 Computational Chemistry Comparison and Benchmark DataBase Release 21 (August 2020) Standard Reference Database 101 National Institute of Standards and Technology Home All data for one species Geometry Experimental Calculated Comparisons Bad Calculations Tutorials and Explanations Vibrations Experimental Calculated Scale factors Reactions Entropies Ions List Ions Energy Electron Affinity Proton Affinity Ionization changes point group Experimental One molecule all properties One property a few molecules Geometry Vibrations Energy Electrostatics Reference Data Calculated Energy Optimized Reaction Internal Rotation Orbital Nuclear repulsion energy Correlation Ion Excited State Basis Set Extrapolation Geometry Vibrations Frequencies Zero point energy (ZPE) Scale Factors Bad Calculations Electrostatics Charges Dipole Quadrupole Polarizability Spin Entropy and Heat Capacity Reaction Lookup by property Comparisons Geometry Vibrations Energy Entropy Electrostatics Ion Resources Info on Results Calculations Done Basis functions used I/O files Glossary Conversion Forms Links NIST Links External links Thermochemistry Tutorials Vibrations Entropy Energy Electrostatics Geometry Cost Bad Calculations FAQ Help Units Choose Units Explanations Credits Just show me Summary Using List Recent molecules Molecules Geometry Vibrations Energy Similar molecules Ions, Dipoles, etc. Index of CCCBDB Feedback You are here: FAQ Help > Units > Explanations > cm-1

# What's a cm-1?

A reciprocal centimeter (or wavenumber) 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 c = 2.99792458 x 1010 cm s-1 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.)