In your chemistry courses and labs, you have frequently made use of various "spectroscopic" tools to identify the molecular structure of unknown compounds. You are probably familiar with the idea that molecules can attain a higher energy state by absorbing light: Before After N (no more photon) photon Molecule in Molecule in ground state excited state Because different molecules absorb light of different frequencies, one can obtain spectra that show how much light is absorbed by a particular compound as a function of the frequency (or wavelength) of the light. Spectroscopic data like these are sometimes called "molecular fingerprints" because each com- pound has a unique absorption spectrum by which its identity can be discovered. Spectroscopy is an in- credibly powerful tool for investigating the physical structures of objects we cannot see with our normal vision! For example, here's an infrared absorption spectrum for an ester: Acetic Acid, Methyl Ester Infrared Spectrum 0.8 Transmitance 0.2 0.0 3500 3000 2500 2000 1500 1000 500 Wavenumbers (cm-1) But what determines the molecular fingerprint? Why is it that some molecules absorb strongly at a particular frequency of light while others do not? Let's try to answer these questions...