1. Purpose

In many experiments one wants to selectively observe protons that are attached to ¹³C or ¹⁵N. The strong signals of protons attached to ¹²C or ¹⁴N need to be suppressed in order to be able to detect the weak desired signals. One method of discriminating between these two kinds of signals is to use the double-quantum filter. It is applied in many different pulse sequences and consists essentially of two 90° pulses. Double-quantum magnetization passes through this filter, whereas single-quantum magnetization is filtered out by the phase cycle. In this educational experiment the use of a heteronuclear double-quantum filter is shown for chloroform, an experiment which, in addition, reveals any instability of the spectrometer. In the homonuclear case one type of application is the suppression of a strong solvent signal, e. g. that of water (see Exp. 10.8), or of the central signal in 1D-INADEQUATE (see Exp. 6.13).

2. Literature

R. Freeman, A Handbook of Nuclear Magnetic Resonance, Longman Scientific Technical, Harlow, 1987, p. 128–133.
S. W. Homans, A Dictionary of Concepts in NMR, Revised Edition, Clarendon Press, Oxford, 1993, 93–106.

3. Pulse Scheme and Phase Cycle

Heteronuclear double-quantum filter pulse sequence and phase cycle

4. Acquisition

Time requirement: 0.5 h

Sample: 10% CHCl₃ in [D₆]acetone.