Calibrate 90º ¹H pulse

Proper pulse widths are critical in 2D experiments. A 90º pulse is when the signal is maximum and positive, a 180º is when the signal is null, a 270º pulse is when the signal is maximum and negative, a 360º pulse is again a null. ¹H pulse widths are normally calibrated on 1 pulse (s2pul) experiments, looking for the null (180º or 360º pulse); it is alot easier to see a null than a maximum signal. The length of the 90º pulse is primarily dependent upon 4 factors: the probe (particularly the quality or age), the tuning (bad tuning = longer 90º pulse), the power of the pulse (the higher the power, the shorter the pulse), and the solvent (lots of salt = long 90º pulse). Just because your pulse width was X ms in CDCl₃ in another probe does not mean that is what it is in D₂O in the probe you are using, so calibrate it.

First guess at a pulse width shorter than the expected 90º pulse at the power that you want (tpwr = 60 for high power), collect a 1pulse 1D spectrum, phase the spectrum so everything is positive, wait enough time for your sample to relax back to steady state (~30-45 seconds after the pulse). Set up an array for pw where pw is approximately 3-5 times the expected 90º pulse (approximately a 360º); you can array pw by pw=25,27,29,31 (etc. or whatever values you want). Set d1 + at = ~20s, since you are looking for the 360, relaxation back to steady state should be a little faster than normal. Type go, after spectrum is acquired type wft(1), do not phase, but do adjust so the height of the peaks and the proper region of the spectrum is zoomed in on, then type wft dssh, this will display the full array of spectra- look for the null, this is the 360º pulse. Rerun the experiment at 1/2 that pw value, this should be 180 and again null, and then 1/2 of that value, which should be 90º and maximum signal- if it is still null, the first pulse was the 540º pulse and this is the 180.

Many 2D experiments require not only a high power (tpwr=60) pulse but also lower power/longer 90º pulses, these also need to be calibrated the same way, except now set tpwr to the desired lower value.