Constant-time period

During a constant-time (CT) period of lenght 2T, chemical shift of the selected X spin evolves during a variable t₁ period whereas homonuclear couplings, J_XX , only evolves during the fixed 2T period and all heteronuclear couplings, J_IX, do not evolve.

Evolution of X chemical shift: (t₁/2)+T-(T-t₁/2)=t₁
Evolution of J_IX coupling constant: (t₁/2)-T+(T-t₁/2)=0
Evolution of J_XX' coupling constant: (t₁/2)+T+(T-t₁/2)=2T

This is widely used in multidimensional NMR experiments of ¹³C-labeled biomolecules in which the undesired J_CC coupling constants are removed in the indirect dimension.

Easy implementation on AVANCE spectrometers.

This experiment provides information on the number of aliphatic carbons attached to a given ¹³C nucleus:

• If T=1/J_CC (about 27 ms) carbons coupled to an odd number of other aliphatic carbons have opposite intensity with those coupled with an even number.
• If T=2/J_CC (about 54 ms) all cross-peaks have the same sign.

The most standard way to implement a CT t₁ period in a ¹H-detected experiment is:

... d0 (p2 ph1) d20 (p4 ph2):f2 d6 ...

in which:

• d0 is the incremented delay (in seconds).
• d20 is the constant time period (in seconds).
• d6 is d20-d0.
• p2 is the 180º ¹H pulse (in microseconds) applied at a power level pl1 from the f1 channel.
• p4 is the 180º X pulse (in microseconds) applied at a power level pl2 from the f2 channel.
• ph1 is the phase specified at the end of pulse program.