NMR Spectroscopy Questions and Answers

Answer: A

HSQC gives a cross peak for every directly bonded carbon and proton pair, so it assigns the carbon spectrum from the proton assignments at a stroke and separates overlapping proton signals by their carbon shifts. Two or three bond correlations belong to HMBC, carbon to carbon correlation needs the insensitive INADEQUATE experiment, and through space proximity is NOESY. Because the experiment detects the proton rather than the carbon, it is far more sensitive than any carbon detected equivalent, which is why inverse detection became standard.

Answer: B

HMBC shows long range proton to carbon correlations, typically two and three bonds, and so links fragments across quaternary carbons and heteroatoms that no proton to proton experiment can bridge. It is the experiment that joins the pieces of a structure together. HSQC is the one bond version, COSY is proton to proton, and DEPT counts attached hydrogens without giving any correlation at all. Using HSQC and HMBC together is the standard route to an unknown structure at natural abundance.

Answer: C

A NOESY cross peak means the two protons are within roughly five angstroms of each other, whether or not they are close through bonds. That is what makes it the key to stereochemistry and to fold determination, since it distinguishes cis from trans and identifies which residues touch. Three bond relationships appear in COSY, identical shifts would put a peak on the diagonal, and chemical exchange does give cross peaks in the related EXSY experiment, which is why exchange must be excluded before a NOE is trusted.

Answer: D

The indirect dimension is created by running the pulse sequence many times with the evolution period lengthened by a fixed step each time, then transforming along that series. Each increment costs a full experiment, which is why a two dimensional data set takes minutes to hours where a one dimensional one takes seconds. There is only one receiver and one coil in the usual arrangement, and the field is held rigidly constant by the lock. Understanding the increment explains why the resolution in the indirect dimension is always the poorer of the two.

Answer: A

Both statements swap the two experiments. COSY reports scalar coupling through bonds, and NOESY reports dipolar proximity through space. Protons three bonds apart are usually also close in space, so the two often agree, but the inference runs the wrong way in each statement as written. The distinction matters most when it is being relied on: a NOE between protons on opposite ends of a molecule is powerful conformational evidence precisely because no coupling links them.

Answer: B

Because signal area is proportional to the number of nuclei, a weighed certified standard in the same tube gives the purity of the analyte without needing a pure sample of the analyte itself. That is a real advantage over chromatography, which needs a calibrated reference for every compound measured. Fragment ions belong to mass spectrometry, trace metals to atomic spectroscopy, and NMR separates nothing. The method demands a long relaxation delay and careful integration, but it has become a primary technique for reference material certification.

Answer: C

Full recovery between pulses is the requirement: waiting about five T1 leaves less than one per cent of the magnetisation unrecovered, so every signal is measured on the same footing. Decoupling in proton work would introduce Overhauser distortions rather than remove them, spinning at high rate adds sidebands that steal area, and a short acquisition truncates the decay and distorts lineshapes. Getting the delay right is tedious but it is the single condition that most often separates a quantitative result from an approximate one.

Answer: D

MRI applies a controlled gradient across the subject so that resonance frequency maps onto position, and the image is reconstructed from the spatially encoded signal, usually of water protons. Contrast comes chiefly from differences in proton density and in the relaxation times of tissues. Solid state NMR is a different specialisation, low field benchtop instruments are another, and electron spin resonance is a separate technique altogether. The gradient is the single idea that turns a spectrometer into a camera.

Answer: A

When a molecule exchanges between two environments, the lineshape changes with the rate: two sharp lines at slow exchange, a broad coalesced hump at intermediate rate and a single sharp averaged line when fast. Measuring the temperature of coalescence yields the activation energy of the process, which is how hindered rotation about an amide bond is quantified. The other options describe spinning artefacts, ordinary relaxation and magnet behaviour rather than any study of rates. It is one of the few spectroscopic methods to measure a rate constant directly.

Answer: B

At coalescence the exchange rate is fixed by the shift separation, and the Eyring equation converts that rate at that temperature into the free energy of activation for the rotation. This is the standard measurement of amide rotation barriers, typically around eighty kilojoules per mole. The equilibrium constant would come from the relative intensities at slow exchange instead, extrapolating a shift to absolute zero is meaningless, and T1 is measured by inversion recovery. Coalescence is a rate measurement, not an equilibrium one.