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41
Resolution is the minimum range of frequencies or wavelengths that
can be distinguished under given conditions for recording spectra.
Wavenumber is the number of wavelengths of electromagnetic radia-
tion in one centimeter.
The analytically useful IR-region ranges from 4000 sm-1 (2500 nm) to
300 sm-1 (33000 nm) and is called the mid or fundamental IR-region. Usually, vibrational spectra are obtained in coordinates
Т,% - , cm-1.
The ratio I / I0 of the intensity of the light flux passing through the substance to the initial intensity of the light flux is called the transmittance or
transmission T.
More often, transmission is expressed not in fractions of a unit, but as
a percentage:
.
6 .1. O r i g i n o f a b s o r p t i o n b a n d s
i n t h e I R - s p e c t r u m
The basis for obtaining IR-spectra is the absorption of electromagnetic
radiation when it passes through a layer of matter. The absorption of electromagnetic radiation is associated with an increase in the internal energy of
the molecule – excitation.
The internal energy of the
can be represented as the sum of four
components:
,
where is the energy of the movement of electrons around the nuclei;
is the energy of the oscillation of atoms near their equilibrium position;
is the energy of the reorientation of the spins of the nuclei;
is
the energy of translational motion.
The first three types of energy are quantized (that is, they take not any,
but certain discrete values). The energy of translational motion can change
42
continuously; at room temperature, this component is smaller than the others,
and it can be neglected. In the absence of excitation, the molecule is characterized by the lowest energy, and when radiation is absorbed, a transition
to a higher energy level occurs. However, only those quanta are absorbed
whose energy is equal to the difference between the energy levels – the “energy gap” of the molecule.
The “energy gaps” of electronic, vibrational and rotational transitions
are unequal:
It is possible to calculate the energy of its quanta by knowing the wavelength of electromagnetic radiation:
,
where c = 3·10-8 m/s is the speed of light in vacuum; h = 4.6 ·10
-13
kJ·s/mol
is Planck's constant.
The average IR-region corresponds to the quantum energy
of 4–55 kJ/mol. This energy is not enough to excite electronic transitions.
Infrared spectra are vibrational-rotational in nature; their occurrence is
associated with the excitation of vibrational and rotational energy levels of
the molecule.
6 .2. T h e m a i n t y p e s o f o s c i l l a t i o n s a n d b a n d s
in t h e I R - s p e c t r a
All vibrations in the molecule are divided into two types: valence and
deformation. Valence vibrations are called, which are carried out along
the bond axis of two atoms without changing the angle between them. Deformation vibrations are vibrations associated with changes in valence angles
(bond lengths do not actually change) (Fig. 6.2).
43
a b c d e
Fig. 6.2. Some types of vibrations of formaldehyde molecules:
a – deformation symmetric
s
; b – deformation antisymmetric
аs
;
c – valence vibration of carbonyl; d – valence symmetric C-H
s
;
e – valence antisymmetric C-H
аs
For linear n-atomic molecules, the number of basic vibrations is (3n – 6).
In addition to them, the IR-absorption spectra contain:
– combination bands, which are the result of adding or subtracting
the frequencies of other vibrations (x + y) or (x - y);
– degenerate bands – vibration bands of several atomic groups with
the same or close frequency;
– skeletal bands – vibration bands of atomic groups;
– overtones – bands with frequencies that are multiples of the frequen-
cies of the main vibrations (2x, 3x, etc.)
It should be remembered that not all vibrations appear in the IR-spectra,
but only those at which a change in the dipole moment of the molecule oc-
curs. For example, symmetric stretching vibrations с=с of ethylene and
other symmetrically constructed compounds are spectrally inactive.
6 .3. Q u a l i t a t i v e I R - a n a l y s i s
Bands, the presence of which in the spectrum proves the presence of
certain structural elements in the substance under study, are called charac-
teristic.
Comparison of their frequencies with the frequencies in the IR-spectrum of this compound is the basis for a qualitative analysis based on vibrational IR-spectra (Fig. 6.3).
44
Fig. 6.3. Assignment of bands in the IR-spectrum of 3-methylpentene-1
Each compound has its own unique IR-absorption pattern – a characteristic spectrum.
For various classes of compounds, there are numerous atlases
of IR-spectra, data banks, computer-based information retrieval systems.
