Quick estimation of T₁s (Derome p. 166)

Many 2D experiments require a knowledge of the T₁s of the nuclei involved for optimum conditions to be set up.

A ball park figure (usually all that's needed) can be estimated from an inversion recovery sequence:

The Δ delay is manually adjusted until the signal(s) of interest are nulled in the resulting spectrum. (If Δ short relative to T₁, signals negative; if Δ long relative to T₁ signals positive).

It can be shown that Δ delay for null = T₁ln2 i.e. T₁ = 1.44*Δnull.

For protons, usually only one scan is required for each attempt to find the null.

Care must be taken to ensure that at least 5T₁s are left between successive attempts to find the null point (or scans if more than one needed) and that the same phase correction is always applied to determine sign of the signals.

Prior calibration of the 90° pulse is also a good idea so that an accurate 180° pulse can be applied (probably will need to be known for the 2D experiment also).

Numerous reasons exist for wanting to know the approximate T₁.

e.g. Optimum repetition rate for best sensitivity is 1.3 T₁ usually - useful for simple COSY experiments.

NOESY on small molecules requires the mixing time to be set to the order of the T₁s of the protons of interest. Where there is a range of T₁s, a mixing time of the shortest T₁ is often most useful. A recycle delay time of 3 times the longest T₁ is often a good compromise.

Inverse detected proton carbon correlations (i.e. proton observation) require the repetition time to be 1.3-2 times the shortest proton T₁, for fast acquisition.

The appropriate pulse programs is quickt1 on the DMX/DPX. This is most easily done using the quickt1 macro in XWINMMR:

Procedure:

1. Run a normal 1H spectrum then calibrate the 90° pulse
2. Type "quickt1". This loads the pulse program then requests some parameters:
   pl1 - power level for pulse. This will be the same if the same dataset as the pulse calibration is used
   p1 - value for 90° pulse calibrated in 1).
   d7 - d7 corresponds to the Δ delay. Start with a small value, e.g., 100μs
3. Run a 1 scan spectrum and process with fp or efp. For a short D7 all peaks should be negative but in phase.
4. Repeat with various values of D7 so that peaks of interest are nulled (zero). Ignore solvent and impurity peaks. D7 = 1s is a good first guess.
5. Remember T₁ = 1.44*D7. This rough method is most likely to understimate the T₁ by a few percent.