In the figure a is the normal ¹H-coupled ¹³C NMR spectrum of ethyl crotonate in the area of the two overlapping methyl group quadruplets, obtained on an ARX-200 spectrometer. In b and c the DANTE spectra of the two selected methyl groups are shown.

7. Comments

The flip angle α caused by an r.f. pulse of duration p and field strength B₁ is given by Equation (1).

α = γB₁p (1)

If n very short pulses exactly on resonance are applied in a pulse train, their net effect is given by (2)

α = nγB₁p (2)

However, if the frequency is offset from resonance by an amount Δν, then during each pulse cycle the nuclei precess in the rotating frame through an angle 2kπ + λ given by Equation (3), where k is an integer, τ is the repetition time of the pulses, and λ is a phase angle less than 2π.

2kπ + λ = 2πΔντ (3)

Thus the DANTE sequence produces signal responses at the sidebands of order k. In quadrature detection it is most convenient to set k = 0 with the transmitter directly on the resonance of the desired signal. However, excitation also occurs at k = 1 and 2.

Very roughly, one can estimate the selectivity of a selective pulse as the reciprocal of its length; thus the DANTE excitation pulse train of 25 ms length used here corresponds to a selectivity of about 40 Hz. Attenuation of the transmitter is necessary, since the normal pulse programmers are not able to produce very short pulses; therefore the 90° pulse should be in the order of 90 μs to give a 1° pulse angle for a pulse duration of 1 μs.

8. Own Observations