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Positive —X Negative +X Positive —X Negative

If we choose the +X axis as our reference, we will have positive peaks for the Cq and CH2 groups, and negative peaks for the CH and CH3 groups. Usually, the APT spectrum is phased the other way, using the — X axis for the reference axis, so that the CH and CH3 peaks are positive and the Cq and CH2 peaks are negative.

There is, however, one very important assumption we have made that is not practical: The carbon resonance was assumed to be on-resonance. Obviously, we cannot guarantee this for all carbon resonances in a spectrum because different13 C peaks have different resonant frequencies. What happens if the carbon chemical shift is not at the center of the spectral window? The magnetization, which starts on the y' axis, will precess in the x-y plane at an angular frequency Av, where Av is the position of the 13C resonance relative to the center of the spectral window on a hertz scale. Components of the multiplet will rotate a little faster or a little slower relative to the central component; for example, the three components of a triplet will rotate at angular frequencies Av + J, Av, and Av — J. This additional rotation due to the chemical shift (Av) will affect the phase of the magnetization when acquisition is started, and the phase information of interest, which is determined by the number of attached protons, will be hopelessly lost in the jumble of chemical shift effects. What we need is a trick that will allow the /-coupling precession ("evolution") to proceed but will somehow cancel out the effect of the chemical shift evolution. In other words, we need some control over what kind of evolution occurs during a delay. Such a trick exists, of course, and it is called the spin echo.

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