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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 ( delta+ delta-).

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.

In molecules of high symmetry, a particular normal mode of vibration sometimes appears only in the IR spectrum, or only in the Raman spectrum. If the molecule contains a center of inversion, the rule of mutual exclusion applies, which means that it is not possible for a normal mode of vibration to be active in both the IR and the Raman spectrum. In this case a normal mode can be active in the IR, active in the Raman, or occasionally inactive in both IR and Raman. For example SF6 has one normal mode which is silent: inactive in both IR and Raman. High symmetry molecules may exhibit some less extreme symmetry restrictions even if they lack a center of inversion. The tetrahedral molecule CCl₄ is of this type.

In this experiment we will work with tetrachloroethene (Cl₂C=CCl₂), carbon tetrachloride (CCl₄), carbon disulfide (CS₂), nitrogen (N₂), and benzene (C₆H₆). Each of these except CCl₄ has a center of inversion. CCl₄ is tetrahedral.

The symmetry analysis of molecules, in relation to their IR and Raman spectra, is discussed in a number of sources, including for example Herzberg, Cotton, and Nakamoto. (Ref. 1-3)

We also encourage quantum chemistry calculations on these molecules, along with symmetry analysis, in order to interpret the IR and Raman spectra through the normal modes of vibration. Some useful software tools in computational quantum chemistry include HyperChem, Spartan, Gaussian 98 for Windows, PC GAMESS, together with visualization tools such as gOpenMol and the Chime 2 plugin for Netscape and other web browsers. (Ref. 4-10) A GUI which facilitates use of PC GAMESS is under development by Wayne P. Anderson. (Ref. 8)

An excellent source for qualitative interpretation of both Raman and IR spectra of organic molecules is Lambert et al., Organic Structural Spectroscopy. (Ref. 11)

Benefits of this experiment include greater insight into vibrational spectra and normal modes of vibration in small molecules. At this point in time, IR spectroscopy is common at the undergraduate level, but experimental work with Raman spectra in undergraduate chemistry courses is not at all common. So this is an opportunity at the frontier.

This work is related to and based in part on a paper which we presented in April 2000 and have submitted for publication. (Ref. 12)

1. Select one or more substances from this list, to work with in this experiment. The set may be extended if desired.

CCl₄ (liquid)
Cl₂C=CCl₂, (liquid), tetrachloroethene
CS₂ (liquid)
Benzene (liquid), C₆H₆
Nitrogen (liquid), N₂

2. Observe the Raman spectrum of the substance, using the Ocean Optics R2000 or R2001 Raman spectrometer. Measure frequencies of major peaks in cm-1 and print a copy of the spectrum. You may want to capture a screen image, using Alt-PrintScrn followed by pasting the image into appropriate software, such as IrfanView, and save it as a *.gif file. (Ref. 13) Save the *.ram file for further use.

3. Convert the Ocean Optics *.ram file(s) for your Raman spectra to JCAMP-DX *.jdx file format. To accomplish this, use utility software Raman2Jdx.exe. The procedure is described in detail in Raman Utility Software Exercise on a PC. (Ref. 13)

4. Observe the IR spectrum of the substance, using an appropriate FTIR instrument. (Note that there is no IR spectrum for nitrogen, since there cannot be a changing dipole moment in this homonuclear diatomic molecule. Generally N2 is used to purge the FTIR instrument) Measure frequencies of major peaks in cm-1 and print a copy of the spectrum. Save the IR spectrum in the native file format of the FTIR, and also in JCAMP-DX format as a *.jdx file. You may want to capture a screen image, using Alt-PrintScrn followed by pasting the image into appropriate software, such as IrfanView, and save it as a *.gif file. (Ref. 14)

5. Choose the type of computational quantum chemistry software which you plan to apply to your selected molecule(s). The present experiment cannot reasonably supply all of the background needed for quantum chemistry calculations. Theoretical foundations are provided by textbooks in physical chemistry and quantum chemistry. (Ref. 15-18) However some reasonable possibilities are listed here, along with references to supplement software manuals, to help in getting started. A Mac version of GAMESS also exists (Ref. 19)

HyperChem (PC) (Ref. 4, 20)
Spartan (Unix, PC, or Mac version) (Ref. 5, 21)
Gaussian 98 for Windows (Ref. 6, 22)
PC GAMESS (Ref. 7, 8)

6. Using your chosen quantum chemistry software, build your molecule and save it, including as a *.pdb file. HyperChem calls this file format *.ent.

