4. Acquisition

Time requirement: 80 min for one carbon nucleus

Sample: 3% strychnine in CDCl₃.

Record normal ¹H and ¹³C NMR spectra of the sample, optimize the spectral widths and determine the offset of the carbon nucleus, for which the long-range C,H spin coupling is to be measured. Load the gs-SELINCOR-TOCSY pulse program. You have to set:

td: 32 k data points
sw: 10 ppm
o1: middle of ¹H NMR spectrum [4.5 ppm]
o2: on resonance of selected carbon nucleus [31.98 ppm]
p1, p4, p5: 90° ¹H transmitter pulse [8 μs, 5 dB]
p2: 180° ¹H transmitter pulse [16 μs, 5 dB]
p3: ¹H decoupler pulse during selective pulse on carbon, same length as p7 [10 ms, 12 dB]
p6: 180° ¹³C decoupler pulse [28 μs, 0 dB]
p7: 90° selective ¹³C decoupler pulse, Gaussian shape [10 ms, 50 dB]
p8: 180° selective ¹³C decoupler pulse, Gaussian shape [20 ms, 50 dB], offset + ½ J(CH) for first and − ½ J(C,H) for second experiment
duration of spin-lock ≈ 1/ⁿJ(C,H) = 132 ms, calculated from ⁿJ = 7.5 Hz, individual pulse length 25 μs at 14.5 dB
d1: relaxation delay 2 s
d2: 1/[2 ¹J(C,H)] minus effective gradient duration = 2.4 ms
g1–g4: sinusoidal-shaped field gradients, 1 ms duration and ca. 0.2 T/m strength, with gradient loop counters, ring-down delays (50 μs), lock blanking and gradient coil blanking switches according to actual instrumentation used, gradient ratio 5 : 5 : 30 : 7.543
ds: 4
ns: 512

Record one spectrum with the second selective pulse adjusted on the high-frequency part of the chosen C,H doublet and another spectrum adjusted to the low-frequency part.

5. Processing

Use standard 1D processing (see Exp. 3.1) applying an exponential window with a line-broadening factor lb = 1 Hz. After Fourier transformation, phase the desired multiplets individually or apply magnitude processing. View the two spectra in the dual-display routine. The frequency difference between the two signals of a chosen proton corresponds to the desired long-range C,H spin coupling constant.