What is infrared spectroscopy?

(mainly copied from Wikipedia)

Infrared spectroscopy exploits the fact that molecules absorb frequencies that are characteristic of their structure. These absorptions occur at resonant frequencies, i.e. the frequency of the absorbed radiation matches the vibrational frequency. The energies are affected by the shape of the molecular potential energy surfaces, the masses of the atoms, and the associated vibronic coupling.

3D animation of the symmetric stretching of the C–H bonds of bromomethane

In particular, in the Born–Oppenheimer and harmonic approximations, i.e. when the molecular Hamiltonian corresponding to the electronic ground state can be approximated by a harmonic oscillator in the neighborhood of the equilibrium molecular geometry, the resonant frequencies are associated with the normal modes corresponding to the molecular electronic ground state potential energy surface. The resonant frequencies are also related to the strength of the bond and the mass of the atoms at either end of it. Thus, the frequency of the vibrations are associated with a particular normal mode of motion and a particular bond type.

Number of vibrational modes:

In order for a vibrational mode in a sample to be "IR active", it must be associated with changes in the dipole moment. A permanent dipole is not necessary, as the rule requires only a change in dipole moment. A molecule can vibrate in many ways, and each way is called a vibrational mode. For molecules with $N$ number of atoms, linear molecules have $3N – 5$ degrees of vibrational modes, whereas nonlinear molecules have $3N – 6$ degrees of vibrational modes (also called vibrational degrees of freedom). As an example $\ce{H2O}$, a non-linear molecule, will have $3 × 3 – 6 = 3$ degrees of vibrational freedom, or modes.

Simple diatomic molecules have only one bond and only one vibrational band. If the molecule is symmetrical, e.g. $\ce{N2}$, the band is not observed in the IR spectrum, but only in the Raman spectrum. Asymmetrical diatomic molecules, e.g. $\ce{CO}$, absorb in the IR spectrum. More complex molecules have many bonds, and their vibrational spectra are correspondingly more complex, i.e. big molecules have many peaks in their IR spectra.

The atoms in a $\ce{CH2X2}$ group, commonly found in organic compounds and where X can represent any other atom, can vibrate in nine different ways. Six of these vibrations involve only the $\ce{CH2}$ portion: symmetric and antisymmetric stretching, scissoring, rocking, wagging and twisting, as shown below. Structures that do not have the two additional X groups attached have fewer modes because some modes are defined by specific relationships to those other attached groups. For example, in water, the rocking, wagging, and twisting modes do not exist because these types of motions of the H represent simple rotation of the whole molecule rather than vibrations within it.

Symmetric stretching

Asymmetric stretching

Scissoring

Rocking

Wagging

Twisting

The above figures do not represent the "recoil" of the C atoms, which, though necessarily present to balance the overall movements of the molecule, are much smaller than the movements of the lighter H atoms.

Where do I use this tag?

You will use this tag for questions regarding the principles and application of infrared spectroscopy. Don't use it for questions regarding other spectroscopic techniques. Use appropriate tags for that instead.

I love infrared spectroscopy! Where do I learn more about it?

There are plenty of books, but we'll reference a few from the resources page.

• Pavia, D. L.; Lampman, G. M.; Kriz, G. S.; Vyvyan, J. A. Introduction to Spectroscopy, 5th ed.; Cengage Learning: Stamford, CT, 2015. Excellent introductory text with good coverage of all the typical structure determination techniques: elemental analysis, NMR, IR, MS, and UV-Vis.

• Silverstein, R. M.; Webster, F. X.; Kiemle, D. J.; Bryce, D.L. Spectrometric Identification of Organic Compounds, 8th ed.; Wiley: Hoboken, NJ, 2014. Perhaps a slightly more in-depth discussion than Pavia, but without sacrificing any clarity. Includes a large number of tables and charts of spectroscopic data, making it also very valuable as a reference.