Infrared (Ir) Spectroscopy (Spectrophotometry)

INFRARED (IR) SPECTROSCOPY (SPECTROPHOTOMETRY)

The interaction of Electromagnetic (EM) radiation with matter is one of the most useful phenomena available to chemists for the identification and quantization of unknown substances. You have already studied spectrophotometry involving the visible spectrum (colourimetry). Because most organic compounds do not absorb in the visible region (most are not colored), organic chemists commonly use non visible EM radiation to carry out spectrophotometric analysis, e.g., IR spectroscopy uses IR radiation, UV spectroscopy uses UV radiation, and Nuclear Magnetic Resonance (NMR) uses radio wave radiation. The Beer-Lambert relationship between absorbance and concentration holds true in IR and UV spectrophotometry. We will be studying these 3 methods of spectral analysis as they apply to organic compounds.

It is important that the student know the location (sequence) and wavelengths of the major bands of the EM spectrum.

Electromagnetic Radiations:

Electromagnetic radiations are forms of radiant energy, that possess no mass or weight and are electrically neutral. They also share 4 other common characteristics:

1)all pass through a vacuum in wavelike motion;

2)all travel at the speed of light (2.998  108 m/s in a vacuum)

3)all emit electric and magnetic fields

4)all have different energies, wavelengths, and frequencies

The spectrum of electromagnetic radiation, in order of increasing energy (decreasing wavelength), includes radio & TV waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays, and gamma rays.

No clear-cut separation exists between the bands so overlap of wavelengths, as shown below, is reported in various literature sources.

Wavelength ‘‘ (m)

10-14 10-12 10-10 10-8 10-6 10-4 10-2 1 102 104

Gamma raysVisible light Microwaves AM radio

Ultraviolet

X-rays Infrared (heat)TV and FM radio

1022 1020 1018 1016 1014 1012 1010 108 106 104

Frequency ‘‘(Hz)

Electromagnetic waves travel as photons, which are massless packets of energy. The energy of a photon is proportional to its frequency and inversely proportional to its wavelength.
A photon of frequency  (or wavelength ) has an energy given by...

where h = Plank’s constant, 1.58  10-37 kcals
or6.6210-37 kJs

Multiplying by Avogadro’ number (N) gives the energy (E) of a mole of photons of frequency  or wavelength  given in centimeters.

Do problems 12.5, 12.6, and 12.7

Despite the overlap of the spectral bands, analytical chemists have assigned boundaries (wavelength ranges) for each group and these must be memorized by the student.

Wavelength Boundaries of Important EM Radiation Bands

 (m) /  (m) /  (m) /  (cm-1 )
Vacuum UV / 10 - 200 / .01 - .20
Near UV / 200 - 400 / .20 - .40
Visible / 400 - 800 / .40 - .80
Near IR / .80 - 2.5 / 13000 - 4000
Mid IR / 2.5 - 25 / 4000 - 400
Far IR / 25 - 1000 / 400 - 10
Microwave / 103 - 106 / .001 - 1
Radio & TV / 1 - 104

The frequency of IR absorption by molecules is stated not in Hz but in ‘wavenumbers’ (), often called ‘reciprocal centimeters’ (cm-1). The wavenumber is simply the number of wavelengths per centimeter, which equals the reciprocal of wavelength in cm.

• Calculate the wavenumbers for the boundaries of the Near, Mid, and Far IR regions for the above table.
• Note that wavenumber is directly proportional to frequency. Mathematically, it differs from frequency by a constant multiplier, i.e., ‘c’, the speed of light in a vacuum...

compare

Do problem 12.8

Principles of absorption spectroscopy:

Much information regarding the structure of a molecule can be obtained from its absorption spectrum. The position of its absorption bands depend upon the configuration of its atom and electrons.

To a first approximation, the internal energy E of a molecule is composed of additive contributions from ...

• the electronic motions within the molecule (Ee)
• the vibrational motions of its atoms (Ev)
• the rotational motion of the whole molecule (Er)

E = Ee + Ev + Er

The energies of these contributions are ‘quantized’, i.e., they are a set of discrete values- not continuous.

