23.6 Fluorescence, Phosphorescence, and Photochemistry 979
PROBLEMS
Section 23.5: Spectra of Polyatomic Molecules
23.35The infrared spectrum of HCN shows strong bands at
712.1 cm−^1 and at 3312.0cm−^1. There is a strong
Raman band at 2089.0cm−^1. There are weaker infrared
bands at 1412.0cm−^1 ,at2116.7cm−^1 , at 2800.3cm−^1 ,
at 4004.5cm−^1 , at 5394 cm−^1 , and at 6521.7cm−^1.
Identify these bands as fundamental, overtone, or
combination bands, and give the shape of the
molecule.
23.36The N 2 O molecule has three strong bands in its IR
spectrum, at 588.8cm−^1 , at 1285.0cm−^1 , and at
2223 .5cm−^1. The band at 588.8cm−^1 has aQbranch.
All three bands are fundamentals, and the molecule has
been shown to be linear. Explain why CO 2 , which is also
linear, has only two fundamental IR bands whereas N 2 O
has three.
23.37Using the frequencies in Problem 23.36, tell where to
look for overtone and combination bands in the spectrum
of N 2 O.
23.38The H 2 S molecule has strong infrared bands at
1290 cm−^1 , 2610.8cm−^1 , and 2684 cm−^1. There are
weaker bands at 2422 cm−^1 , 3789 cm−^1 , and 5154 cm−^1.
Assign these bands as fundamentals, overtones, or
combination bands, and specify which normal mode
corresponds to each fundamental.
23.39The NO 2 molecule has strong infrared bands at
648 cm−^1 , 1320 cm−^1 , and 1621 cm−^1. There are weaker
bands at 1373 cm−^1 , 2220 cm−^1 , and 2910 cm−^1. Assign
these bands as fundamentals, overtones, or combination
bands, and specify which normal mode corresponds to
each fundamental.
23.40a.Predict the shape of the CS 2 molecule.
b.Draw sketches depicting the vibrational modes of this
molecule. Tell which will be seen in the Raman
spectrum and which will be seen in the infrared
spectrum.
c.Describe the microwave and rotational Raman spectra
of this substance.23.6 Fluorescence, Phosphorescence, and
Photochemistry
In this section we discuss various processes that involve emission or absorption of
photons. The material in this section is somewhat separate from spectroscopy, and the
entire section can be skipped without loss of continuity.Fluorescence and Phosphorescence
We discuss an example substance, benzophenone (Ph 2 CO, where Ph stands for the
phenyl group, C 6 H 5 ). Figure 23.15 shows schematically some low-lying electronic
energy levels of benzophenone. The excited levels that are shown correspond to exci-
tation of electrons in the carbonyl group. An excited level that is reached from the
ground level by ann→π∗transition is labeled (n,π∗) and an excited level that is
reached by aπ→π∗transition is labeled (π,π∗).
There are two levels labeled (n,π∗), a singlet level (S0) and a triplet level
(S1). There are two levels labeled (π,π∗), also a singlet and a triplet. The ground
level is a singlet level. The singlet levels are labeled asS 0 ,S 1 ,S 2 , and so on, in order
of increasing energy. We reserve the subscript 0 for the ground level, so the triplet
levels are labeledT 1 ,T 2 , and so on. The selection rules allow absorption of a photon