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Infrared and Raman spectroscopy are complementary forms of vibrational spectroscopy. IR involves absorption of infrared light, and requires a changing dipole moment during the vibration. A dipole moment involves a partial charge separation in a molecule ( + -). An example is an asymmetric C-H stretch of ethene. Experimentally, this is an intense band nu(C-H) = 3108 cm^-1 in the IR, and is absent from the Raman spectrum.

Raman spectroscopy involves scattering (usually visible) light with a frequency shift, and requires a changing polarizability. The polarizability may be regarded as the "squishiness," the ease of deformation in an electric field, of the electron cloud of the molecule. An example is the symmetric C=C stretch of ethene. Experimentally, this is an intense band nu(C=C) = 1623 cm^-1 in the Raman spectrum, and is absent from the IR spectrum. (Ref. 1, 2)

In this laboratory experiment, each student identifies an organic unknown on the basis of its Raman and IR spectra, from a set of 18 possible compounds representing a variety of structural types. Students tabulate the principal vibrations in Raman and IR, attempt to assign the bands to particular types of vibrations, compare and contrast the Raman and IR intensity patterns, and attempt to identify the unknown. Students are encouraged to work in pairs, discussing their observations and interpretation with each other, and submitting their lab reports together. However each student has her/his own unknown to identify and interpret.

An excellent source for interpreting both Raman and IR spectra of organic molecules is Lambert et al., Organic Structural Spectroscopy. (Ref. 3)

Benefits of this experiment include greater insight into vibrational spectra and vibrational motions in organic molecules. At this point in time, discussion of Raman spectra in organic chemistry courses is not at all common. So this is an opportunity at the frontier.

IR and Raman spectroscopy are complementary forms of vibrational spectroscopy, since IR requires a changing dipole moment and Raman requires a changing polarizability. Thus vibrations can be active in the IR, active in the Raman, active in both, or occasionally (in high symmetry cases such as SF6) active in neither. This lab is designed to introduce you to Raman spectroscopy, and to differences in intensity patterns between IR and Raman spectra of organic molecules.

Students will work together in pairs. Each pair will obtain two unknowns (one per student). The students will take an IR and a Raman spectrum for each unknown. Students will take their own FTIR spectrum under the supervision of a lab assistant. For liquid unknowns, run the sample as a neat liquid beteen NaCl salt plates. For a solid unknown, prepare the sample as a KBr pellet. To take the Raman spectrum, work with the instructor or a lab assistant assigned to the instrument. Measure frequencies of the major IR bands. Save the spectrum in the directory, and with the file format(s) and naming conventions, established by your instructor.

Safety note: it is essential to wear safety goggles while working with the Ocean Optics Raman instrument. For example if four laser safety goggles are available, and if the instructor and a lab assistant are each wearing a pair of laser safety goggles, then two students can wear the other two laser safety goggles in gathering the Raman spectrum. In addition, the laser is turned on ONLY when the probe tip is positioned in a vial or small bottle of sample, inside a covered opaque plastic can. Think about what you are doing, and take these precautions in order to avoid getting the dangerous laser beam, or reflections from it, in your eye.

As another practical note, if the unknown sample is a solid, an extra filter must be in the laser fiber optic assembly, in order to avoid overloading the detector. The instructor and lab assistant are familiar with this procedure. If it is a liquid, filter and/or out-gas it to avoid scattering by impurities.

Save the Raman spectrum in the directory, and with the file format and naming conventions, established by your instructor. Measure frequencies (actually Raman shifts in cm^-1) for the major Raman bands. Print out the Raman spectrum.

After the IR and Raman spectra are obtained., each student will identify her/his unknown, from the following list of possibilities. (Of course your instructor may add to or alter the list.)

Examine the IR spectra for the usual patterns and clues with which you are familiar. For the Raman spectra, begin by working with Lambert Table 8-4: Characteristic Frequencies of Functional Groups in the Raman Spectra of Complex Molecules. Then explore more broadly in Chapters 8 and 9 of Lambert. (Ref. 3)

Fill out the attached worksheets and identify your unknown.

Characteristic Raman frequencies for a few functional groups are shown here, from Lambert Table 8-4. (Ref. 3).

Student names ________________________________________________________________

Unknown # _______                                                            Identified as: ___________________

RAMAN SPECTRUM

IR SPECTRUM

A set of organic unknowns, such as the following list. Although the amount is not critical, 25 mL per student is recommended. The unknowns should be collected at the end of the experiment for reuse.

A suitable IR or FTIR spectrometer, with a range of 400 - 4000 cm-1 or thereabouts. NaCl salt plates. KBr and a pellet press to make KBr pellets with occasional solid unknowns.

Ocean Optics R2000 or R2001 Raman spectrometer, and a suitable PC on which to install it.

.Copy of Lambert for student use. (Ref. 3)

Technical cautions:

Emphasize laser safety. Wear laser safety goggles consistently.

As noted, liquids may need to be filtered, or degassed (placed underr a vacuum), to reduce bacground scattering. If this step is skipped and the background seems too high, then seriously consider doing it.

Raman spectra of solids are reasonably possible, but require an extra laser line rejection filter in the fiber optic light path. This is Ocean Optics item code R-2000-FLT. Without it, background scattering (reflections) of solid samples will make the spectrum unuseable, and may overload and possibly damage the detector.

It is obvious that the instructor and lab assistant need to be fully conversant with the setup and operating instructions and manual for the Raman instrument. Although reasonably straightforward, the instructions and setup may not be intuitively obvious. In particular, check the setup parameters periodically, especially if the PC which hosts the Raman instrument is used for other purposes as well. This is especially true if more than one Ocean Optics instrument is installed on that PC, since it is conceible that their setup parameters will conflict or interact.

Know your instrument thoroughly!

These questions may be used to demonstrate or assess students' understanding of the experiment.

1.    Discuss issues that arise in this experiment with respect to safety, and with respect to appropriate care of the Ocean Optics Raman instrument.

2.    Identify one spectrum which suggests the presence of a C=C double bond. Is this more evident in the IR spectrum or the Raman Spectrum, or equally obvious in both?

3.    Identify the two unknowns that your instructor assigns to you and your lab partner. Compare and contrast their Raman and IR spectra, and relate several bands to the structure of each of the two unknowns.

1.    Kazuo Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, 4th ed., Wiley, New York, p. 387.

2.    Gerhard Herzberg, Molecular Spectra and Molecular Structure. II. Infrared and Raman Spectra of Polyatomic Molecules, Van Nostrand, New York, 1945, p. 151, 326.

3.    Joseph B..Lambert, Herbert F. Shurvell, David A. Lightner, and R. Graham Cooks, Organic Structural Spectroscopy, Prentice-Hall, Upper Saddle River, NJ, 1998, Chapter 8, 9.