When a molecule absorbs light, its energy is momentarily increased by an amount equal to that of the energy of the photons of electromagnetic radiation it absorbs. The absorbed energy changes one or more of the electronic, vibrational, or rotational states. The relative magnitudes of electronic : vibrational : rotational excitations are ca. 1000 : 50 : 1. It is thus possible to excite changes in rotational energy without exciting the vibrational or electronic states (Er but not Ev or Ee). It is also possible to excite changes in rotational and vibrational states without exciting the electronic states (Er and Ev but not Ee).

Absorption by molecules in the IR region involves changes in rotational and vibrational energies only; absorption by molecules in the UV and Visible regions produce changes in all 3 energies. For these reasons, molecular absorption in the UV, Visible and IR give rise to bands rather than sharp lines which are obtained with atomic absorption, e.g. AA.

Despite the breadth of IR bands, the IR spectra of molecules are so complex that no
2 compounds with different structures have the same IR spectrum except for optical isomers and certain high molecular weight polymers differing only slightly in molecular weight.

IR spectrometers are scanning instruments; emitting radiation from 4000 cm-1 to 650 cm-1,
(2.5 m  16 m), i.e., most of but not all of the Mid IR region. Organic compounds placed in the radiation path absorb various frequencies of IR producing a printout of
% transmittance versus wavenumber (cm-1) and wavelength (m).

• Absorption of Far IR radiation causes molecular rotation and low energy C-C vibrations. These are quantized absorptions and are measured using a different instrument called a Raman Spectrometer.
• Absorption of Mid IR radiation induces strong molecular vibrations and is measured by IR spectroscopy. The Mid IR region of the EM spectrum is also called the ‘fundamental’ region.
• Absorptions of Near IR radiation occur as a result of ‘overtones’ (resonance multiples) of H-stretching vibrations generated by Mid IR radiation.

Mid IR (2.5-25 m, i.e., 4000-400 cm-1)

To understand the kinds of molecular vibrations which occur when this type of radiation is absorbed, we need to picture a molecule and its bonds as an assembly of spheres (atoms) and springs (bonds).

There are 2 kinds of molecular vibrations; i.e., stretching and bending.

1. Stretching vibrations involve bond length change, i.e., the distance between atoms increase or decreases but the bond angle remains unchanged.

1. Bending vibrations involve a change in bond angle, i.e., atoms change positions while maintaining a constant distance between atoms. Bending vibrations are also called ‘deformations’.

The frequency () of vibrations in molecules is not a continuum but is quantized and determined by several factors... a) mass of the atoms, b) EN of atoms (dipole strength) and c) bond strength and type (i.e., single, double or triple).

When IR radiation of the same  as the  of vibration of a bond strikes a bond, the intensity (amplitude) of the vibration of the bond increases, i.e., energy is absorbed. If the  of the radiation is not equal to the  of vibration of the bond, the radiation is not absorbed.

When one considers the number of different kinds of atoms, bond lengths and dipole strengths that exist in a molecule, it turns out that there are several (perhaps many) different stretching and bending 's so that there are several (perhaps many) different 's (and 's) of radiation absorbed by a single molecule.

In contrast to UV and VIS spectroscopy, where only some compounds absorb, almost all known molecules are IR-active. Exceptions include monatomics (He, Ne, Ar, Kr, etc.) and some simple diatomics (O2, N2, H2, etc.). Consequently, IR is unexcelled as a tool for molecular identification but is rather poor for quantitative analysis. In general IR cannot accurately detect substances at less than about 1% wt.

Theoretical Number of Fundamental Vibrations:

Every atom has the ability to move in 3 directions, i.e., along its x, y, & z Cartesian coordinates. A change in an atom’s position along any coordinate represents a type of motion (or ‘degree of freedom’). Theoretically, a molecule of N atoms can have 3N different types of motion ('degrees of freedom'). Translational motion involves 3 degrees of freedom. Rotational motion involves another 3 degrees of freedom. This leaves (3N-6) forms of vibrational motion for non-linear molecules or (3N-5) possible vibrations for linear molecules.

