UV-Visible and Infrared Spectroscopic Methods in Environmental Analysis - Fundamentals of Environmental Sampling and Analysis - page 198

Table 8.2 Examples of absorption due to n ! s* transition (Skoog
et al., 1997) a

Compound l max (nm) e max

H 2 O 167 1480

CH 3 OH 184 150

CH 3 Cl 173 200

CH 3 I 258 365

(CH 3 ) 2 S 229 140

(CH 3 )O 184 2520

CH 3 NH 2 215 600

(CH 3 ) 3 N 227 900

a Sample in vapor phase except (CH 3 ) 2 S in ethanol; e max in L/mol/cm.

(the UV-absorbing functional group in a molecule) in the UV range due to
n ! p * and p ! p * transitions.
The UV chromophores listed in Table 8.3 reveal important information
regarding which chemicals can absorb UV and, therefore, can be analyzed by UV
spectroscopy skoog et al., 1997. Compounds with UV chromophores include dienes
and polyenes (C

C), carbonyl compounds (C O), and benzene derivatives. Dienes

are hydrocarbons that contain two double bonds. If the double bonds are separated
by two or more single bonds, they are unconjugated dienes . If the double bonds are
separated by one single bond, they are conjugated dienes . The series of
conjugated double bonds typically absorb strongly in the UV range of the
electromagnetic spectrum.

Table 8.3 Examples of absorption due to n ! p* and p ! p* transitions

Chromophore Example Excitation l e Solvent

C

C

Ethene p ! p* 171 15,000 Hexane

C

C

1-Hexyne p ! p* 180 10,000 Hexane

C

O

Ethanal n ! p* 290 15 Hexane

p ! p* 180 10,000 Hexane

N

O

Nitromethane n ! p* 275 17 Ethanol

p ! p* 200 5,000 Ethanol

IR Absorption and Vibrational and Rotational Transitions

From the above discussion, we understand the origin of UV sorption from the
molecular orbitals standpoint. Now let us try to understand the origin of IR
absorption and how this is related to vibrational and rotational transitions (Jang,
1996; Pecsok and shield, 1968).
Molecular vibrations can be understood by imagining a diatomic molecule as
two spheres connected by a spring. When the molecule vibrates, the atoms move