Page 276 - Handbook of Instrumental Techniques for Analytical Chemistry

266 Handbook of Instrumental Techniques for Analytical Chemistry

IR absorption at specific, narrow frequency ranges regardless of their relationship with the rest of the
molecule. Generalized tables of the positions and relative intensities of absorption bands (Fig. 15.13)
have been established and used to determine the functional groups present or absent in a molecule. The
CRC Handbook of Chemistry and Physics (2), Silverstein, Bassler, and Morrill’s book (3), and a num-
ber of other publications all contain useful correlation charts.
Multiple functional groups may absorb at one particular frequency range, but a functional group
often gives rise to multiple-characteristic absorption. Thus, the spectral interpretations should not be
confined to one or two bands and the whole spectrum should be examined. To confirm or better eluci-
date the structure of an unknown substance, other analytical information provided by nuclear magnetic
resonance (NMR), mass spectrometry (MS), or other chemical analysis should also be used where pos-
sible. For systematic evaluation, the IR spectrum is commonly divided into three regions.
–1
The Functional Group Region, 4000 to 1300 cm The appearance of strong absorption bands in the
–1
region of 4000 to 2500 cm usually comes from stretching vibrations between hydrogen and some oth-
er atoms with a mass of 19 or less. The O-H and N-H stretching frequencies fall in the 3700 to 2500

–1
cm region, with various intensities. Hydrogen bonding has a significant influence on the peak shape
and intensity, generally causing peak broadening and shifts in absorption to lower frequencies. The C-
–1
H stretching bands occur in the region of 3300 to 2800 cm . The acetylenic C-H exhibits strong ab-
–1 –1
sorption at about 3300 cm . Alkene and aromatic C-H stretch vibrations absorb at 3100 to 3000 cm .
–1
Most aliphatic (saturated) C-H stretching bands occur at 3000 to 2850 cm , with generally prominent
intensities that are proportional to the number of C-H bonds. Aldehydes often show two sharp aldehy-
–1
dic C-H stretching absorption bands at 2900 to 2700 cm .
–1
The absorption bands at the 2700 to 1850 cm region usually come only from triple bonds and oth-
–1 –1
er limited types of functional groups, such as C”C at 2260 to 2100 cm , C”N at 2260 to 2220 cm ,
+ –1 –1
diazonium salts –N ”N at approximately 2260 cm , allenes C=C=C at 2000 to 1900 cm , S-H at 2600
–1 –1 –1
to 2550 cm , P-H at 2440 to 2275 cm , Si-H at 2250 to 2100 cm .
–1
The 1950 to 1450 cm region exhibits IR absorption from a wide variety of double-bonded func-
–
tional groups. Almost all the carbonyl C=O stretching bands are strong and occur at 1870 to 1550 cm
1 –1
. Acid chlorides and acid anhydrides give rise to IR bands at 1850 to 1750 cm . Whereas ketones,
–1
aldehydes, carboxylic acids, amides, and esters generally show IR absorption at 1750 to 1650 cm ,
-1
carboxylate ions usually display stretching bands at 1610 to 1550 and 1420 to 1300 cm . Conjugation,
ring size, hydrogen bonding, and steric and electronic effects often result in significant shifts in ab-
sorption frequencies. Nonconjugated aliphatic C=C and C=N have absorption bands at 1690 to 1620
–1
cm , with variable intensities. Aromatic compounds contain delocalized p electrons from the reso-
nance-stabilized double bonds, showing skeletal vibrations (including C-C stretchings within the ring)
–1 –1
in the 1650 to 1400 cm region and weak combination and overtone bands in the 2000 to 1650 cm
region. Valuable information about the substitution pattern on an aromatic ring can be obtained by
careful examination of absorption bands in these two regions. Molecules containing NO 2 groups, such
as nitro compounds, nitrates, and nitramines, commonly exhibit asymmetric and symmetric stretching
–1
vibrations of the NO 2 group at 1660 to 1500 and 1390 to 1260 cm regions.
–1
The Fingerprint Region, 1300 to 910 cm Absorptions in this region include the contributions from
complex interacting vibrations, giving rise to the generally unique fingerprint for each compound. A
good match between the IR spectra of two compounds in all frequency ranges, particularly in the fin-
gerprint region, strongly indicates that they have the same molecular structures.
Detailed interpretation of IR bands in this region is difficult. However, some assignments of bands
in the fingerprint region to a few important vibrational frequencies of functional groups can be done
when IR absorptions in other regions are correlated together. For example, esters not only show their