Infrared Spectroscopy

Spectroscopy is the study of how light interacts with matter — specifically, how a molecule absorbs electromagnetic radiation in discrete amounts that correspond to the energy gap between two quantized states. Molecules can only occupy certain allowed energy levels, so absorption happens only when the energy of incoming radiation matches the gap between two of those levels. This chapter covers three spectroscopic techniques built on that same idea: infrared (IR) spectroscopy, which probes vibrational transitions; UV-Vis spectroscopy, which probes electronic transitions; and nuclear magnetic resonance (NMR) spectroscopy, which probes nuclear spin transitions in a magnetic field. This article covers infrared spectroscopy — what it measures, how to read an IR spectrum, and the characteristic absorptions used to identify functional groups.

Key Takeaways

• Spectroscopy measures the absorption of electromagnetic radiation by a molecule in discrete amounts that match the energy gap between two quantized states; IR spectroscopy specifically probes vibrational transitions.

• IR spectroscopy detects molecular vibrations — bond stretching (symmetric/asymmetric) and bending (symmetric/asymmetric).

• IR radiation is low-energy enough to avoid exciting electrons or causing ionization, but matched to the energy needed for vibrational transitions.

• Each bond type absorbs a characteristic frequency because ΔE between vibrational states depends on bond strength and atomic mass.

• IR spectra plot percent transmittance vs. wavenumber (cm⁻¹, reciprocal of wavelength, proportional to energy) across the standard 4000–400 cm⁻¹ range; a dip (peak) marks an absorbed frequency.

• Peaks are interpreted by three characteristics: wavenumber (bond strength/atomic mass), intensity (dipole moment change), and shape (hydrogen bonding broadens peaks).

• The spectrum splits into the fingerprint region (below ~1500 cm⁻¹, molecule-specific, not an MCAT focus) and the diagnostic region (~4000–1500 cm⁻¹, where functional groups produce identifiable peaks).

• Characteristic absorptions — like C=O around 1700–1750 cm⁻¹ and broad O–H around 2800–3500 cm⁻¹ — let you identify functional groups directly from an IR spectrum.

What IR Spectroscopy Detects

IR spectroscopy detects molecular vibrations — periodic motions of atoms within a molecule. These vibrations take two basic forms:

• Stretching: atoms move closer together or farther apart along the bond axis. Stretching can be symmetric (both atoms move in or out together) or asymmetric (one atom moves in while the other moves out).

• Bending: the angle between atoms changes. Bending motions can also be symmetric or asymmetric, and they're especially common in molecules with three or more atoms bonded to a central atom.

Many molecules exhibit more complex vibrations that are combinations of these basic motions.

Why Infrared Radiation

On the electromagnetic spectrum, infrared radiation lies just beyond red light in the visible region — it has longer wavelengths and lower energy than visible or ultraviolet light. That lower energy means IR radiation isn't energetic enough to excite electrons or cause ionization, but it's exactly the right amount of energy to induce vibrational transitions in covalent bonds. That's what makes IR radiation the right tool for probing molecular vibrations specifically, rather than electronic structure.

Quantized Vibrations and Characteristic Absorption

Molecular vibrational energy is quantized: a molecule doesn't have access to a continuous range of vibrational energies, only discrete vibrational energy levels. When infrared radiation is absorbed, it's because the photon's energy matches the energy difference between two vibrational states — commonly written as ΔE. If there's no match, the photon simply passes through without being absorbed.

Because ΔE depends on both bond strength and the masses of the atoms involved, ΔE is different for every bond type — so each bond absorbs a different, characteristic frequency of IR light. A C=O bond, for example, absorbs a different frequency than a C–H bond, because their vibrational energy levels are spaced differently. This is the core principle behind IR spectroscopy: by recording which frequencies a compound absorbs, you can identify the types of bonds present in it.

Reading an IR Spectrum

When IR radiation is passed through a compound, it probes all the bonds in the molecule. Any frequency that matches a bond's vibrational transition gets absorbed; any frequency that doesn't match passes through unaffected. The result is an IR spectrum — a graph of how much light is absorbed at each frequency, which reveals which bonds are present in the molecule.

Rather than plotting frequency directly, IR spectra are reported in wavenumber — the reciprocal of wavelength, measured in inverse centimeters (cm⁻¹). Wavenumber is directly proportional to energy, so higher wavenumbers correspond to higher-energy vibrations. The standard IR region spans roughly 4000 to 400 cm⁻¹, covering nearly all useful vibrations for common organic functional groups.

An IR spectrum is typically plotted as percent transmittance versus wavenumber. A dip in the spectrum — a peak — marks a frequency where light was absorbed, meaning a vibrational transition occurred. For example, a sharp absorption near 1715 cm⁻¹ typically indicates a carbonyl (C=O) stretch, while an absorption near 2900 cm⁻¹ is associated with C–H stretching.

Interpreting Peaks: Wavenumber, Intensity, and Shape

Fully interpreting an IR spectrum means looking at three characteristics of each peak:

Wavenumber (position). This is where a peak falls on the x-axis, and it tells you the frequency — and therefore the energy — of the absorbed vibration. Wavenumber is determined by bond strength and the masses of the atoms involved: stronger bonds and lighter atoms vibrate at higher frequency, and therefore absorb at higher wavenumber. A C–H bond, for instance, absorbs at a higher wavenumber than a C–C bond.

