Infrared Spectroscopy (OCR A-Level Chemistry A): Revision Notes

Infrared spectroscopy

How infrared radiation interacts with molecules

Covalent bonds connect atoms within molecules, creating a structure that is constantly in motion. These bonds possess energy and vibrate around a central equilibrium point. As temperature increases, the amount of vibration increases correspondingly. This molecular motion allows bonds to absorb infrared (IR) radiation at specific frequencies, causing them to stretch or bend more vigorously.

There are two main types of molecular vibration that occur when bonds absorb IR radiation. The first type is stretching, which involves a rhythmic movement along the line connecting two atomic centres. During this vibration, the distance between the bonded atoms increases and decreases in a periodic fashion, similar to how a spring oscillates.

The second type of vibration is bending, which results in a change in the bond angle rather than the bond length. When bending occurs, the angle between bonds at an atom changes repeatedly.

Several factors determine how fast a particular bond vibrates. The mass of the atoms involved plays a crucial role - heavier atoms vibrate more slowly than lighter atoms because they have greater inertia. Additionally, the strength of the chemical bond matters significantly. Stronger bonds (such as triple bonds) vibrate at higher frequencies than weaker bonds (such as single bonds), as more energy is required to stretch or compress them.

A bond can only absorb IR radiation when the radiation's frequency matches the natural vibrational frequency of that bond. Since frequency values for molecular vibrations are very large numbers, chemists use a more convenient scale called wavenumber (measured in $\text{cm}^{-1}$). Wavenumber is directly proportional to frequency, making it easier to work with in practice. The vibrations of most chemical bonds occur in the IR wavenumber range of $200 \text{ cm}^{-1}$ to $4000 \text{ cm}^{-1}$.

The technique of infrared spectroscopy

Infrared spectroscopy is an analytical technique that organic chemists use extensively to identify functional groups present in organic molecules. The process involves several key steps that convert the absorption of IR radiation into useful analytical information.

The experimental procedure begins when the sample under investigation is placed inside an IR spectrometer. A beam of IR radiation spanning the range of $200\text{-}4000 \text{ cm}^{-1}$ is then passed through the sample. As the radiation passes through, the molecule absorbs some specific IR frequencies that match the natural frequencies of its bonds. The emerging beam of radiation is analysed to identify which frequencies have been absorbed by the sample.

Modern IR spectrometers are typically connected to computers that plot a graph showing transmittance (the percentage of radiation that passes through) against wavenumber. This produces the characteristic IR spectrum of the compound.

The fingerprint region

A typical IR spectrum contains a complex pattern of absorption peaks across the entire range. The region below $1500 \text{ cm}^{-1}$ is particularly important and is known as the fingerprint region. This section of the spectrum contains numerous unique peaks that are characteristic of the specific molecule being analysed. Just as human fingerprints are unique to each individual, the fingerprint region pattern is unique to each different molecule.

Identifying functional groups using IR spectroscopy

While the fingerprint region is excellent for confirming the identity of a known compound, identifying functional groups is more straightforward when examining peaks outside this region. Chemists have studied the IR spectra of thousands of organic molecules and compiled extensive data tables that correlate specific wavenumber ranges with particular bonds and functional groups.

Key absorption peaks you need to know

The OCR specification requires you to be familiar with several important functional groups and their characteristic IR absorptions. The most critical ones for identification are:

The O-H group in alcohols produces an absorption peak in the range $3200\text{-}3600 \text{ cm}^{-1}$. This peak is relatively sharp and strong, appearing at high wavenumbers. Phenols also show O-H absorption in this same range.

The C=O group (carbonyl group) found in aldehydes, ketones, and carboxylic acids creates a key absorption peak in the range $1630\text{-}1820 \text{ cm}^{-1}$. This peak is very strong and sharp, typically absorbing close to $1700 \text{ cm}^{-1}$. The C=O peak is one of the most useful for identifying organic compounds because it is strong, clear, and appears in a relatively uncrowded region of the spectrum.

The COOH group in carboxylic acids produces two distinctive peaks that together confirm the presence of a carboxylic acid. First, there is the C=O peak around $1630\text{-}1820 \text{ cm}^{-1}$ (typically near $1700 \text{ cm}^{-1}$). Second, and most characteristically, there is a very broad O-H absorption in the range $2500\text{-}3300 \text{ cm}^{-1}$. This broad O-H peak is the key identifier for carboxylic acids - it is much broader than the sharp O-H peak seen in alcohols.

There is also a C-O single bond peak in the range $1000\text{-}1300 \text{ cm}^{-1}$, but this falls in the fingerprint region and is not always reliable for identification purposes.

Complete IR data table for functional groups

The table below shows the characteristic IR absorptions for bonds within various functional groups. You should be familiar with these ranges, particularly for the groups emphasized above.

Bond	Location	Wavenumber / $\text{cm}^{-1}$
C-C	alkanes, alkyl chains	750-1100
C-X	haloalkanes (X = Cl, Br, I)	500-800
C-F	fluoroalkanes	1000-1350
C-O	alcohols, esters, carboxylic acids	1000-1300
C=C	alkenes	1620-1680
C=O	aldehydes, ketones, carboxylic acids, esters, amides, acyl chlorides, acid anhydrides	1630-1820
aromatic C=C	arenes	Several peaks in range 1450-1650 (variable)
C≡N	nitriles	2220-2260
C-H	alkyl groups, alkenes, arenes	2850-3100
O-H	carboxylic acids	2500-3300 (broad)
N-H	amines, amides	3300-3500
O-H	alcohols, phenols	3200-3600

Worked examples of spectrum interpretation

Applications of infrared spectroscopy

Infrared spectroscopy has numerous practical applications beyond the chemistry laboratory, particularly in environmental monitoring and law enforcement.

Breathalysers for detecting ethanol

IR-based breathalysers are used by police to detect alcohol (ethanol) in a person's breath. The device works by passing a beam of IR radiation through the captured breath sample in a testing chamber. The characteristic bonds present in ethanol molecules absorb specific frequencies of the IR radiation. By measuring which frequencies and how much radiation is absorbed, the breathalyser can detect the presence of ethanol and quantify its concentration. The more IR radiation absorbed at ethanol's characteristic frequencies, the higher the reading and the greater the ethanol concentration in the breath.

Environmental monitoring and pollution detection

Many atmospheric pollutants can be identified and measured using their distinctive IR spectral fingerprints. Remote sensors analyse the IR spectra of vehicle emissions to detect and measure carbon monoxide (CO), carbon dioxide (CO₂), and various hydrocarbons in busy urban areas or at motorways. This technology allows for continuous monitoring of localized air pollution without requiring physical collection of air samples.

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