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.