However, the role of the researcher remains significant, since it is not always
possible to take into account all the numerous factors that affect the nature of
the IR-spectrum of a compound.
If the characteristic frequency of some oscillation is unknown, the corresponding band can be approximately found using simple relations.
1. The stronger the bond, the higher the oscillation frequency:
с-с
1000 cm-1
с=с
1600 cm-1
сс
2200 cm-1
2. The greater the mass of bound atoms, the lower the oscillation fre-
quency:
3. The frequency of stretching vibrations is greater than the frequency
of deformation
с=с
1700 cm-1
с-с
1000 cm-1
с-н
3000 cm
-1
с-о
1100 cm-1
45
4. Vibrations of a more polar bond generate more intense bands in
the spectrum:
С=О > C=N >C=C
For the convenience of decoding the spectrum of an unknown compound, the entire area of 675–3650 cm-1 can be divided into more intervals
and analyzed each of them separately.
1. 2700–3650 cm-1. The region of stretching vibrations of X-H bonds
(X=C, O, N). All vibrations are quite intense. Weak bands may be overtones
of bands in the 1600 cm-1 region (for example,
с-о
), or may arise due to
the presence of a small amount of water in the sample.
Aliphatic hydrocarbons absorb in the range of 2800–3000 cm-1, aromatic and unsaturated 3100 cm-1. The highest frequency
3300 cm-1
belongs to the –С=С-Н fragment.
The last band already falls into the region of NH and OH vibrations 3200–3600 cm-1, which, as a rule, corresponds to broadened bands of
medium intensity.
Organic acids are easily distinguished by wide asymmetric absorption
throughout the region, sometimes extending up to 2000 cm-1, and ammonium
salts (NH4+) by the presence of several narrow bands.
2. 1850–2700 сm-1. “Region of Transparency”. In addition to the al-
ready noted vibrations, stretching vibrations of triple and cumulated (located
in a row, for example, R2C=C=CR2 or RN=C=S) bonds are manifested,
mainly in the range of 1900–2400 cm-1. There are few stripes, so they are
easily identified.
3. 1450–1850 cm-1. The region of vibrations of the double bond.
The most common and characteristic vibrations of the carbonyl group are:
– 1750–1850 cm-1 for anhydrides and acid halides;
– 1600–1780 cm-1 for ketones, aldehydes, carboxylic acids, esters and
amides.
In this region, aromatic compounds with a low degree of substitution
also absorb (3–4 sharp bands), which correspond to a weaker absorption of
1000–1200 cm-1 and out-of-plane deformation vibrations С-Н 670–900 cm-1.
In the region of 1400–1650 cm-1, bands of bending vibrations of NH are observed.
4. 900–1500 сm-1. “Dactyloscopic region”. The spectrum pattern in this
region is strictly individual for each compound, like fingerprints, since it contains a large number of bands of the most diverse vibrations.
46
5. 675–900 сm-1. This region contains bands of C-H bending vibrations
in alkenes and benzene derivatives, as well as C-Cl stretching vibrations.
Especially useful for identifying aromatic compounds.
Fig. 6.4. IR-absorption spectra of oxygen-containing salts: a – K2CrO4;
b – KClO3; c – K2SrO4; d – КСlO4
Fig. 6.5. IR-absorption spectra of fluorides: а – CaF2; b – MgF2; c – NH4F
The information obtained on the basis of consideration of each area
complements each other well. However, a complete decoding of the spectrum
is extremely time-consuming and not always possible. Therefore, depending
on the task, a larger or smaller number of bands are considered.
It should be remembered that with IR-spectral analysis, important information can be obtained not only on the basis of knowledge about the position of the absorption bands, but also the ratio of their intensities. Therefore,
reference books almost always provide qualitative information about the intensity of absorption of vibrations.
47
Inorganic compounds also absorb IR-radiation, giving characteristic
patterns of spectra (Fig. 6.4, 6.5). However, the IR-analysis of various inorganic objects is much worse developed than organic ones, and relatively little
is used.
