NMR Spectroscopy Questions and Answers

Answer: C

The first two are sound. Equivalence of the neighbours is exactly the condition for a single coupling constant and a binomial pattern, and the intensities follow Pascal's triangle. The third is where the rule fails: once the shift difference falls towards the coupling constant the spectrum becomes second order, intensities distort and extra lines appear. Coupling to two inequivalent neighbours gives a doublet of doublets rather than a triplet, which is the other common way the naive rule misleads.

Answer: D

With two different coupling constants the signal splits twice, giving four lines of equal intensity, a doublet of doublets, whose two spacings are the two coupling constants. Only if the couplings happen to be equal do the inner lines coincide and produce the familiar 1:2:1 triplet. Recognising a doublet of doublets that has nearly collapsed is a common source of error, and expanding the region usually resolves it. Couplings never cancel, and the quartet needs three equivalent neighbours.

Answer: A

The first three are right and the last is wrong. A quintet, five lines with intensities 1:4:6:4:1, needs four equivalent neighbours, not three; three neighbours give a quartet. The rule is simply that n equivalent neighbours give n plus one lines, so the middle methylene of a propyl chain flanked by a methyl and a methylene shows a multiplet approaching a sextet when the two couplings are similar. Counting neighbours correctly is the first habit to build when reading a proton spectrum.

Answer: B

T1 is the time constant for the longitudinal magnetisation to return towards its equilibrium value after a pulse, energy passing from the spins to their surroundings, the lattice. It sets how long you must wait between pulses: about five times T1 for essentially complete recovery. Loss of transverse coherence is T2, the sample reaches room temperature long before any of this, and the pulse interval is chosen by the operator in the light of T1. A long T1 is the reason quaternary carbons are weak in a fast experiment.

Answer: C

T2 measures the decay of transverse coherence, and the natural width of a line at half height is one divided by pi times T2. A short T2 therefore gives a broad line, which is why large molecules, viscous solutions and quadrupolar nuclei give broad spectra. In practice the observed width also includes field inhomogeneity, and the combined constant is written T2 star. Shift is a shielding matter, the Overhauser effect depends on dipolar relaxation pathways, and the scan count follows from sensitivity. Linewidth is the visible face of T2.

Answer: D

The natural width at half height is the reciprocal of pi times T2:

\[ \Delta\nu = \tfrac{1}{\pi \times 0.32} \approx 1 \]

so the line is about 1 Hz wide, which is a good high resolution figure. Observed widths are usually larger because shimming is never perfect. Knowing the natural width tells you the best resolution the sample itself will allow, and therefore whether a failure to resolve a small coupling is the fault of the instrument or of the chemistry.

Answer: A

Slow molecular tumbling makes the dipolar interaction average less completely, so transverse relaxation is efficient, T2 is short and lines are broad. The effect grows with molecular mass and is the principal barrier to NMR of very large assemblies, addressed by deuteration, high field and specialised experiments. Quadrupolar nuclei are no more common in proteins, shift dispersion is if anything an advantage in a folded protein, and concentration affects viscosity only indirectly. Correlation time is the governing quantity throughout.

Answer: B

Saturating one nucleus perturbs the populations of another close in space through dipolar relaxation, so the intensity of the second changes, positively for small molecules. Because the effect falls off as the inverse sixth power of distance it is short range and gives genuine geometric information, which is what makes NOESY central to structure determination. It does not move peaks or split them, and it is quite distinct from exchange broadening. It is also the reason a proton decoupled carbon spectrum is more intense than the decoupling alone would explain.

Answer: C

The inverse sixth power dependence means the effect is measurable only out to roughly five angstroms, which conveniently spans a hydrogen bond, a ring stacking contact or the width of a peptide turn. Fifty angstroms is far beyond reach, the effect certainly does not require a bond between the two nuclei, which is the whole point, and conjugation is irrelevant since the mechanism is through space. A set of such distance constraints is what allows a protein fold to be computed from an NMR data set.

Answer: D

COSY places a cross peak wherever two protons share a scalar coupling, so the off-diagonal peaks trace the coupling network and let a spin system be walked from one end to the other. Spatial proximity without coupling is NOESY, the one bond carbon to proton correlation is HSQC or HMQC, and the longer range carbon correlation is HMBC. Reading a COSY is largely a matter of stepping from the diagonal to a cross peak, across to the diagonal again, and repeating, which assembles the chain.