For example, methane (CH4 ), which has 5 atoms, can exhibit 3(5)-6 = 9 different vibrations.

Calculate the number of possible modes of vibration of benzene and ethanol.

Since each type of vibration usually occurs at a different frequency, it is apparent that many frequencies of IR may be absorbed by even simple molecules. As a result, the IR spectra of most organic compounds shows many absorption bands.

Terminology and Types of Fundamental Vibrations:

Consider a methylene group-CH2

1. symmetrical stretching (s CH2 ~ 2853 cm-1)
   (same symbol as used for frequency)
   

1. asymmetrical stretching (as CH2 ~ 2926 cm-1)
   (Note that asymmetric stretching occurs at slightly higher frequencies
   than symmetric stretching)
   

1. in-plane bending (scissoring) (s CH2 ~ 1465 cm-1)
   (Greek symbol 'delta')
   

1. in-plane bending (rocking) ( CH2 ~ 720 cm-1)
   (Greek symbol 'rho')
   

1. out-of-plane bending (wagging) ( CH2 ~ 1350-1150 cm-1)
   (Greek symbol 'omega')
   

1. out-of-plane bending (twisting) ( CH2 ~ 1350-1150 cm-1)
   (Greek symbol 'tau')
   

Requirements for Absorption of IR by Matter:

1. Radiation must be of correct energy, i.e., same  as the  of vibration of the bonds in a molecule.

1. There must be a change in the dipole moment when the atom vibrates. If not, the radiation is not absorbed even though the  is correct. Such a vibration is said to be "infrared-inactive".

The dipole moment () is given by  = q  r

q = the product of the magnitudes of the +'ve and -'ve charges in the dipole

r = the distance between the centers of +'ve and -'ve charges

Consider CO2:

symmetric stretch asymmetric stretch

O==C==O O==C==O

O===C===O O=C====O

sas
effective center of charge unchangedeffective center of charge changes
q 0 but  = 0 because r = 0,q 0 & r  0, so  0
no absorption occurs (IR inactive) absorption occurs (IR active)

The Theoretical Number of Fundamental Vibrations will Seldom be Observed:

The following phenomena will reduce the number of bands observed ...

1.  are outside the 4000 - 650 cm-1 range.

1. bands are too weak to be observed.

1. two bands are so close in  that they are not resolved by the spectrophotometer.

1. two or more bands absorbed at the same  (are 'degenerate') because sections of the molecule are symmetrical.

1. lack of change of dipole occurs.

It occasionally happens that extra bands are observed. Bands may occur which are not fundamental vibrations but are ‘overtones’.

Overtones are integral (whole number) multiples of a fundamental vibration, , e.g., IR absorption at 600 cm-1 may be accompanied by weak overtone absorptions at 1200 cm-1 (2), 1800 cm-1 (3), etc. Overtone absorptions are less intense than the fundamental bands by a factor of 10 to 100 times. Most absorption bands in the Near IR (0.75 - 2.5 m) are overtones of H-stretching vibrations. Overtones are commonly seen in the 5-6 m (2000-1650 cm-1) range of the Mid IR.

Phenomena which alter the position () and shape of IR absorption bands are ‘vibrational coupling’ of peaks, ‘Fermi resonance’ and ‘hydrogen bonding’.

• Vibrational Coupling may result when 2 vibrating bonds share a common atom. For example, in a C-C-O group, both C-C stretch and C-O stretch can interact (combine) to produce a single IR absorption band whose position is different than the positions of either of individual stretches.
• Fermi Resonance is a special case of vibrational coupling in which the frequency of a fundamental vibration and an overtone of another vibration happen to coincide. For example, in cyclopentanone, the C=O stretch at 1750 cm-1 coincides with the overtone of an -methylene (-CH2) bend. With good resolution, the apparently single absorption band is shown to be a ‘doublet’, i.e., it’s absorption band has a split end.
• Hydrogen Bonding occurs most frequently in alcohols (-OH groups), amines (-NH groups) and amides (CONH groups) all of which contain electropositive H and electronegative O, or N atoms. Hydrogen bonding alters the strength of these bonds and results in increased band intensity and band widening. H bonding is increased when samples are run in polar solvents and is reduced when run in nonpolar solvents. Other proton acceptor groups, beside O and N are the halogens (X), and unsaturated groups (C=C).

