Monday, June 2, 2008
The Effect of Spinning Speed on Dipolar Dephasing
Dipolar dephasing is a very simple and effective tool to help assign the 13C CPMAS spectra of solid organic compounds. The technique is based on turning off the high power 1H decoupler for a period of time immediately after cross polarization but prior to the collection of the FID. During this dephasing delay, the 13C - 1H heteronuclear dipolar interaction is averaged only by the magic angle spinning. The degree of averaging of the 13C - 1H dipolar interaction and hence the amount of dipolar dephasing, depends on the MAS spinning speed. The faster the spinning speed, the smaller the amount of dephasing for a fixed period of time. This is illustrated in the figure below. The lower trace shows a 13C CPMAS spectrum with a spinning speed of 12 kHz. The center trace shows the 13C CPMAS spectrum with a 40 microsecond dephasing delay with a spinning speed of 4 kHz. The upper trace is identical to the center trace except the rotor was spinning at 12 kHz. One can see that many of the resonances which did not survive the dephasing delay at 4 kHz are present in the dipolar dephasing spectrum at 12 kHz. When fast spinning speeds are employed, longer dephasing delays (and the complications encountered with them) are required.