Resumen de An Investigation into the Absorption of Infrared Light by Small Molecules:: A General Chemistry Experiment
William B. Heuer, Edward Koubek
An introductory, two-part classroom/laboratory activity demonstrating the mechanism of absorption of infrared light by small molecules is described. A model for molecular vibration is introduced during pre-lab discussion, and a mechanism by which such vibrations may be excited by infrared radiation light is postulated. Students then explore the physics of simple harmonic motion (SHM) on the laboratory scale through several quantitative experiments using a simple spring oscillator, which serves as a macroscopic model for a vibrating chemical bond. The infrared spectrum of CHCl3 is recorded, and the results of the preceding laboratory investigation are used to predict the shift in the C-H stretching and bending frequencies upon deuteration. The validity of applying classical SHM equations to a molecular system is tested by recording the spectrum of CDCl3 and comparing the observed frequency shifts with those predicted by the model. The observed correspondence between infrared absorption frequencies and the frequencies of molecular vibration provides convenient entry into discussion of such topics as analytical applications of IR spectroscopy and the greenhouse effect. The successful application of a macroscopic physical model to a molecular system also enhances the ability of students to visualize chemical processes occurring on a microscopic level.
