Wednesday, January 16, 2008
13C Cross Polarization with Dipolar Dephasing
A very simple modification to the standard cross polarization sequence is to put a delay at the end of the contact time just before the acquisition where the proton amplifier is turned off. During this delay, the signals for carbons with strong dipolar coupling to protons will decay quickly while those with weak diploar coupling to protons will decay very slowly.The duration of the delay can be chosen such that CH and CH2 signals will decay entirely while those from quaternary or carbonyl carbons will decay very little. Methyl carbons will behave like carbonyl or quaternary carbons as the dipolar coupling is averaged by the fast rotation of the methyl group. The delay is approximately 30 -50 microseconds when spinning speeds of 3-6 kHz are used. When faster spinning speeds are necessary, the heteronuclear dipolar coupling is more efficiently averaged by the magic angle spinning and the delay must be made longer. For long dephasing delays, a 180 degree 13C pulse may have to be inserted at the midpoint of the delay to enable proper phasing. Running a 13C CPMAS spectrum with dipolar dephasing will produce a spectrum with only quaternary, carbonyl or methyl carbons present. This method can also be used to show if there is a high degree of molecular motion causing the dipolar coupling to protons to be averaged.