Chapter 2

Determination of Pulse-Duration

In pulsed Fourier transform NMR spectroscopy there is nothing more important than the use of radiofrequency pulses of correct duration. This applies not only for advanced multipulse and multidimensional methods but for the most simple routine experiments as well. The use of a wrong excitation pulse can render all FT experiments insensitive and useless. Regular determinations of pulse-duration (also often called pulse-width or pulse-length) are also necessary for instruments working in an automatic mode with a sample changer. Aging or malfunctioning components can increase the pulse-duration corresponding to the normal setting (e.g. 90°) and therefore degrade the performance of these spectrometers, if not corrected.

In this first experimental chapter we therefore provide five basic methods for measuring the pulse-duration. First we describe the calibration of transmitter pulse-width both for ¹H and ¹³C (Exps. 2.1–2.2) and of the ¹H decoupler pulse-width (Exp. 2.3). Corresponding experiments are then performed for the inverse mode of operation (Exps. 2.4–2.5).

We demonstrate further the use of composite pulses (Exp. 2.6) and the effect of radiation damping which renders the determination of the 90° pulse duration in normal water difficult (Exp. 2.7). At the end of the chapter we show the relationship between pulse and receiver phases (Exp. 2.8) and how the pulse-length is connected to radiofrequency power and the excitation bandwidth (Exp. 2.9).

This provides important knowledge for the setting up of more advanced experiments such as those using spin-locks or selective pulses. The calibration of selective pulses is demonstrated in Chapter 7.

Literature

T. D. W. Claridge, High-Resolution NMR Techniques in Organic Chemistry, Pergamon, Oxford, 1999, 94–99.
E. Fukushima, S. B. W. Roeder, Experimental Pulse NMR, Addison-Wesley, Reading, 1981, 47–60.