In HyperChem or Spartan, building the molecule includes an approximate optimization of molecular geometry using molecular mechanics.

HyperChem is a particularly convenient builder for molecules in *.pdb (*.ent) format. A *.pdb file from any source is a conventient starting point for calculations using PC GAMESS or Gaussian 98 W as well.

7. Semiempirical or ab initio molecular orbital calculation. Using AM1 or PM3 (semiempirtical) or 6-31G* [also known as 6-31G(d)] (ab initio), carry out a geometry optimization for your molecule.

With molecules of the size suggested, this is is a rather fast calculation for semiempirical (most likely less than a minute), but considerably slower for ab initio. In fact it is wise to do the semiempirical calculation first, to provide a good starting point for the ab initio calculation.

Extend the calculation to calculate vibrational frequencies and normal modes. If you are using HyperChem, calculate the vibrational spectrum, which includes and displays IR intensities for each normal mode. In HyperChem or Spartan, animate the normal modes for your own visualization.

With PC GAMESS and Gaussian 98W, a good approach to visualizing the normal modes of vibration is to use the gOpenMol software package. (Ref. 9) This in turn makes use of Xvibs utility software to convert the normal modes into *.XYZ files for the animation. (Ref. 23)

8. Vibrational frequencies are generally calculated a little too high in both semiempirical and ab initio calculations. (Ref 24, 25)

Apply a scaling factor to your frequencies, as indicated in these references. AM1 0.92 PM3 0.94 6-31G(d) 0.89

Then attempt to match up your observed Raman and IR frequencies with the quantum chemistry results for your calculated normal modes. Even after scaling, discrepancies of 100 cm-1 may well arise.

The quantum chemistry results will generally indicate relative IR intensities of the normal modes. Gaussian 98W output for the 6-31G(d) calculations will also indicate relative Raman intensities, which is extremely valuable in helping to assihn the observed bands to normal modes.

9. If your molecule is included in Herzberg or Nakamoto, read the discussion there. (Ref. 1,3) Herzberg in particular is very thorough, including discussion of overtone and combination bands. Overtone and combination bands are nominally forbidden in the harmonic oscillator model, but may gain some intensity through anharmonicity if the symmetry works out appropriately.

10. It is possible, though challenging, to post your results on your web site. The spectra are embedded as *.jdx files. The normal modes are animated as *.XYZ files. The procedures involved in obtaining and displaying the normal modes are discussed by Lancashire, Lahti, Motyka, and Donohoue, (Ref. 26, 27, 28, 29)

For Gaussian or HyperChem, extract vibrational modes using VIBREAD98, a program written by P. M. Lahti (Ref. 27)

For Gaussian or PC GAMESS, gOpenMol and Xvibs work well to generate *.XYZ files and visualize normal modes. (Ref. 9, 23)

For examples of normal modes animated (through Chime) from HyperChem results, for a molecule not assigned in this exercise, see the S₄²⁺ page. That example also illustrates placing vibrational vectors on the atoms to show the normal modes quite clearly.(Ref. 30)

We invite you to examine the code used in Ref. 29 and 30, as a starting point for building your own. A typical segment of code in one cell of the table in Ref 30 is as follows.

<CENTER><P><EMBED SRC="S4_277.xyz" display3d="ball&stick" animfps="10"
startanim="true"

WIDTH="250" HEIGHT="150" ALIGN="BOTTOM" frank="false" animmode="loop"></P></CENTER>

<CENTER><P>B<SUB>1g</SUB> (371 cm<SUP>-1</SUP>, Raman)</P></CENTER>

<CENTER><P><IMG SRC="S4_277.gif" ALT="(S4)2+ normal mode" HEIGHT=200 WIDTH=165></P></CENTER>

The CCl₄ page of Deanna Donohoue nicely illustrates the integration of JCAMP-DX spectral display with the animation of the normal modes, triggered by clicking on a band in the Raman or IR spectrum. We invite you to look at the html code on the page, particularly the <embed> tag, and the structure of the *.jdx file. (Ref. 29)

Note particular the following section of the file
http://ed.augie.edu/~dldonoho/ccl4_2_15.jdx
which represents the Raman spectrum of CCl4.

