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  Home > JCE Print > Journal of Chemical Education > Issues > 2002  > January  >
In the Laboratory
Using the Asymmetric Stretch Band of Atmospheric CO2 to Obtain the C=O Bond Length
Paul J. Ogren
Department of Chemistry, Earlham College, Richmond, IN 47374-4095
Cover
January 2002
Vol. 79 No. 1
p. 117
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.
Supplement
A spreadsheet containing the data and analysis used to obtain the results given in Figure 2 are available. The supplemental material also provides further description of the fitting methodology, a discussion of possible 13CO2 interferences, and further discussion of notation for the vibrational levels. Spreadsheet results are available for the case where point weights are variable and for the case where the De parameter is included in the data fit.
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*  Citation
 Ogren, Paul J. J. Chem. Educ. 2002 79 117.
*  Keywords
 IR Spectroscopy; Laboratory Computing / Interfacing; Molecular Properties / Structure; Physical Chemistry
*  History
 Created:
Last Updated:		 December 3, 2001
March 16, 2005
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