Intensity. This is how strong or tall a peak appears. Intensity depends on how effectively a bond absorbs IR light, which in turn depends on the size of the change in dipole moment during the vibration. Bonds with a large dipole moment change — like O–H or C=O — absorb strongly. Bonds like C–H, which undergo a smaller dipole change, tend to show weaker absorptions.

Shape. Peaks can be sharp and narrow or broad and rounded. Shape is influenced by several factors, most notably hydrogen bonding, which can broaden a peak significantly. The O–H stretch in alcohols, for example, typically appears as a broad signal because of intermolecular hydrogen bonding.

Fingerprint Region vs. Diagnostic Region

With these three characteristics in mind, the IR spectrum can be divided into two main regions:

MCAT Callout — Fingerprint Region: Below about 1500 cm⁻¹ is the fingerprint region — a dense collection of peaks from complex bending vibrations and skeletal motions. It's difficult to analyze peak-by-peak, but the overall pattern is unique to each molecule, like a molecular barcode, which makes it useful for confirming a match between a known and an unknown sample. The MCAT does not require detailed analysis of this region.

The diagnostic region, from about 4000 to 1500 cm⁻¹, contains most of the important and easily interpretable signals. This is where functional groups produce absorptions — typically strong and distinct peaks that allow you to identify specific bond types and functional groups.

Characteristic IR Absorptions by Functional Group

The diagnostic region is where the practical work of IR interpretation happens. Below are the most important characteristic absorption ranges for common functional groups:

Getting familiar with these regions — even without memorizing every exact number — lets you look at an IR spectrum and quickly narrow down the possible functional groups present in a molecule. A carbonyl-region peak around 1700–1750 cm⁻¹ points to an aldehyde, ketone, or carboxylic acid; adding a broad O–H stretch around 2800–3200 cm⁻¹ narrows that down specifically to a carboxylic acid.

Common MCAT Mistakes

• Treating wavenumber like wavelength. Wavenumber (cm⁻¹) is the reciprocal of wavelength and is proportional to energy — a higher wavenumber means higher energy, the opposite of how wavelength scales with energy. Mixing the two up leads to reading the x-axis backward.

• Assuming a tall peak means a strong bond. Peak intensity depends on the size of the dipole moment change during the vibration, not on bond strength. A strong, nonpolar bond like C–H can produce a comparatively weak peak, while a highly polar bond like O–H produces a strong one.

• Trying to assign individual peaks in the fingerprint region. Below ~1500 cm⁻¹, the pattern is molecule-specific and not meant to be interpreted bond-by-bond. The MCAT tests the diagnostic region (~4000–1500 cm⁻¹); time spent decoding fingerprint-region peaks is time misspent.

• Confusing a broad O–H stretch with a sharp N–H stretch. Both fall in a similar wavenumber range (roughly 3100–3500 cm⁻¹), but hydrogen bonding broadens the O–H peak while the N–H peak stays comparatively sharp — shape, not just position, is what separates them.

MCAT-Style Concept Check

Question: An IR spectrum shows a strong, sharp absorption at 1715 cm⁻¹ and no broad absorption anywhere between 2500 and 3300 cm⁻¹. Which functional group is most consistent with this spectrum?

• A) Carboxylic acid

• B) Alcohol

• C) Ketone

• D) Amine

Answer: C

Explanation: The peak at 1715 cm⁻¹ falls in the carbonyl (C=O) stretch range shared by aldehydes, ketones, and carboxylic acids. A carboxylic acid would also show a broad O–H stretch between roughly 2800 and 3200 cm⁻¹ from hydrogen bonding — since that broad absorption is absent, carboxylic acid (A) is ruled out. Alcohols (B) and amines (D) don't produce a carbonyl stretch at all, since neither contains a C=O bond. That leaves a ketone (C) as the functional group most consistent with the spectrum.

FAQ

What does an IR spectrum actually measure?

It measures how much infrared light a compound absorbs at each frequency (reported as wavenumber). Absorption occurs when a photon's energy matches the energy gap between two vibrational states of a bond, so the resulting pattern of peaks reveals which bonds — and therefore which functional groups — are present in the molecule.

What's the difference between the fingerprint region and the diagnostic region?

The diagnostic region (~4000–1500 cm⁻¹) contains strong, distinct peaks that correspond to specific functional groups and is where most IR interpretation happens. The fingerprint region (below ~1500 cm⁻¹) is a dense, molecule-specific pattern from complex bending and skeletal vibrations — useful for confirming an exact match between samples, but not meant to be interpreted peak-by-peak.

Why does bond strength affect where a peak appears on an IR spectrum?

A peak's wavenumber reflects the energy gap (ΔE) between vibrational states, and that gap depends on bond strength and the masses of the atoms involved. Stronger bonds and lighter atoms vibrate at higher frequency, which shows up as a higher wavenumber.

Why is the O–H stretch of an alcohol broad instead of sharp?

Peak shape is heavily influenced by hydrogen bonding. Alcohols form intermolecular hydrogen bonds through their O–H group, and that hydrogen bonding broadens the O–H stretch into a wide signal rather than a narrow, sharp one.