1. Purpose

In “inverse” experiments one observes protons and “decouples” heteronuclei X (e.g. ¹³C, ¹⁵N). On NMR instruments built later than 1991 the pulse-lengths in the normal and the inverse mode are usually quite similar. Older instruments, however, use different r.f. sources and signal routings in these two configurations. Therefore, before starting an inverse experiment, such as an HMQC experiment (Exp. 10.13), the pulse-durations for protons and X nuclei have to be determined in this spectrometer configuration. For ¹H this can be done exactly as described in Experiment 2.1. Here, however, we describe the 360° method using only the FID and not transforming the spectra.

2. Literature

E. Fukushima, S. B. W. Roeder, Experimental Pulse NMR, Addison-Wesley, Reading, 1981, 47–60.

3. Pulse Scheme and Phase Cycle

Inverse-mode proton pulse calibration sequence

4. Acquisition

Time requirement: 10 min

Sample: 3% CHCl₃ in [D₆]acetone, do not use a degassed and sealed sample since that would make the relaxation times of the protons exceedingly long.

An inverse probe-head (inner coil ¹H, outer coil ¹³C or other heteronuclei) is placed into the magnet; both coils are tuned to the sample. On older instruments change to the inverse set-up. Since the inverse set-up varies widely depending on the year of the spectrometer's construction it cannot be discussed here. Load the correct pulse program to obtain a ¹H NMR spectrum in the inverse mode. You have to set: