UV-Visible and Infrared Spectroscopic Methods in Environmental Analysis - Fundamentals of Environmental Sampling and Analysis - page 200

For CO 2 molecule described above, infrared radiation at 2349 cm 1

m)
excites this particular vibration. The symmetric stretch is not infrared active and so
this vibration is not observed in the infrared spectrum of CO 2 . The two equal-energy
bending vibrations in CO 2 (Fig. 8.9c and d) are identical except that one bending
mode is in the plane of the paper and the other is out of the plane. Infrared
radiation at 667 cm 1

(4.26 m

m) excites these vibrations. Figure 8.10 is the IR
spectrum of CO 2 clearly showing two bands of IR absorption due to the
molecular vibration—one at 4.26

(15 m

m m due to asymmetric stretching and the

other at 15 m m due to bending.

Figure 8.10 Infrared spectrum of carbon dioxide (CO 2 ). Source: NIST Mass Spec Data Center, S.E.
Stein, director, ''Infrared Spectra,'' in NIST Chemistry WebBook, NIST Standard Reference Database
Number 69, Eds. P.J. Linstrom and W.G. Mallard, June 2005, National Institute of Standards and
Technology, Gaithersburg MD, 20899 (http://webbook.nist.gov)

Besides stretching and bending of a chemical bond, more complicated
molecules vibrate in rocking, twisting, scissoring, and wagging modes, which
arise from combinations of bond bending in adjacent portions of a molecule.
Without revealing the details of molecular vibrations, some general trends can
be stated. That is, the stronger the bond the more energy will be required to
excite the stretching vibration. Thus, stretches of organic compounds with triple
bonds such as C CandC

N occur at higher frequencies than stretches for

double bonds (C

C, C N, C O), which are in turn at higher frequencies than

single bonds (C C, C N, C H, O H, or N H). The heavier an atom the
lower the frequencies for vibrations that involve that atom.

8.1.3 Molecular Structure and UV-Visible/Infrared
Spectra

A spectrum is a plot of absorption vs. wavelength. Spectra of various types are
the basis to deduce structures for chemicals of unknown identities to confirm