11.2 Ultraviolet Spectroscopy

LEARNING GOALS

After   Chapter 11.2,   you will    be  able    to:

Predict whether a   given   molecule    can be  detected    via UV  spectroscopy

Describe    the relationship    between HOMO,   LUMO,   and absorption  wavelength

Although you will never have to interpret ultraviolet (UV) spectroscopy data on the MCAT, it is fair
game for discussion. A basic understanding of how it works and when it is used will suffice. UV
spectra are obtained by passing ultraviolet light through a sample that is usually dissolved in an
inert, nonabsorbing solvent, and recording the absorbance. The absorbance is then plotted against
wavelength. The absorbance is caused by electronic transitions between orbitals. The biggest piece
of information we get from this technique is the wavelength of maximum absorbance, which tells us
the extent of conjugation within conjugated systems: the more conjugated the compound, the
lower the energy of the transition and the greater the wavelength of maximum absorbance.

ELECTRON TRANSITIONS

UV spectroscopy works because molecules with π-electrons or nonbonding electrons can be excited
by ultraviolet light to higher-energy antibonding orbitals. Molecules with a lower energy gap
between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital
(LUMO) are more easily excited and can absorb longer wavelengths (lower frequencies) with lower
energy.

MCAT EXPERTISE