NMR Spectroscopy Questions and Answers

Answer: A

COSY maps the proton coupling network, HSQC ties each proton to the carbon it sits on, NOESY reports which protons are near in space, and DEPT sorts carbons by attached hydrogen count. Together these four cover connectivity, assignment, geometry and multiplicity, which is most of what a structural problem needs. Adding HMBC for the long range carbon correlations completes the standard set used on an unknown natural product.

Answer: B

INEPT moves the larger proton polarisation onto carbon or nitrogen, enhancing the weak nucleus by roughly the ratio of the gyromagnetic ratios and allowing repetition at the faster proton relaxation rate. It is the building block behind DEPT, HSQC and much of modern heteronuclear NMR. Solvent suppression, selective inversion and phase correction are each achieved by other means. Polarisation transfer, together with inverse detection, is why nitrogen-15 work at natural abundance became feasible at all.

Answer: C

Inverse detection acquires the signal on the sensitive proton channel while the carbon or nitrogen frequency is stepped in the indirect dimension, which gains an order of magnitude or more in sensitivity over detecting the heteronucleus directly. It required probes with the proton coil innermost, which is why inverse probes were a distinct development. Field inversion is not done, receiver phase cycling is a separate and universal technique, and plotting convention has nothing to do with it.

Answer: D

Repeating a sequence with the phases of pulses and receiver stepped in a set pattern means unwanted signals add destructively while the wanted pathway adds constructively, removing quadrature images, axial peaks and much instrumental imperfection. It is the reason many sequences require multiples of eight or sixteen scans even on a strong sample. Flip angle depends on pulse power and duration, the relaxation delay is set by T1, and bandwidth by pulse length. Gradient selection now achieves some of the same ends in a single scan.

Answer: A

The first two describe the lock exactly: it watches the solvent deuterium line and adjusts the field to hold the ratio of field to frequency constant. The third confuses locking with decoupling, which is a separate function on a separate channel and is not normally applied to deuterium at all in routine work. Locking is also what allows the shims to be optimised automatically, since the height of the lock signal is a direct measure of homogeneity.

Answer: B

Field drift during a long accumulation means each scan is recorded at a slightly different frequency, and the co-added result is a smeared, broadened and often lopsided line. Short experiments on a stable magnet can survive without a lock, which is why an unlocked proton spectrum of a non-deuterated sample is sometimes acceptable. Drift never improves resolution, does not move all peaks uniformly in a way that matters, and does not invert phase. The lock is the cheapest insurance against a wasted overnight run.

Answer: C

The gyromagnetic ratio of carbon-13 is close to a quarter that of the proton, so the two frequencies stand in about that ratio:

\[ \nu_{H} \approx 4 \times 100 = 400 \]

making it a 400 MHz spectrometer. The same rule puts carbon at 125 MHz on a 500 and at 150 on a 600. Remembering the four to one ratio lets you name any instrument from either of its two commonest channels, and it also explains the sensitivity gap between the nuclei.

Answer: D

Any variation in wall thickness or straightness distorts the field across the sample and no shim can fully compensate, so precision tubes are worth their price for high resolution work while cheap ones cost resolution. Spinning stresses are modest at twenty hertz, borosilicate glass is transparent to the radiofrequencies used, and volume calculation is not a routine concern. A scratched, bent or cheap tube is a frequent and easily missed cause of a spectrum that refuses to shim properly.

Answer: A

COSY, HSQC and NOESY all build a second frequency dimension by incrementing an evolution period, whereas DEPT is a one dimensional experiment that uses a variable final pulse angle to edit the carbon spectrum by attached proton count. Its output is a single spectrum with peaks pointing up or down, not a contour map. The distinction matters when planning instrument time, since DEPT takes minutes and a good HMBC can take hours.

Answer: B

A ninety degree pulse tips the equilibrium magnetisation fully into the transverse plane, where it precesses and induces the maximum signal in the coil. A one hundred and eighty degree pulse inverts it along the negative field direction and gives no signal at all, which is why it is used as the first step of an inversion recovery T1 measurement. A three hundred and sixty degree pulse returns it to equilibrium. Smaller angles such as thirty degrees are used deliberately when rapid pulsing is wanted with incomplete relaxation.