information of interest to chemists. For example, the width of the 1 H NMR resonance in the spectrum of water is ca. 0.1 Hz, while the line from a static sample of ice is ca. 100 kHz wide, i.e., a million times broader.Andrew et al. [12], and independently Lowe [13], had the idea of substituting the insufficient molecular motion in solids for the macro-scopic rotation of the sample. Consider a pair of protons, separated by distance r, in a rigid crystal im-mersed in an external magnetic field B o . Each nuclear magnetic dipole pro-duces a magnetic field in its neighbourhood. Therefore, depending on its quan-Topics in Current Chemistry (2004) 246: 1–14 DOI 10.1007/b98646 © Springer-Verlag Berlin Heidelberg 2004 tum state (1/2 or –1/2), each of the two protons slightly increases or decreases the magnetic field acting on its neighbour, and the NMR line splits into a dou-blet. The splitting expressed in frequency units is – 3 2 dh –1 (1 – 3cos 2 q), where d is the dipolar coupling constant proportional to r –3 , and g 2 , where g is the gy-romagnetic ratio of the proton and q is the angle between the direction of the magnetic field B o and the vector connecting the two protons. The factor (1–3cos 2 q), known as the second-order Legendre polynomial, comes from the secular part of the Hamiltonian of the dipolar interaction [14], which has the form 1 H = – 3 d [I