Tuesday, April 15, 2008
Dipolar Dephasing with Long Dephasing Delays
The combination of cross polarization with dipolar dephasing is very useful as an assignment tool in 13C CPMAS spectra. The technique works well with dephasing delays of 40 to 60 microseconds when low MAS spinning speeds ( 2 - 5 kHz) are employed, as these speeds are insufficient to effectively average the heteronuclear dipolar coupling interaction. At higher fields, faster MAS rates are often needed to avoid an undue number of spinning sidebands. At faster spinning speeds the heteronuclear dipolar coupling interaction is more effectively averaged and much longer dephasing delays (hundreds of microseconds) must be used. The use of such long delays makes phasing the NMR spectrum impossible. In these cases a 180 degree 13C pulse can be applied at the midpoint of the dephasing delay to refocus the 13C chemical shifts and permit proper phase correction. This scheme is shown below.