Wednesday, February 27, 2008
Hartman-Hahn Match as a Function of MAS Spinning Speed
Cross polarization with magic angle spinning (CPMAS) is a means of obtaining high resolution, high sensitivity NMR spectra of dilute isotopes in solids. The dipolar coupling between the protons and the dilute isotope to be observed ( typically 13C, 15N, 29Si etc....) is exploited as a means for magnetization transfer from the abundant protons to the dilute isotope. The transfer is only possible when the product of the gyromagnetic ratio and applied power for the dilute isotope equals the product of the gyromagnetic ratio and applied power for the protons. For static or slow spinning samples, one can vary either the X or 1H power level during the contact time and one will observe a maximum signal when the Hartman-Hahn condition is met. The situation is more complicated when a sample is spinning at a rate comparable to or faster than the magnitude of the heteronuclear dipolar coupling used for the magnetization transfer. In such a case the MAS interferes with the dipolar coupling. The effect is that the Harman Hahn matching curve (intensity vs X or 1H power) is split into a series of maxima and minima separated by the spinning speed. This is illustrated in the figure below. When fast MAS CP experiments are to be used, it is important to set up the Hartman-Hahn condition on a maximum for a standard sample at the same spinning speed to be used for the sample of interest.