Vibrational spectra care sensitive to changes in the molecule. For example, isomeric (Fig. 6.6). IR-spectra are sensitive to phase transformations (Fig. 6.7), association (Fig. 6.8), formation of hydrogen bonds of
complexes (Fig. 6.9) and crystallohydrates (Fig. 6.10), etc. Therefore,
the study of IR-spectra provides additional information not only about their
composition, but also about the state of their molecules
а
b
c
Fig. 6.6. Absorption of trans-substituted benzene derivatives with different
substituents in the ring: a – 1,2,3-; b – 1,3,5-; c – 1,2,4-
а
b
Fig. 6.7. IR-spectra of polypropylene with different degrees of crystallinity:
a – 52 %; b – 75 %
48
Fig. 6.8. Fragments of the IR-spectra of dimethyl ether
-oxy--phenyl--carboethoxyethylphosphonic acid: under the conditions
of the formation of intermolecular associates A (all hydroxyl groups are
the same – one band); under the conditions of the formation of
intramolecular associates B-C and free from hydrogen binding molecules
(different hydroxyl groups give several absorption bands)
Fig. 6.9. The region of valence vibrations of OH bonds in complexes:
a – O-H...XRn (strong hydrogen bond with heteroatom X, R-alkyl);
b – O-H...XPhn (less strong hydrogen bond with heteroatom X);
c – O-H...XPh3CH (incomplete H-binding to the aromatic system)
Fig. 6.10. IR-spectra of calcium sulfate (, cm-1): a – CaSO4 (600, 615,
685, 1010, 1150, 1400); b – CaSO4·2H2O (607, 673, 1012, 1120, 1155,
1627, 1682, 3412, 3555)
49
6 .4. D e v i c e o f I R - s p e c t r o m e t e r s
All IR-spectrometers have common elements: radiation source, monochromator, optical system, receiver, amplifier (Fig. 6.11).
Fig. 6.11. Schematic representation of a two-beam IR-spectrometer
Radiation source. Usually, sources emitting a continuous spectrum ap-
proaching the radiation of a completely black body are used. In the area of
100–4000 cm-1, globars (silicon carbide rods) or a Nernst pin (a mixture of
rare earth metal oxides) are most common, sometimes a nichrome or plati-
num-bearing spiral wound on a ceramic rod is used. The source is heated by
an electric current.
Optical system and cuvettes. The optical system serves to direct the radiation along the desired path. The devices can be constructed according to
a single-beam scheme (the useful signal is compared with zero when the light
is blocked) and according to a two-beam scheme (the signal received after
the light passes through the test substance is based on a signal passing
through the channel of the device, in which the studied sample is missing
(Fig. 6.11). Such a comparison is necessary in order to distinguish a useful
signal from noise.
Focusing parts are made mainly with mirror metal coatings, lenses and
cuvette windows are more often made of potassium bromide, one of the most
transparent materials for the wavelength range used (Table 6.1). Silver chloride is used to work with aqueous solutions.
Neither glass nor quartz parts can be used in the IR-region, since glass
and quartz absorb infrared radiation.
50
Table 6.1
Materials for IR-spectroscopy
Material
Transparency area, nm
Glass
400–2600
Quartz
160–4000
LiF
120–9000
Si
1200–15000
Ge
1800–23000
NaCl
200–25000
KBr
250–40000
Monochromator. Diffraction gratings, prisms made of single crystals
of alkali metal halides, narrow-band filters made of alternating layers of dielectrics with different refractive indices can be used as a monochromator.
Detectors. Detectors can be thermal and photonic. The first group includes thermoelements – contacting layers of dissimilar metals, in which EMF
occurs when heated; bolometers – resistances with a large temperature coefficient; pneumatic receivers – devices that use thermal expansion of a non-absorbing gas located in a blackened chamber with a back wall in the form of
a flexible mirror film, the movement of which is recorded using a photocell.
The second group includes receivers that use an internal photo effect. To increase sensitivity, the receivers are usually placed in a vacuum chamber.
Registrars. The IR-region is characterized by low radiation energy,
which causes a change in the temperature of the thermoelectric element in
several thousandths of a degree. Therefore, an electronic signal amplification
circuit is used. However, the amplifier is an auxiliary unit. The recorder itself
is usually a recorder or a similar device.
All modern spectrophotometers are equipped with computers that perform primary processing of spectra: accumulation of signals, subtraction of
the background and comparison spectrum (solvent spectrum), changing
the recording scale, etc.
6 . 5 . M e t h o d s f o r p r e p a r i n g s a m p l e s f o r a n a l y s i s
Special preparation of the sample is required to ensure uniform distribution of the analyte particles in the beam. For these purposes, various
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