Doc Brown's Advanced Chemistry: PART 15.2.1 Infrared Spectroscopy Theory

15.2.2 Examples of the uses and applications of infrared spectroscopy

15.2.3 Index of infrared spectra (on this page, and added links to relevant organic section indexes)

Some simple NMR-IR problem solving questions

15.2.1 The theory of infrared spectroscopy

Sub-index for this page

(a) The basis of infrared molecular spectroscopy

(b) Vibrational modes - deformations of molecules

(c) How an infrared spectrometer works

(a) The basis of infrared molecular spectroscopy

Which radiation causes vibrational excitation and why?

One of the ways electromagnetic radiation (emr) interacts with matter is to resonate with the natural vibrational frequencies of the atoms and their bonds e.g. in an organic molecule.

However, to absorb infrared photons, the molecular vibrations must involve the oscillation of a δ+ δ- dipole aspect of the molecule - this is needed to produce an oscillating electromagnetic field which interacts (resonates) with the incoming infrared electromagnetic radiation.

The vibrations (distortions-deformations) of atoms-bond arrangements in molecules are quantised and absorption of photons of the appropriate frequency-energy will raise the vibrational level to a higher level in the molecule.

These vibrational frequencies of molecules coincide with the frequencies of the infrared region of the emr spectrum.

wavelength (λ): ~7 x 10^-7 to 10^-4 m, ~700 - ~10⁵ nm; frequency (): ~3 x 10¹² to ~4.3 x 10¹⁴ Hz

The wavelength range corresponds to ~7 x 10^-5 to ~10^-2 cm.

It is normal to quote infrared absorptions as their 'wavenumber' (1/λ) which corresponds to the reciprocal of the absorption wavelengths in cm^-1.

Therefore the wavelength range of 7 x 10^-5 to ~10^-2 cm, corresponds to 14285 to 100 cm^-1.

In practice, the region of 4000 to 400 cm^-1 is typical for the recorded infrared spectrum of organic molecules.

What causes variation in the infrared absorption frequencies?

The energy needed to excite a vibrational mode depends on bond length, the number of bonds or atoms involved and also the mass of the atoms e.g.

for the simple diatomic molecules like the hydrogen halides, as the bond enthalpy decreases, there is a clear trend of decreasing wavenumber and decreasing energy of infrared photon needed to excite the symmetrical stretching vibration of the H-X bond (which is the only vibrational mode here).

You can think of the two atoms vibrating as if the covalent bond is acting as a spring.

The weaker the bond, 'the weaker the spring', so less energy is needed to excite the vibrational quantum level.

This is a very simple example, with one vibration mode for a simple diatomic H-X molecule, but even a simple triatomic molecule like water (H-O-H) has several fundamental modes of vibration and there are extra harmonic vibrations too (see below).

What causes variation in the absorption intensity?

Absorption is usually described as weak, moderate/medium or strong depending on the decrease in % transmission (increase in absorption) - see (c) explaining % transmission.

Generally speaking, the more polar the bond the greater the intensity of absorption because of the greater intensity of the dipole oscillation e.g. O-H, C-O and C=O absorptions tend to be more intense than non-polar C-H, C-C or C=C bond stretching vibrations.

Useful characteristic frequencies

Stretching vibrations (see section (b) an A<=>B bond vibration) are often the most useful for identifying structural features in a molecule, though many are common to a large number of organic molecules e.g.

Strength of absorption key: weak W), moderate/MEDIUM (M), strong (S) or variable (V).

