In the Laboratory
IR Spectroscopy or “Hooke’s Law at the Molecular Level” A Joint Freshman Physics–Chemistry Experience Jeffrey T. Burke Department of Chemistry, Cumberland County College, Vineland, NJ 08362-0517
There is a growing trend to introduce students at the freshman level to modern spectroscopic techniques, especially IR spectroscopy. The availability of Fourier transform instruments with their fast acquisition times (typically less than 30 s) has made IR spectroscopy a practical alternative even for the large lab sections typical of the freshman level. Recently, several papers describing activities with IR spectroscopy at the freshman level have appeared in this Journal (1–4). In an effort to help students make the connection between physics and chemistry at Cumberland County College, I have developed and describe here a brief laboratory exercise on IR spectroscopy and Hooke’s law at the molecular level. Most of our students enrolled in the Physics 101–102 sequence also enroll in the General Chemistry 101–102 sequence. In the physics laboratory, Hooke’s law is studied using various masses and several springs varying in stiffness. In general, the effects of mass and spring stiffness on vibrational periods are measured and interpreted. Several weeks later these students revisit Hooke’s law, but instead of the mechanical springs and masses, the IR spectra of chloroform and chloroform-d are used to study Hooke’s law at the molecular level. The exercise begins with a minilecture to acquaint these students with the fundamentals of IR spectroscopy. The major ideas emphasized are: 1. To a reasonable approximation, the chemical bond mimics the familiar mechanical spring and simple harmonic motion. 2. Provided the IR frequency matches the molecular vibrational frequency and the vibration is accompanied by a change in molecular dipole moment, absorption of specific frequencies of IR will occur. 3. To give the connection between physics and chemistry a human element, it is mentioned that William Coblentz, educated as a physicist, was the first to make detailed studies of the IR spectra of organic compounds (5). 4. The equation 1 ν- = 2πc
k µ
is derived from Hooke’s law, where ν- = 1/ λ, k = the force constant, and µ is the reduced mass of the system. After this introduction we take to the lab, where groups of students acquire the IR spectra of chloroform and chloroform-d as neat liquids between KCl windows on our
Perkin-Elmer 1615 FT-IR. The region from 4000 to 500 cm {1 is used. To get things started it is pointed out that the band at 3018.5 cm{1 on the chloroform spectrum is the result of C–H bond stretching and the other three bands at 1215.4, 762.3, and 669.0 cm {1 are the results of H–C–Cl bending, asymmetric C–Cl stretching, and symmetric C–Cl stretching, respectively. With their spectra students are required to do the following: 1. Use the chloroform spectrum and the band at 3018.5 cm{1 to calculate the C–H bond force constant. 2. Predict the frequency (cm{1) of the IR band due to C–D stretching (this is the molecular analog of hanging a new mass twice the original mass on the same spring). The predicted frequency is 2223 cm{1 and the observed is 2253 cm{1. 3. To demonstrate an understanding of the necessary conditions for IR absorption, determine the periods of the C–H and C–D stretching vibrations. Many students express amazement when they discover that the periods of molecular vibrations are on the order of fractions of a picosecond! It is hoped that students will come away from this experience with a better understanding of simple harmonic motion—and even more importantly, an understanding that the boundaries between physics and chemistry are often contrived. Acknowledgment I wish to thank my colleague Paul G. Menz for the privilege of working with his physics 101-102 students at Cumberland County College on this project. Literature Cited 1. Swartz, J. E.; Schladetzky, K. J. Chem. Educ. 1996, 73, 188– 189. 2. Anderson, J. S.; Hayes, D. M.; Werner, T. C. J. Chem. Educ. 1995, 72, 653–655. 3. Hess, K. R.; Smith, W. S.; Thomsen, M. W.; Yoder, C. H. J. Chem. Educ. 1995, 72, 655–656. 4. Heuer, W. B.; Koubek, E. J. Chem. Educ. 1997, 74, 313– 315. 5. Hellemans, A.; Bunch, B. The Timetables of Science: A Chronology of the Most Important People and Events in the History of Science; Simon and Schuster: New York, 1988; p 347.
Vol. 74 No. 10 October 1997 • Journal of Chemical Education
1213
