4. Acquisition

Time requirement: 2 h

Sample: 80% ethyl crotonate in CDCl₃, not degassed.

Set up the spectrometer for ¹³C NMR and load a pulse program for inversion recovery with ¹H broad-band decoupling. Current program versions handle the data as 2D data: for example, the results of eight experiments with different delays d2 are stored as rows in a 2D matrix. So you have to create a 2D file and to set:

decoupler attenuation and 90° pulse for CPD
td1	8
td2	32 k
sw	200 ppm
o1	middle of ¹³C NMR spectrum
o2	middle of ¹H NMR spectrum
p1	180° ¹³C transmitter pulse
p2	90° ¹³C transmitter pulse
d1	60 s (> 5 T₁ in order to achieve the equilibrium z-magnetization)
d2	create a list with the following values [s]: 0.5, 1, 3, 6, 10, 16, 24, 50
ds	2
ns	8

5. Processing

If a 2D file has been created, Fourier transformation has to be performed in F₂, using a line-broadening factor lb = 2 Hz. In order to adjust the phase, read spectrum number 8 in which all signals have a positive phase, and transfer this phase correction to all other spectra.

6. Result

In the figure the results obtained on an ARX-300 spectrometer with a 5 mm dual probe-head are presented as a stacked plot for qualitative inspection. In the first two spectra all signals have a negative intensity, since after the first 180° pulse and the short delays of 0.5 and 1 s all spin vectors are still in the −z direction. After a delay d2 of 3 s spin–lattice relaxation has reduced the intensity of the signal of C-6 nearly to zero, whereas those of all other signals are still negative, showing that C-6 must have the shortest T₁. For a rough estimation of the T₁-values from these spectra you may use Equation (1), where t_null is the (interpolated) delay d2 at which the intensity of a signal is zero.

T₁ = 1.44t_null(1)