Skip to ContentGo to accessibility pageKeyboard shortcuts menu
OpenStax Logo
Organic Chemistry

12.5 Spectroscopy and the Electromagnetic Spectrum

Organic Chemistry12.5 Spectroscopy and the Electromagnetic Spectrum

12.5 • Spectroscopy and the Electromagnetic Spectrum

Infrared, ultraviolet, and nuclear magnetic resonance spectroscopies differ from mass spectrometry in that they are nondestructive and involve the interaction of molecules with electromagnetic energy rather than with an ionizing source. Before beginning a study of these techniques, however, let’s briefly review the nature of radiant energy and the electromagnetic spectrum.

Visible light, X rays, microwaves, radio waves, and so forth are all different kinds of electromagnetic radiation. Collectively, they make up the electromagnetic spectrum, shown in Figure 12.17. The electromagnetic spectrum is arbitrarily divided into regions, with the familiar visible region accounting for only a small portion, from 3.8 × 10–7 m to 7.8 × 10–7 m in wavelength. The visible region is flanked by the infrared and ultraviolet regions.

A representation of the wavelengths, frequencies, and energies of radiation types from gamma radiation (highest frequency) to radio waves (lowest frequency). Other types of radiation are between these extremes.
Figure 12.17 The electromagnetic spectrum covers a continuous range of wavelengths and frequencies, from radio waves at the low-frequency end to gamma (γ) rays at the high-frequency end. The familiar visible region accounts for only a small portion near the middle of the spectrum.

Electromagnetic radiation is often said to have dual behavior. In some respects, it has the properties of a particle, called a photon, yet in other respects it behaves as an energy wave. Like all waves, electromagnetic radiation is characterized by a wavelength, a frequency, and an amplitude (Figure 12.18). The wavelength, λ (Greek lambda), is the distance from one wave maximum to the next. The frequency, ν (Greek nu), is the number of waves that pass by a fixed point per unit time, usually given in reciprocal seconds (s–1), or hertz, Hz (1 Hz = 1 s–1). The amplitude is the height of a wave, measured from midpoint to peak. The intensity of radiant energy, whether a feeble glow or a blinding glare, is proportional to the square of the wave’s amplitude.

A representation of a typical waveform including indications of how wavelength and amplitude are measured along a wave, as well as specific measurements for violet light and I R radiation.
Figure 12.18 Electromagnetic waves are characterized by a wavelength, a frequency, and an amplitude. (a) Wavelength (λ) is the distance between two successive wave maxima. Amplitude is the height of the wave measured from the center. (b)–(c) What we perceive as different kinds of electromagnetic radiation are simply waves with different wavelengths and frequencies.

Multiplying the wavelength of a wave in meters (m) by its frequency in reciprocal seconds (s–1) gives the speed of the wave in meters per second (m/s). The rate of travel of all electromagnetic radiation in a vacuum is a constant value, commonly called the “speed of light” and abbreviated c. Its numerical value is defined as exactly 2.997 924 58 × 108 m/s, usually rounded off to 3.00 × 108 m/s.


Just as matter comes only in discrete units called atoms, electromagnetic energy is transmitted only in discrete amounts called quanta. The amount of energy ε corresponding to 1 quantum of energy (1 photon) of a given frequency ν is expressed by the Planck equation


where h = Planck’s constant (6.62 × 10–34 J ‧ s = 1.58 × 10–34 cal ‧ s).

The Planck equation says that the energy of a given photon varies directly with its frequency ν but inversely with its wavelength λ. High frequencies and short wavelengths correspond to high-energy radiation such as gamma rays; low frequencies and long wavelengths correspond to low-energy radiation such as radio waves. Multiplying ε by Avogadro’s number NA gives the same equation in more familiar units, where E represents the energy of Avogadro’s number (one “mole”) of photons of wavelength λ:


When an organic compound is exposed to a beam of electromagnetic radiation, it absorbs energy of some wavelengths but passes, or transmits, energy of other wavelengths. If we irradiate the sample with energy of many different wavelengths and determine which are absorbed and which are transmitted, we can measure the absorption spectrum of the compound.

An example of an absorption spectrum—that of ethanol exposed to infrared radiation—is shown in Figure 12.19. The horizontal axis records the wavelength, and the vertical axis records the intensity of the various energy absorptions in percent transmittance. The baseline corresponding to 0% absorption (or 100% transmittance) runs along the top of the chart, so a downward spike means that energy absorption has occurred at that wavelength.

An infrared spectrum of ethanol, showing significant absorption between wavenumbers 3500 and 3100, and just below wavenumber 3000. Other significant absorption spikes also appear.
Figure 12.19 An infrared absorption spectrum for ethanol, CH3CH2OH. A transmittance of 100% means that all the energy is passing through the sample, whereas a lower transmittance means that some energy is being absorbed. Thus, each downward spike corresponds to an energy absorption.

The energy a molecule gains when it absorbs radiation must be distributed over the molecule in some way. With infrared radiation, the absorbed energy causes bonds to stretch and bend more vigorously. With ultraviolet radiation, the energy causes an electron to jump from a lower-energy orbital to a higher-energy one. Different radiation frequencies affect molecules in different ways, but each provides structural information when the results are interpreted.

There are many kinds of spectroscopies, which differ according to the region of the electromagnetic spectrum used. We’ll look at three: infrared spectroscopy, ultraviolet spectroscopy, and nuclear magnetic resonance spectroscopy. Let’s begin by seeing what happens when an organic sample absorbs infrared energy.

Worked Example 12.3

Correlating Energy and Frequency of Radiation

Which is higher in energy, FM radio waves with a frequency of 1.015 × 108 Hz (101.5 MHz) or visible green light with a frequency of 5 × 1014 Hz?


Remember the equations ε = and ε = hc/λ, which say that energy increases as frequency increases and as wavelength decreases.


Since visible light has a higher frequency than radio waves, it is higher in energy.
Problem 12-5
Which has higher energy, infrared radiation with λ = 1.0 × 10–6 m or an X ray with λ = 3.0 × 10–9 m? Radiation with ν = 4.0 × 109 Hz or with λ = 9.0 × 10–6 m?
Problem 12-6
It’s useful to develop a feeling for the amounts of energy that correspond to different parts of the electromagnetic spectrum. Calculate the energies in kJ/mol of each of the following kinds of radiation:
A gamma ray with λ = 5.0 × 10–11 m
An X ray with λ = 3.0 × 10–9 m
Ultraviolet light with ν = 6.0 × 1015 Hz
Visible light with ν = 7.0 × 1014 Hz
Infrared radiation with λ = 2.0 × 10–5 m
Microwave radiation with ν = 1.0 × 1011 Hz
Order a print copy

As an Amazon Associate we earn from qualifying purchases.


This book may not be used in the training of large language models or otherwise be ingested into large language models or generative AI offerings without OpenStax's permission.

Want to cite, share, or modify this book? This book uses the Creative Commons Attribution-NonCommercial-ShareAlike License and you must attribute OpenStax.

Attribution information
  • If you are redistributing all or part of this book in a print format, then you must include on every physical page the following attribution:
    Access for free at
  • If you are redistributing all or part of this book in a digital format, then you must include on every digital page view the following attribution:
    Access for free at
Citation information

© Jan 9, 2024 OpenStax. Textbook content produced by OpenStax is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike License . The OpenStax name, OpenStax logo, OpenStax book covers, OpenStax CNX name, and OpenStax CNX logo are not subject to the Creative Commons license and may not be reproduced without the prior and express written consent of Rice University.