Infrared and UV-Visible Spectroscopy

Infrared spectroscopy identifies functional groups through their vibrational absorptions, while ultraviolet–visible spectroscopy probes conjugated and electronic systems, together giving a rapid read on the functionality of an organic molecule.

Definition

Infrared spectroscopy measures absorption arising from molecular bond vibrations, and UV–visible spectroscopy measures absorption from electronic transitions; both relate spectral features to functional groups and conjugation.

Scope

This topic covers the basis of vibrational (infrared) absorption and characteristic group frequencies, the fingerprint region, electronic transitions in UV–visible spectroscopy, the effect of conjugation on absorption wavelength, and chromophores.

Core questions

• How do infrared absorptions identify functional groups such as carbonyls and hydroxyls?
• Why does extended conjugation shift UV–visible absorption to longer wavelengths?
• What complementary information do these two techniques give about a molecule?

Key theories

Vibrational (infrared) group frequencies

Each bond type absorbs infrared radiation at a characteristic frequency determined by bond strength and atomic masses, so the spectrum reveals which functional groups are present.

Electronic transitions and conjugation

UV–visible absorption arises from promotion of electrons between molecular orbitals; increasing conjugation lowers the transition energy and shifts absorption toward longer wavelengths.

Mechanisms

Infrared photons excite vibrational modes of bonds that change dipole moment; the resulting absorption bands, especially in the diagnostic region above the fingerprint, pinpoint functional groups. UV–visible photons promote electrons from bonding or non-bonding to antibonding orbitals, with the energy gap — and hence the wavelength of maximum absorption — decreasing as a conjugated chromophore is extended.

Clinical relevance

Infrared and UV–visible spectroscopy are routine for identifying and quantifying drug substances and for monitoring purity; UV–visible absorbance underlies many quantitative assays in clinical and pharmaceutical laboratories.

History

Coblentz's early twentieth-century catalog of infrared absorptions established the link between vibrational bands and chemical structure, and the maturation of commercial spectrometers made infrared and UV–visible spectroscopy standard tools for routine organic analysis.

Key figures

• William Coblentz
• Arthur Adamson

Seminal works

• pavia2015
• silverstein2014

Frequently asked questions

What is the fingerprint region of an IR spectrum?

The fingerprint region is the lower-frequency part of the infrared spectrum where complex, molecule-specific patterns of vibrations appear; while hard to assign band by band, it is highly characteristic and useful for confirming that two samples are the same compound.

Why does a colored compound absorb visible light?

A compound appears colored when its conjugated electronic system has a small enough energy gap to absorb light in the visible range; the transmitted or reflected complementary wavelengths give it its color.