Infrared Spectroscopy Questions and Answers

Answer: C

The first two are correct and the third reverses the two regions: functional groups are identified above 1500, and the fingerprint region below it is used to confirm identity by comparison. Getting the two the wrong way round leads to attempting assignments in the region least suited to them. Two regions answering two different questions is the structure of the whole technique.

Answer: D

The first two are correct and the third describes the dispersive instrument these replaced: an interferometer has no monochromator at all. Removing the wavelength-selecting element is precisely what produces both the multiplex and the throughput advantages, so the third statement contradicts the first two rather than merely being wrong.

Answer: A

Both are true and the second is the reason: the two bond dipoles cancel throughout the symmetric motion, so there is nothing for the oscillating field to couple to. The asymmetric stretch and the bend both do change the dipole and both appear strongly. That a molecule can vibrate vigorously and absorb nothing is the selection rule's whole content, and carbon dioxide is the standard illustration of it.

Answer: B

The reason is true but the assertion has the direction wrong: delocalisation reduces the double bond character, lowering the force constant and hence the frequency by twenty or thirty reciprocal centimetres. A correct mechanism applied in the wrong direction is the most easily missed kind of error, and tracing it through the force constant settles it.

Answer: C

The pairing is A-P, B-Q, C-R, and each entry carries a shape as well as a position - broad, strong and sharp respectively. Reading both together is what distinguishes a hydroxyl from an amine at nearly the same wavenumber, and it is why a table of group frequencies always quotes intensity and shape alongside.

Answer: D

The pairing is A-P, B-Q, C-R. Each technique introduces an artefact of its own, and knowing which artefact to expect is what prevents it being reported as a structural feature. That every preparation method leaves its own signature on the spectrum is a general truth of the technique, and the choice between them is partly a choice of which artefact is easiest to live with.

Answer: A

The first three are correct and the fourth is badly wrong: an amide absorbs near 1650, the lowest of the family, because the nitrogen's lone pair delocalises into the carbonyl. Ordering the family by how much the adjacent atom donates or withdraws - anhydride highest, amide lowest - turns eight remembered numbers into one pattern that can be reconstructed when a value is forgotten. Naming the cost of a choice alongside its benefit is what turns a fact into a design decision.

Answer: B

This is the fingerprint region, used to confirm identity by comparison with a reference rather than to assign bands individually. The group frequency region lies above it, overtones appear at roughly twice a fundamental's wavenumber, and the far infrared is below 400. A region whose very complexity makes it useless for assignment and ideal for identification is worth understanding as a deliberate division of labour. The shared properties explain why these belong to one family, and the differences explain why more than one exists.

Answer: C

Nitrogen is a homonuclear diatomic with no dipole moment, and stretching it changes nothing, so its single vibration is inactive. Water, carbon monoxide and methane all have modes that change the dipole and all absorb. That the two main constituents of air are both transparent is what allows the technique to be used at all without evacuating the instrument.

Answer: D

Raman scattering depends on a change in polarisability, so modes that are infrared inactive because they are symmetric are often Raman active and the reverse. Electronic transitions, spin transitions and molecular weight belong to other techniques. In a molecule with a centre of symmetry the two techniques are mutually exclusive, which makes running both a direct test of whether that symmetry is present.