Bond	Wavenumber/cm-1	Examples and comments on the stretching vibration
C-H alkanes	2850-2960 (M-S)	CH3-CH2-, often S due to several of these groups
C-H alkenes	3010-3095 (V)	-CH=CH-, W, M or S
C-H alkynes	3250-3300 (S)
C-H arenes	3030-3080 (W-M)	benzene ring hydrogens, can be S due to several coincident vibrations
C-C	750-1100 (W)	saturated carbon chain
C=C alkene	620-1680 (V)	W, M or S
C≡C alkyne	2100-2250 (M)
arenes	1500-1600 (V)
C-O	1000-1300 (S)	alcohols, carboxylic acids, esters, ethers
>C=O carbonyl	1630-1750 (S)	aldehydes, amides, carboxylic acids, esters, ketones
C-N amines	1180-1360 (V)	aliphatic (W-M), aromatic (S)
C≡N nitriles	2210-2260 (M)
O-H hydroxyl	3580-3670 (S)	non-hydrogen bonded alcohols and phenols
O-H hydroxyl	3230-3550 (S-broad)	Hδ+llllδ-O hydrogen bonded alcohols and phenols
O-H hydroxyl	2500-3000 (M-broad)	Hδ+llllδ-O hydrogen bonded carboxylic acids
N-H	3320-3560 (M-S)	non-hydrogen bonded amines and amides
N-H	3100-3400 (M-S)	Hδ+llllδ-N hydrogen bonded primary and secondary amines, amides
C-Cl chloro-	600-800 (S)	chloroalkanes (chlorohalogenoalkanes)
C-Br bromo-	500-600 (S)	bromoalkanes (bromohalogenoalkanes)
C-I iodo-	~500 (S)	iodoalkanes (iodohalogenoalkanes)

Notes

(1) Hydrogen bonding affects the O-H and N-H stretching vibrations

Hydrogen bonded O-H or N-H tend to give a broader peak compared to a sharper peak for non-hydrogen bonded stretching vibrations e.g. the difference between alcohol liquid and alcohol vapour - where the molecules are not in close contact.

(2)

TOP OF PAGE and sub-index

(b) Vibrational modes - deformations of molecules

The vibrations of molecules involve some kind of distortion (deformation) of the bonding arrangement, but no bond is broken.

There are lots of stretching and twisting vibrations involving at least two atoms and one bond, but often involving at least three or more atoms and two or more bonds.

Some of the possible vibrational modes are illustrated below.

Three vibrational modes of the carbon dioxide molecule.

Some possible vibrations of an AX2 bond arrangement in a more complex molecule.

Three basic vibrational modes of the water molecule (H-O-H).

The above diagram shows the C-H, C-O and O-H stretching vibrations of the ethanol molecule.

There are lots of vibrational modes in 'modestly' complex molecules like ethanol including rocking, scissoring, stretching, twisting and wagging vibrations, and, combined with different bond strengths and harmonics, the result is a very complex infrared spectrum. 

Although the previous diagram illustrated three stretching vibrations absorptions of ethanol, which are pointed out in the above diagram of the complete infrared spectrum of ethanol.

However there are other vibrations and many harmonic vibrations derived from the fundamental vibrations so the infrared spectra of multi-atom molecules become very complex.

It is this complexity that actually underlies the identification of an organic molecule from its infrared fingerprint pattern.

Some good animation of various infrared vibrations on the webpage https://en.wikipedia.org/wiki/Infrared_spectroscopy

TOP OF PAGE and sub-index

(c) How an infrared spectrometer works

Below is a schematic diagram of one type of infrared spectrometer

From the infrared energy source (e.g. a heated filament), the beam is split into two identical beams.

One beam passes through a reference cell ('control') and the other beam passes through the sample cell and each beam is alternately sampled via the beam chopper.

The reference cell is needed to eliminate absorption by any water vapour or carbon dioxide in the air or other absorbing molecules e.g. if a solvent is used.

In air, nitrogen and oxygen do not absorb infrared photons because the vibration does not involve oscillation of a dipole (no oscillating δ+ δ- aspect to the molecular vibrations).

The sample in the cell can be liquid or vapour and a thin film of a solid can be analysed when compressed between potassium bromide discs (KBr doesn't absorb infrared radiation).

The final signal is the difference between the two beams and then passes onto the diffraction grating which is rotated to sweep through the infrared frequencies.

The thermocouple detects the signal and the output recorded on paper or computer screen.

The infrared spectrum is usually graphically 'portrayed' as % transmittance (0 to 100 for the vertical y axis) versus wavenumber (usually 4000 to 400 cm^-1 on the horizontal x axis).

100% transmission means no absorption,

Diagram from https://www.researchgate.net/figure/A-Schematic-diagram-of-a-dispersive-infrared-spectrometer-6_fig2_342953117