4. Acquisition

Time requirement: 15 min

Sample: 5% ethyl crotonate in CDCl₃.

Set the instrument to inverse operation for ¹³C and to ¹H NMR detection. First record a normal ¹H NMR spectrum and optimize the spectral width and offset. Load the 1D HMBC pulse program with the dual step low-pass filter as shown above, corresponding to spectrum d. You have to set:

td	32 k
sw	10 ppm
o1	middle of ¹H NMR spectrum [5 ppm]
o2	middle of ¹³C NMR spectrum [100 ppm]
p1	90° ¹H transmitter pulse
p2	180° ¹H transmitter pulse
p3, p4, p5, p6	90° ¹³C decoupler pulse
d1	2 s
d2	1/[2J_min + 0.146(J_max − J_min)] − effective gradient length = 2.73 ms, calculated from ¹J(C,H_max) = 163 Hz and ¹J(C,H_min) = 127 Hz and effective gradient length of 1.05 ms
d3	1/[2J_max − 0.146(J_max − J_min)] − effective gradient length = 2.1 ms, calculated from ¹J(C,H_max) = 163 Hz and ¹J(C,H_min) = 127 Hz and effective gradient length of 1.05 ms
d4	HMBC delay 1/[2ⁿJ(C,H)] set to 60 ms − effective gradient length
g1–g6	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 15 : −10 : −5 : 50 : 30 : 40
ds	4
ns	16

Since this is meant as an educational sequence of experiments a–d, first record a 1D HMQC spectrum a (you may use the same pulse sequence by omitting the pulses p3 and p4, the delays d3 and d4, and the gradients g1–g3). Be sure to have the pulse phases as given above for a 1D version of HMQC. Set the delay between p1 and p5 to 1/[2¹J(C,H)] with ¹J(C,H) = 145 Hz. From this spectrum the different ¹J(C,H) spin coupling constants can be measured.

After this, record for comparison a 1D HMBC spectrum b, using the same pulse sequence as shown above, but omitting p4, delay d3, and the gradients g1–g3. Set the delay d2 to 1/[2¹J(C,H)] with ¹J(C,H) = 145 Hz = 0.345 ms and the long-range delay d4 to 60 ms. Then perform the latter sequence, but with two gradients g1 = −g2 in experiment c. This corresponds to an HMBC with a single pulse low-pass filter, in this