NMR Spectroscopy Questions and Answers

Answer: C

Fluorine-19 is one hundred per cent abundant, spin half and almost as sensitive as the proton, so it gives excellent spectra over a very wide shift range and is heavily used in pharmaceutical work. Oxygen-16 has zero spin and is silent. Chlorine-35 and nitrogen-14 are quadrupolar and give broad lines, which is why nitrogen work uses the scarce spin half nitrogen-15, usually through an indirectly detected experiment. Phosphorus-31 is the other favourable nucleus, for the same reasons as fluorine.

Answer: D

Phosphorus-31 is the only natural isotope of phosphorus, has spin half and gives sharp lines across a shift range of several hundred ppm, so nucleotides, phosphines and organophosphates are readily distinguished. The proton keeps the largest gyromagnetic ratio, the shift range is wide rather than absent, which is exactly its usefulness, and slow relaxation would require longer delays rather than fewer scans. In biological work phosphorus NMR follows ATP and phosphocreatine in living tissue.

Answer: A

The solution must extend well beyond the receiver coil at both ends, which for a five millimetre tube means about four to five centimetres, so that the coil sees a uniform column and the shims can compensate. Filling the tube to the top wastes sample and makes spinning unstable, filling only to the coil height leaves severe end effects that no shimming can remove, and the coil certainly does not sense the whole tube equally. Poor filling is one of the commonest causes of a spectrum that will not shim.

Answer: B

A lineshape fault common to every peak points at the field rather than the chemistry, and a poorly adjusted shim, typically a low order z shim, gives exactly this asymmetric shoulder. Insufficient scans would raise the noise without deforming the lines, a wrong spectral width folds peaks back into the spectrum at false positions, and an air supply fault affects spinning or temperature. The rule of thumb is that a fault shared by all peaks is instrumental and one confined to a few is chemical.

Answer: C

If a resonance lies beyond the chosen spectral width, the digitiser cannot represent its frequency and it reappears inside the spectrum at a wrong position, often with odd phase. Changing the spectral width moves it while genuine peaks stay put, which is the test. Unrecognised coupling, exchange broadening and transmitter artefacts all exist but produce different signatures. The cure is simply to widen the window, and the habit of running a wide survey spectrum first avoids the trap.

Answer: D

Pulse duration and excitation bandwidth are reciprocally related, so a pulse of a few microseconds covers tens of kilohertz and excites the whole carbon range nearly uniformly. A long soft pulse is selective and is used deliberately when only one resonance is wanted, as in solvent suppression. Energy, repetition rate and amplifier loading are secondary considerations. This reciprocal relationship between time and frequency is the same principle that underlies the Fourier transform itself.

Answer: A

In aqueous biological samples the water proton concentration is around a hundred molar against millimolar analyte, so a selective sequence such as presaturation or WATERGATE is used to knock the water resonance down before acquisition. Without it the receiver would be dominated by water and the dynamic range of the digitiser would leave nothing for the sample. The lock signal is on another channel entirely, sidebands are cured by stopping the spin, and the solvent is not coupled to the analyte. It is what makes protein NMR in water possible.

Answer: B

Zero filling appends zeros so the transform yields more points per hertz and the line is defined more finely. It improves digital resolution without adding information, and doubling the data size once is generally worthwhile while further doubling gains little. Phasing corrects the mixture of absorption and dispersion in the displayed lineshape, line broadening is the exponential apodisation, and baseline correction fits and subtracts a slow roll. The four operations are separate steps of the same processing chain.

Answer: C

The complex transform yields absorption and dispersion parts, and if the phase correction is wrong some dispersion leaks into the displayed spectrum, giving the characteristic dip on one side of each peak. Zero order phase corrects all peaks equally and first order corrects a frequency dependent error. Excessive receiver gain clips the signal and produces baseline distortion instead, a short delay reduces intensity, and a paramagnetic impurity broadens lines. Phasing before integrating is essential, since a dispersive component integrates to nothing.

Answer: D

Unpaired electrons relax nuclei very efficiently, so both T1 and T2 fall and lines broaden, an effect large enough that degassing is worthwhile for high resolution work and essential for measuring nuclear Overhauser effects. The same mechanism is turned to advantage when a chromium relaxation agent is added deliberately to shorten long carbon T1 values in quantitative work. Paramagnetic shift reagents do move peaks, but that is a different and much larger contact and pseudocontact effect, and no impurity splits a peak.