Physical Chemistry , 1st ed.

Rovibrational spectra for nonlinear polyatomic molecules are more com-
plicated. For polyatomic molecules that have a permanent dipole moment,
there can be up to three independentrotational lines superimposed on a single
normal vibration of the molecule. Figure 14.38 shows a rovibrational spectrum
of H 2 O in the gas phase, where the molecules can rotate freely. Even though
we have increased the size of our molecular system by only one atom, the com-
plexity of the spectrum has increased dramatically. But because of the large
amount of molecular data that can be derived from such spectra, the detailed

14.17 Rotational-Vibrational Spectroscopy 509

0.15

Q branch

0.00

700

Wavenumbers

630

Absorbance

690 680 670 660 650 640

0.10

0.05

R branch P branch

Figure 14.37 Rovibrational spectrum of CO 2 , showing a Qbranch.

0.25

0.00

2000

Wavenumbers

1300

Absorbance

1900 1800 1700 1600 1500 1400

0.20

0.15

0.10

0.05

Figure 14.38 Part of the rovibrational spectrum of H 2 O vapor. Compare this with Figure
14.37. Although both CO 2 and H 2 O are triatomic, H 2 O’s spectrum is much more complicated
than CO 2 ’s because it is a nonlinear, asymmetric top molecule.