##$ASSIGNMENT TYPE= CHIME
##$CHIME TARGET= MOLECULE
##PEAK ASSIGNMENT= (XYWA)
(770, -1, 30, <load "ccl4ram007.xyz"; select *;
wireframe 40; animation on>)
(454, -1, 30, <load "ccl4ram006.xyz"; select *;
wireframe 40; animation on>)
(312, -1, 30, <load "ccl4ram003.xyz"; select *;
wireframe 40; animation on>)
(220, -1, 30, <load "ccl4ram001.xyz"; select *;
wireframe 40; animation on>)
(2400, -1, 2000, <load "ccl4ram.pdb"; wireframe 60;>)
##XUNITS=1/CM

Quoting from R. J. Lancashire, the meaning of these lines is: "In the present context, ##PEAK ASSIGNMENT has the following syntax: (X value, Y value, width, <CHIME SCRIPT>)" (Ref. 28)

Thus in the first peak assignment, clicking the peak at 770 cm^-1, within 30 cm^-1 on either side (740-800 cm^-1), activates the animation of the associated normal mode.

As a matter of interest, the last "peak assignment" simply represents the default result that if you click on a region which does not have a peak connected with a normal mode, animation ceases and the static structure of CCl₄ is displayed again.

Samples of the materials selected for spectral investigation, such as those recommended..

CCl₄ (liquid)
Cl₂C=CCl₂, (liquid), tetrachloroethene
CS₂ (liquid)
Benzene (liquid), C₆H₆
Nitrogen (liquid), N₂

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.

Computational chemistry software, such as one of the following:

HyperChem,
Spartan,
Gaussian 98W,
PC GAMESS.

Carbon tetrachloride, CCl4, is a tetrahedral molecule, point group Td. It has nine normal modes of vibration, of which 1 (A1) at 460 cm^-1 is a Raman active symmetric stretch, 2 (E) at 214 cm^-1 is a Raman active ClCCl bend, 3 (T2) at 793 cm^-1 is an asymmetric stretch active in both IR and Raman, and 4 (T2) at 314 cm^-1 is a bend active in both both IR and Raman (Ref, 3) The Raman spectrum shown was taken with the Ocean Optics R-2000 instrument, with spectral range ca 200-2850 cm^-1, and the FTIR spectrum was recorded with a Nicolet Avatar 360 FT-IR Spectrometer, with spectral range 400-4000 cm^-1.

All four vibrations are clearly shown in the Raman spectrum, while only 3 appears in this FTIR spectrum. The 1 and 2 vibrations are symmetry forbidden in IR, and 4 is outside the range of our FTIR instrument. Frequencies and intensities were calculated for the normal modes of CCl4 with Gaussian 98 for Windows. At the Hartee-Fock 6-31G(d) level, the frequency scaling factor is 0.8929 (Foresman, 1996). The Raman activities (intensities) calculated for frequencies 1-4 were 19.0 for 449 cm^-1 (A1), 3.4 for 218 cm^-1 (E) , 10.5 for 805 cm-1 (T2), and for 5.1 for 311 cm^-1 (T2), The Gaussian calculations are in good agreement with the observed spectra. Note that the most intense Raman band is the breathing mode (symmetric stretch), which is forbidden in the IR spectrum.

The CCl4 Raman and IR spectra are posted as JCAMP-DX *.jdx files on Deanna Donohoue's web page, and the bands for the vibrational fundamentals are linked to a display which animates the normal modes of vibration. The following section is based principally on her page. (Ref. 29)

Animated vibrations include IR: 794 cm^-1 and Raman: 770, 454, 312, 220 cm^-1

Left click on peak to observe vibrational mode. To zoom-in click and drag, then click and drag again to zoom-out.

For display options for the molecule, right click on the molecule

Chime sometimes goes a little overboard in making and breaking bonds.

It might be instructive to download the *.xyz files and view them in gOpenMol.

To download, right click on the link to the file, select Save Link As, and save the file to a directory on your hard drive.

