Tuesday, June 10, 2008
Calibration of 13C Decoupler Pulses
Proton-carbon HMQC, HSQC, and HMBC experiments rely on knowing the duration of the 90 degree pulse for 13C for at least one power level. These pulses must be known at both high power and low power when 13C decoupling is employed during the acquisition. This calibration is conveniently done with the pulse sequence depicted in the figure below for CHCl3. For protons attached to 12C, the pulse sequence produces an out of phase signal. When no 13C pulse is applied (i.e. 0 degrees) the components of the 1H-13C doublet signal (13C satellites) are antiphase with respect to one another. When a 90 degree 13C pulse is used, the 1H-13C doublet is converted to an unobservable double quantum coherence and therefore is not present in the spectrum. When a 180 degree 13C pulse is employed, the 13C satellites will again be antiphase with one another but in the opposite sense compared to that when no 13C pulse was employed. The 13C decoupler 90 degree pulse is thus measured by collecting a series of spectra with increasing 13C pulse duration. The 90 degree pulse produces a null for the 13C satellites. This is depicted in the figure below. This must be carried out at both high and low 13C power to determine hard pulses to be used in pulse sequences and low power pulses to be used in multiple pulse decoupling schemes.