Intermolecular Hydrogen Bonding:

Molecules with polar hydrogen groups usually associate readily and their spectra are very dependent on the state of the sample.

Consider cyclohexanol: Cyclohexanol normally exists as a polymeric aggregate held together by hydrogen bonds.

If the concentration is decreased, this tends to break up H-bonding into monomers.

Trimers  Dimers  Monomers

The following spectrum of cyclohexanol illustrates hydrogen bonding dependence. The spectra were measured in CCl4 solution.

Intramolecular Hydrogen Bonding:

Intramolecular hydrogen bonds are formed when the proton donor and acceptor are present in the same molecule under conditions that would allow formation of a 5- or 6-membered ring.

For example, o-hydroxyacetophenone exhibits strong intramolecular H-bonding. A broad, medium-to-strong band is observed near 3100 cm-1 even as a dilute solution in a non polar solvent.

The Relative Position of Stretching Frequencies of 2-atom Vibrations can be easily estimated since the main participants in most vibrations are just 2 bonded atoms. The  of their vibrations depend upon the 2 masses and the bond strength between them, i.e.,

1.  is proportional to () the bond strength ('force constant', 'f') which EN & # of bonds

1.  is inversely  the masses of the atoms

For example,

• strong CC bonds absorb at higher wavenumbers than C=C which in turn is higher than C-C bonds.
• C-H absorbs at a higher wavenumber than C-D because the CH group has lower mass.
• The highly polar O-H bond absorbs at a higher wavenumber than N-H which is higher than C-H, even though the mass of these groups is decreasing.

The Exact Position of Stretching Frequencies of 2-atom Vibrations can be calculated (approximately) using Hooke's Law.

c = 3.00  1010cm/s
Mx and My are masses of atoms (g)
f = force constant (bond strength) in
dynes/cm

Values of force constants for these calculations are tabulated below.

1

Bond'f' (dyn/cm)

H-F9.7  105

H-Cl4.8  105

H-Br4.1  105

H-I3.2  105

H-O7.8  105

H-S4.3  105

H-N6.5  105

H-C5  105

Bond'f' (dyn/cm)

Cl-C3.4  105

C-C5  105

C=C10  105

CC15  105

N-N4.5  105

N=N13  105

NN23  105

C-O5.4  105

C=O12.5  105

1

1

Example: Calculate the absorption wavenumber for OH, CH, and CD bond stretching.

1

Generalizations re: IR Absorption Band Positions:

• fundamental stretching range = 4000-800 cm-1 (2.5-12.5 m)
• fundamental bending range = 1700-400 cm-1 (6-25 m)

The MID IR region is divided into 2 regions:

1. The 'group frequency' region = 4000-1250 cm-1 (2.5-8 m)

1. The 'fingerprint' region = 1250-400 cm-1 (8-25 m)

In the group frequency region, absorption bands are characteristic of specific functional groups (OH, NH2, C=O, C-H, etc.). These appear at fairly constant positions, rather independent of the rest of the molecule.

In the fingerprint region, vibrational frequencies are greatly affected by the whole molecular structure and spectra are considered specific for a particular molecule. Some functional group absorption can be identified in the fingerprint region, especially below 1000 cm-1.

The region between 1400-1000 cm-1 is often congested with deformation (bending) bands and is difficult to interpret and many (but not all) bands in this region are often ignored.

Recall the relationship:  f  1/mass and note the general locations of functional group stretches in the functional group region of the IR spectrum.

Intensity of Absorption Bands:

Intensity  change in dipole moment. Bond with weak dipoles absorb weakly, e.g., ...
C=C, extinction coefficient () ~ 5. Similarly, C=N, and C-C give weak stretching bands.
O-H, C=O, C-Cl, C-F and Si-O are intense,  = 100-1000.

