The Forbidden Transition ν 2 in the Infrared Spectrum of Methane

A preliminary discussion is given concerning the measurements on the forbidden transition ν2 in the infrared spectrum of methane, first observed by Coblentz. This band becomes active in the infrared through Coriolis interaction with the frequency ν4. Although no detailed discussion of the structure of this band can be given before the theory has been studied in detail, many of its characteristics can be accounted for. Finally, with the aid of the value of ν2*, the corresponding frequency in the infrared spectrum of CD4, an estimate of the normal frequency ω2 can be made.