The Vibrational Spectrum of a Polyatomic Molecule

The vibrational motion of a molecule is quantized and the resulting energy level spacings give rise to transitions in the mid-IR portion of the electromagnetic spectrum (4000 to ca. 400 cm). As you know from study of the diatomic harmonic oscillator, the energies (or wavenumber positions, cm) of these transitions are related to the bond strength (force constant), bond length, and atomic masses (reduced mass). In polyatomic spectra, the positions and relative intensities of the vibrational modes depend on the symmetry (i.e. shape or structure) of the molecule, as well as the bond strengths and masses. For this reason, vibrational spectra (IR and Raman) can provide detailed structural information. This analysis, or interpretation, of an infrared spectrum to obtain structural information is the objective of this lab.