CCl₄ equilibrium geometry     ccl4raman.pdb

Normal modes of vibration

E    220 cm^-1 Bend    Ccl4ram001.xyz     Ccl4ram002.xyz

T₂    312 cm^-1 Bend   Ccl4ram003.xyz     Ccl4ram004.xyz     Ccl4ram005.xyz

A₁   454 cm^-1 Symmetric stretch     Ccl4ram006.xyz

T₂    770 cm^-1 Asymmetric stretch     Ccl4ram007.xyz     Ccl4ram008.xyz     Ccl4ram009.xyz

Raman spectra were observed for both liquid N₂ and liquid O₂, as shown here.

The observed frequencies are 2329 cm^-1 for liquid N2 and 1538 cm^-1 for liquid O2. From the gas phase data, and using
omega₀ = omega_e - 2 omega_ex_e, literature values of omega₀ are 2330 cm^-1 for N₂ and 1556 cm^-1 for O₂, in satisfactory agreement with the liquid phase data observed here. (Ref. 31, 32)

The Raman and spectra of C₂Cl₄ are shown here.

There is a center of inversion. The molecule is planar, with D2h symmetry. The Rule of Mutual Exclusion applies, and each of the normal modes appears in the Raman or in the IR, but not both.

Note particularly that the C=C stretch is clearly present in the Raman spectrum at 1576 c^m-1 but is absent in the IR The spectra are posted on the web in JCAMP-DX *.jdx format in Experiment for Organic Chemistry II, The Rest of the Vibrations: Raman and Infrared Laboratory Experiment. (Ref. 33) Symmetry analysis, assignments of normal modes of vibration, and Gaussian 98W calculations were carried out for this molecule, and have been presented at the South Dakota Academy of Science. (Ref. 12) The IR and Raman spectra of this molecules are also discussed thoroughly by Herzberg.(Ref. 34)

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

1. For the CCl4 molecule, describe the normal mode of vibration which is associated with the Raman band at 452 cm^-1. Explain why this normal mode is absent from the IR spectrum but present in the Raman spectrum.

2. For the molecule that you worked on yourself, clearly describe the normal mode of vibration which has the greatest intensity in the IR spectrum, and the normal mode of vibration which has the greatest intensity in the Raman spectrum.

3. Examine the IR spectrum and the Raman spectrum of C₂Cl₄ as JCAMP-DX *.jdx files on the web in the document Experiment for Organic Chemistry II, The Rest of the Vibrations: Raman and Infrared Laboratory Experiment. (Ref. 33) at URL
http://ed.augie.edu/~viste/Raman/RamanOrganic.html
Thus first compare with the spectrum shown in the present exercise, in order to decide which unknown is in fact C₂Cl₄. Click on the major peaks, and compare the frequencies that you measure there with those printed on the C₂Cl₄ IR spectrum and Raman spectrum shown earlier in the present experiment.

4. For the CCl₄ molecule, examine the IR spectrum and Raman spectrum in *.jdx form, earlier in this exercise. Carefully click on the *jdx Raman spectrum, and test whether the wavenumber ranges that activate the animation of each normal mode agree with those specified in the partial listing of the *.jdx file shown above.

1. Gerhard Herzberg, Molecular Spectra and Molecular Structure. II. Infrared and Raman Spectra of Polyatomic Molecules, Van Nostrand, New York, 1945.

2. F. Albert Cotton, Chemical Applications of Group Theory, 3rd ed., John Wiley, New York, 1990.

3. Kazuo Nakamoto. Infrared and Raman Spectra of Inorganic and Coordination Compounds, Part A and Part B, 5th ed., Wiley-Interscience, New York, 1997.

4. HyperChem, semiempirical and ab initio quantum mechanical software package. http://www.hyper.com/

5. Spartan, ab initio and semiempirical quantum mechanical software package. http://www.wavefun.com/

6. Gaussian 98 for Windows, ab initio and semiempirical quantum mechanical software package. http://www.gaussian.com/

7. PC GAMESS, ab initio and semiempirical quantum mechanical software package. http://quantum-2.chem.msu.ru/gran/gamess/index.html http://www.msg.ameslab.gov/GAMESS/pcgamess.shtml

8. pcgRun GUI for PC GAMESS, Wayne P. Anderson, Bloomsburg University, Bloomsburg, PA. wpa1@sunlink.net or wpa@husky.bloomu.edu

9. gOpenMol has been developed by Leif Laaksonen, Center for Scientific Computing, Espoo, Finland. http://www.csc.fi/~laaksone/gopenmol/gopenmol.html

10. Chime 2 plugin. MDL Information Systems, Inc, Chime 2 download site. http://www.mdli.com/cgi/dynamic/downloadsect.html?uid=$uid&key=$key&id=1

11. 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.

