Answers to Concept Checks

11.1

IR  spectroscopy    measures    absorption  of  infrared    light   by  specific    bonds,  which   vibrate.    These

vibrations  cause   changes in  the dipole  moment  of  the molecule    that    can be  measured.   Once    the

bonds   in  a   molecule    are determined, one can infer   the presence    of  a   number  of  functional  groups

to  determine   the identity    of  the molecule.

A   carboxylic  acid    would   have    a   broad   O–H peak    around  2800–3200   cm–1    and a   sharp   carbonyl

peak    at  1700–1750   cm–1.

11.2
Molecules with π or nonbonding electrons, and conjugated systems, will give absorbances on a
UV spectroscopy plot.
HOMO is the highest occupied molecular orbital; LUMO is the lowest unoccupied molecular
orbital. The smaller the difference in energy between the two, the longer the wavelengths that
can be absorbed by the molecule.

11.3
NMR measures alignment of the spin of a nucleus with an applied magnetic field. It is most
often used for identifying the different types and magnetic environments of protons in a
molecule, which allows us to infer the connectivity (backbone) of a molecule.
The units for chemical shift with a standardized NMR spectrum are parts per million (ppm).
Deshielding occurs in molecules that have electronegative atoms that pull electron density
away from the hydrogens being measured. This results in a downfield (leftward) shift of the
proton peak.
Spin–spin coupling occurs when two protons close to one another have an effect on the other’s
magnetic environment. This results in the splitting of peaks into doublets, triplets, or multiplets,
depending on the environment.