• Note that the intensity of even 'intense' IR bands are 2-3 orders of magnitude less than those of UV and visible absorption bands. The low energy of IR radiation causes instrumentation problems. For example, the signals measured are often of the same order of magnitude as electronic noise in a detecting circuit. In addition, warm components of the spectrometer radiate IR and this false radiation must be distinguished from the signal.
• The low energies of IR require wide slits in IR spectrometers; often the same width as the absorption band, i.e., several m. As a result, the resolution of IR spectrometers is much poorer than Visible spectrometers where slit widths are around 1 m.
• Quantitation is poor by IR. About  5% accuracy is optimum. Absorption band intensities are reported simply as very strong (vs), strong (s), medium (m), weak (w),
  very weak (vw), or weak to medium (wm).

Physical State of the Sample Affects Group Frequencies and Band Shapes:

IR analysis can be carried out on gas, liquid, or solid samples. The variation of  of absorption bands in various phases is small (only 1-2%) unless specific interactions like hydrogen bonding or solvent effects occur. However, the appearance and complexity of bands of spectra are strongly affected by the physical state of the sample.

• Gas phase IR scans are the simplest (smoothest) since few intermolecular interactions occur. Absorption band tend to broader than in the liquid or solid state.
• Liquid phase IR scans show more squiggles and extra small peaks due to restricted rotation caused by interaction between molecules in the liquid phase. Absorption bands are often symmetrical, similar to the ‘normal’ curve.
• Solid phase IR scans are most sharp and most complex. Many bands are split.
• Dilute solutions of solute in nonpolar solvents give the most reproducible band frequencies and shapes. Polar solvents tend to shift bands to lower  (longer ), e.g., OH from 3620  3300 cm-1. Solvent concentration affects intermolecular hydrogen bonding but not intramolecular.
• Temperature has little effect on IR spectral band position or shapes.

IR Instrumentation:

The IR beam is split; ½ is directed through the sample cell and ½ through the reference cell (e.g. solvent only or air). The beams are chopped by a rotating mirror (chopper) that alternately directs the sample and reference beams to the monochromator.
The monochromator separates the beams into its various wavelengths that strike the detector. A chart recorder prints a plot of %T vs. wavenumber (cm-1) and wavelength (m). A full scan typically requires ~ 5 min.

Note that the sample is placed before the monochromator, so that the monochromator can dispose of extraneous radiation.

In general, lenses are avoided and focusing is accomplished with variously shaped mirrors - usually made of glass or fused quartz with aluminum deposited on the front face. Such mirrors reflect up to 96% of IR.

With more expensive instruments, called Fourier Transform IR spectrometers, the entire IR spectrum is passed through the sample several times per second and the data is averaged. Higher resolution and much faster scan times are achieved.

Radiation Sources:

1. The Nernst Glower or Nernst filament, which is fabricated from a binder and oxides of Zr, Th, and Ce or Y, is shaped into a hollow rod. When electrically connected, the material heats to ~ 1500 C and emits IR (maximum emission at 7100 cm-1).

1. The Glowbar is a small rod of SiC, which when heated to ~1500 C by electrical conduction, emits IR with maximum emission at 5500-5000 cm-1.

Detectors:

1. Thermocouples convert heat (from radiant energy) into emf.

1. 'Bolometers' use a Wheatstone bridge as a detector. The electrical resistance of a filament changes with its temperature, which varies with the intensity of IR radiation transmitted through a sample.

Sample Handling:

Gases are viewed in evacuated cells (up to10 cm. wide).

Liquids are scanned in either pure ('neat') form or in solution with a solvent. Non volatile neat liquids are examined between flat salt (NaCl) plates without a spacer. Capillary thickness is 0.01mm or less. Volatile liquids (b.p. < 70 C) must be examined in very thin sealed cells which have thin spacers between salt plates.

Solutions are handled in cells of 0.1-1 mm path length at concentrations ranging from 0.05 to 10%. All solvents absorb at some part of the MID IR range, so if the entire spectrum is to be studied, several solvents must be used in sequence.
For example:CCl4 is absorbance free above 1333 cm-1
CS2 is absorbance free below 1333 cm-1
Solvent-solute interactions must be avoided, e.g., CS2 cannot be used with 1 or 2 amines and amino alcohols react slowly with CCl4 and CS2.