Examples of Hartmann–Hahn Match Conditions for CP/MAS between Two Half-Integer Quadrupolar Nuclei

Abstract

Hartmann–Hahn match conditions for n2 → M2 CP/MAS between two quadrupolar nuclei, spin-lock signal as a function of effective nutation frequency, and the correlation of effective nutation frequency and radiofrequency field strength are reported for three samples: sodium diborate (Na2B4O7), aluminum boride (AlB2), and lithium aluminate (LiAlO2). Radiofrequency field strengths used for CP/MAS are both greater and less than the sample spinning speed of 10 kHz, resulting in the observation of both zero-quantum and double-quantum matches, which have signals of opposite sign. The match conditions for Na2B4O7 are as expected from published theory and CP/MAS experiments on spins 12 and n2 (n = 3 or 5) with quadrupole frequencies (ωQ) large compared to the radiofrequency field strength of the CP contact pulse, consisting mainly of sideband matches at one and two times the sample spinning frequency, and the correlation of effective nutation frequency and radiofrequency field strength supports the conclusion that ωQ is large for both 11B and 23Na. Aluminum-27 in AlB2 may have either small or intermediate ωQ, and 7Li in LiAlO2 is proposed to have intermediate ωQ in relation to the radiofrequency field strength, and both have curves of the spin-lock signal as a function of effective nutation frequency with central minima, differing from those of the nuclei with large ωQ. The sign of the CP/MAS signal for AlB2 and LiAlO2 appears to vary with the CP field strengths for the two nuclei so that positive or negative signals cannot be consistently correlated with zero- or double-quantum matches. However, it is possible to assign at least some of the matches as close to integral multiples of the sample spinning frequency, and some of these are matches at greater than two times the sample spinning frequency.