return to home page Computational Chemistry Comparison and Benchmark DataBase Release 22 (May 2022) Standard Reference Database 101 National Institute of Standards and Technology
You are here: FAQ Help > Units > Explanations > cm-1

What's a cm-1?

A reciprocal centimeter is used as an energy unit. It is often used for the energy of vibrational levels and transitions, and for electronic energy levels and transitions. The spectra of vibrational and electronic transitions are often measured as an intensity as a function of wavelength. A reciprocal wavelength (cm-1) is a simple convenient unit for comparing energies when dealing with spectra. An actual energy can be obtained by multiplying cm-1 by hc (Planck's constant times the speed of light). To convert from cm-1 to kJ/mol one uses

Planck constant h = 6.62606876 x 10-37 kJ s
Speed of light c = 2.99792458 x 1010 cm s-1
Avogadro constant NA = 6.02214199 x 1023 mol-1
or 1.196266 x 10-2 kJ mol-1 cm.

More on units and conversions can be found near the end of the page discussing thermochemistry (Section I.D.)