Physical Chemistry Third Edition

996 23 Optical Spectroscopy and Photochemistry

The specific rotation often has different signs for two wavelengths between which a

spectral line occurs. Specific rotations of many substances have often been tabulated for

a single wavelength, usually the yellow sodium “D lines” at 589.0 and 589.6 nm, and the

wavelength dependence has often been ignored. However, additional information about

the stereochemical configuration of molecules can be obtained from the dependence of

the specific rotation on wavelength, which is calledoptical rotatory dispersion(ORD).

The absorptivity of an optically active substance can also differ for right-polarized

and left-polarized photons. This phenomenon is calledcircular dichroism(CD), and

is also studied as a function of wavelength. Until the 1970s, only ultraviolet and vis-

ible light were used for ORD and CD. Since then techniques have been developed

for infrared circular dichroism spectroscopy, which is usually calledvibrational circu-

lar dichroism(VCD). In addition, techniques have been invented for determining the

differences in scattering of left- and right-polarized light, and Raman optical activity

(ROA) is now being studied.^28

In addition to the types of spectroscopy that we have discussed, many other types

and techniques of spectroscopy have been developed. A lot of work is being done in

making rapid spectroscopic observations to study molecular species that have short

lifetimes. Almost any issue ofThe Journal of Chemical Physicsor theJournal of

Physical Chemistrycontains one or more articles reporting on spectroscopic techniques

that we have not discussed.

PROBLEMS

Section 23.8: Other Types of Spectroscopy

23.51By analogy with the photoelectron spectrum of N 2 ,
predict what the photoelectron spectrum of O 2 would
look like.

23.52By analogy with the photoelectron spectrum of N 2 ,

predict what the photoelectron spectrum of C 2 would

look like.

Summary of the Chapter

Electromagnetic radiation that is absorbed or emitted by atoms or molecules gives

information about energy level differences through the Bohr frequency rule

Ephotonhν

hc

λ

Eupper−Elower

Each pair of energy levels does not necessarily lead to a spectral line for emission

or absorption. Selection rules tell whether a transition with emission or absorption of

radiation can occur between a given pair of energy levels.

Transitions between rotational states lead to emission or absorption in the microwave

region. For diatomic and linear polyatomic molecules with permanent dipole moments,

the selection rule is

∆J± 1

(^28) S. C. Stinson,Chem. Eng. News, 63 (45), 21 (Nov. 11, 1985).