Effects of sample spinning on cross polarization

Abstract

In solid samples with small dipolar couplings between the abundant spins, sample spinning can effectively modulate the heteronuclear dipolar interaction responsible for cross-polarization (CP) transfer of magnetization from abundant to rare spins. This modulation is most visible with spinning frequencies that are at least comparable to the size of dipolar coupling between the abundant spins. The effective frequency of modulation and the widths of maxima in plots of CP rate vs matching rf field strength depend on the angle between the spinning axis and the direction of the magnetic field. Spinning the sample about an axis that is off the magic angle increases the widths of such maxima, thereby rendering the Hartmann-Hahn match condition easier to achieve and maintain. These effects are demonstrated by spinning samples of adamantane and ( 1HCO 2) 2Cd · 2 2H 2O at angles of 54.7° and at 90°. In samples with large dipolar couplings of the abundant spins, any modulation caused by spinning is largely inconsequential; samples of polyethylene and ( 1HCO 2) 2Cd · 2 1H 2O are used to demonstrate this fact.