Using the Asymmetric Stretch Band of Atmospheric CO2 to Obtain the C=O Bond Length

Abstract

Infrared studies of the vibrational-rotational levels of linear molecules such as HCl are often used to teach principles of spectral interpretation and data analysis. The determination of precise bond lengths is a particularly interesting result of such experiments. The resolution of IR instruments commonly used in the undergraduate laboratory precludes the study of most larger linear molecules. Carbon dioxide is one exception because only even J rotational levels are present in the molecular ground state owing to symmetry requirements, and transitions from the even J levels, separated by 1-2 cm-1, are sufficiently resolved for analysis by many FTIR instruments. This illustration of a symmetry effect makes CO2 a useful additional example for educational purposes, and normal air has enough CO2 to provide good spectra. The analysis of the asymmetric stretch band is presented with results for the rC=O bond length. The supplemental material presents further discussion of alternative data analysis options, the possible impact of 13CO2, and analysis spreadsheets.