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10 xs

Hetero-dec. (waltz-16)

1/10

1/100

100 [xs

Homo-decoupling

1/300

1/90,000

3 ms

Presaturation (90% H2O)

1/600

1/360,000

6 ms

NOE Difference

1/3000

1/90,000,000

30 ms

The important point is that a much longer duration, much lower power pulse can give selectivity of excitation, affecting only one resonance in the 1H spectrum if it is sufficiently separated from other resonances in chemical shift. Very little RF power is required to saturate a resonance (equalize populations), but a bit higher power is necessary for decoupling because the spins must rapidly shuttle between the two spin states to give the "blurring" effect which makes other spins blind to the J coupling phenomenon. Nonselective or "broadband" decoupling (waltz-16 heteronuclear decoupling) requires much higher power levels, near the limits set by amplifier and sample heating.

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