12. Deanna L. Donohoue, Gary W. Earl, and Arlen Viste, "Using the Ocean Optics R-2000 Raman Spectrometer in the Undergraduate Laboratory," presented at the South Dakota Academy of Science, Moorhead, MN, April 29, 2000. Submitted for publication in Proceedings of the South Dakota Academy of Science, Volume 79, 2000. 13. Arlen Viste and Gary Earl, Raman Utility Software Exercise on a PC. http://ed.augie.edu/~viste/Raman/RamanSoftware.html

14. IrfanView
http://stud1.tuwien.ac.at/~e9227474/english.htm

15. Peter W. Atkins, Physical Chemistry, 6th ed., Freeman, New York, 1998.

16. P. W. Atkins and R. S. Friedman, Molecular Quantum Mechanics, 3rd ed., Oxford University Press, New York, 1997.

17. Ira N. Levine, Quantum Chemistry, 5th ed., Prentice Hall, Upper Saddle River, NJ, 2000.

18. John P. Lowe, Quantum Chemistry, 2nd ed., Academic Press, San Diego, 1993.

19. MacGAMESS is maintained by Brett Bode at Ames Lab, Iowa State University.. http://www.msg.ameslab.gov/GAMESS/dist.mac.shtml

20. Mary L. Caffery, Paul A. Dobosh, and Diane M. Richardson, Laboratory Exercises Using HyperChem, Hypercube, Inc., 1998, Gainesville, FL.

21. Warren J. Hehre, Alan J. Shusterman, and W. Wayne Huang, A Laboratory Book of Computational Organic Chemistry, Wavefunction, Inc., Irvine, CA, 1996.

22. James B. Foresman and Æleen Frisch, Exploring Chemistry with Electronic Structure Methods, 2nd ed., Gaussian, Inc., Pittsburgh, PA, 1996

23. Xvibs, written and maintained by Bradley A. Smith. http://members.home.net/yeldar/xvibs/

24. E. Lewars, "Semiempirical Frequency Scaling Summary," CCL List, 21 May 2000. http://www.ccl.net/cgi-bin/ccl/message.cgi?2000+05+21+004

25. Scaling frequencies in ab initio calculations. Ref. 22, p. 64.

26. Robert J. Lancashire, Paul M. Lahti, and Eric Motyka, "Use of Chime (v2) For Infrared Spectral Display -- Integration with Computational Chemistry."
http://www.chem.umass.edu/~nermmw/Spectra/

27. Paul M. Lahti, VIBREAD98 program.
http://www.chem.umass.edu/~nermmw/Spectra/1.htm

28. Robert J. Lancashire et al., "JCAMP-DX Data Viewer for Windows (95,98 and NT)." http://wwwchem.uwimona.edu.jm:1104/software/jcampdx.html

29. Deanna L. Donohoue, "Using Chime for IR and Raman." http://ed.augie.edu/~dldonoho/samplejdx.html

30. Arlen Viste, Normal Modes of Vibration. Example: S42+. http://ed.augie.edu/~viste/302s2000/S4.html

31. K. P. Huber and G. Herzberg, Molecular Spectra and Molecular Structure. IV. Constants of Diatomic Molecules. Van Nostrand, Princeton, NJ, 1979

32. David P. Shoemaker, Carl W. Garland, and Joseph W. Nibler, Experiments in Physical Chemistry, 6th ed. McGraw-Hill, New York, 1996, Expt. 36.

33. Gary Earl, Arlen Viste, and Deanna Donohoue, Experiment for Organic Chemistry II, The Rest of the Vibrations: Raman and Infrared Laboratory Experiment. http://ed.augie.edu/~viste/Raman/RamanOrganic.html

34. Herzberg, Ref. 1, p 107, 329.