Measurement of 13C 90° Pulses in Solids via Cross Polarization

The direct measurement of ¹³C 90° pulses in solids under MAS conditions by the conventional method suffers from the very low inherent sensitivity of ¹³C and is very time consuming due to the typically long ¹³C T₁'s.  These problems can be at least partially overcome by using ¹H - ¹³C cross polarization which has a potential four-fold sensitivity gain and also a time advantage as the repetition rate depends on the ¹H T₁ rather than the ¹³C T₁, the former typically being less than the latter by a factor of ten.  The 90° pulses are measured by carrying out the usual cross polarization contact which leaves the ¹³C magnetization along the -y axis.  The contact is followed by a ¹³C -x phased pulse (φ_-x) which rotates the magnetization towards the z axis in the -yz plane.  The acquisition follows with high power ¹H decoupling.  The sequence is illustrated in the figure below.

When φ_-x = 0°, one observes the usual positively phased CP spectrum.  As φ_-x is increased the signal decreases until φ_-x = 90° at which point the ¹³C magnetization is on the z axis and a null signal is observed.  As φ_-x is increased further, +y magnetization is created and a negative signal is observed until φ_-x = 180° at which point the magnetization is on the y axis and a maximum negative signal is observed, etc..... The 90° pulse can be read directly from the first null or 1/3 of the second null at 270°.  The vector diagrams and a typical measurement (where φ_-x was increased from 0.5 µsec to 20 µsec in 0.5 µsec steps) are illustrated in the figure below.