carbonyl oxygen is electron-withdrawing and deshields the proton. Choice (C) corresponds to a
carboxyl proton and is even further downfield because the acidic proton is deshielded to a
greater degree than the aldehydic proton. Choice (B) corresponds to aromatic protons. Choice
(D) is characteristic of an alkyl proton on an sp^3 -hybridized carbon.5 . C
This isotope has no magnetic moment and will therefore not exhibit resonance with an applied
magnetic field. Nuclei with odd mass numbers (^1 H, ^11 B, ^13 C, ^15 N, ^19 F, and so on) or those with an
even mass number but an odd atomic number (^2 H, ^10 B) will have a nonzero magnetic moment.6 . D
Spin–spin coupling (splitting) is due to influence on the magnetic environment of one proton by
protons on the adjacent atom. These protons are three bonds away from each other. Splitting in
other NMR spectra can include coupling with carbon atoms, but not in ^1 H–NMR.7 . C
Most conjugated alkenes have an intense ultraviolet absorption. Aldehydes, ketones, acids, and
amines, mentioned in choices (A) and (D), all absorb in the ultraviolet range. However, other
forms of spectroscopy (mainly IR and NMR) are more useful for precise identification. Isolated
alkenes, choice (B), can rarely be identified by UV spectroscopy.8 . B
The region in question often gives information about the types of alkyl groups present.
Specifically, ethanol will give a characteristic triplet for the methyl group (which is coupled to –
CH 2 –) and a quartet for –CH 2 – (which is coupled to the methyl group). Isopropanol will have a
septet for the –CH– group (which is coupled to both methyl groups combined) and a doublet for
the two methyl groups (which are coupled to –CH–). In both cases, the proton in the alcohol does
not participate in coupling. The alcohol hydrogen likely lies downfield for both compounds
because it is bonded to such an electronegative element.9 . A
The HOMO is the highest occupied molecular orbital. Only after absorbing ultraviolet light is an
electron is excited from the HOMO to the LUMO, the lowest unoccupied molecular orbital.10 . A
Carbonyl groups (C=O) in conjugation with double bonds tend to absorb at lower wavenumbers