FTIR Vibration-Rotation Spectrum and Analysis of the 13CO2 Asymmetric Stretch Fundamental Band

A physical chemistry experiment involving the acquisition and analysis of the FTIR vibration-rotation spectrum of the asymmetric stretch fundamental band of 13CO2 is presented. Due to the large band strength and 1.1% natural abundance of carbon-13, this band is readily observable in a typical air background spectrum and is shifted sufficiently from the stronger 12CO2 fundamental such that the 13CO2 P-branch lines from P(2) to P(50) are almost completely free of interferences and easily assigned. All of the 13CO2 R-branch lines appear within the lower frequency region of the 12CO2 P-branch, which creates assignment challenges. While preserving all of the pedagogical benefits of the similar 12CO2 experiment, this experiment offers students the realistic opportunity of engaging in the more common spectroscopic practice of predicting additional line positions from analysis of a partial data set and then finding and adding as many of the predicted lines as may be identified to obtain a refined fit of the larger data set. This experiment may best be conducted with a spectral resolution of 0.5 wavenumber or better.