The figure shows the result obtained on an AMX-500 spectrometer with a multinuclear inverse probe-head. Compare the result with the other methods described in this book for achieving a separation between signals of protons bonded to ¹³C and those bonded to ¹²C (see Exps. 6.14–6.16 and 11.7–11.9).

7. Comments

The method can best be understood with the product operator formalism. Neglecting the 180° pulses, which refocus the chemical shifts and the heteronuclear spin coupling, we find, for a proton bonded to ¹³C, the result as given in Equation (1), since by setting the delay 2·d2 = τ = 1/[2J(C,H)] the cosine term becomes zero and the sine term unity.

Spin-lock purging product-operator Equation 1

A proton bonded to ¹²C cannot develop heteronuclear spin coupling and stays as −I_Hy. A spin-lock pulse with x-phase dephases this magnetization depending on the spin-lock strength and length, whereas the wanted magnetization 2I_HyI_Cz stays spin-locked. During acquisition, in-phase magnetization I_Hy sin(πJaq) develops, yielding the antiphase signals as observed in the figure.

As an additional exercise you may change the phase of the spin-lock pulse to y; the ¹³C satellites will disappear and only the main signal remains. In recent literature a “hard” spin-lock of 2 ms at 3 dB is also often used.

8. Own Observations