Frank P. DeHaanl Occidental College 10s Angeles, California 90041 Jack C. Thibeault Cornell University Ithaca, New York 14850 and David K. Ottesen Sandia Laboratories Livermore, California 94550
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Raman Spectra of ZXY3 Compounds A d r y - l a b spectral ana/ysis experiment
One of the largest gaps in the undergraduate education of many chemistry majors, particularly those a t small colleges, continues to be their unfamiliarity with many of the newer, more sophisticated instruments a n d the information which can he gleaned from them. Considering their price tags and today's budgetary crisis in education, the gap is both understandable and predictable for years to come. As a n interim solution to the problem, we have found t h a t students respond enthusiasiically when given some first-class spectra t o interpret a s a "dry lab" problem, and i n so doing, pick u p a much better understanding of the instrumental approach in q u e ~ t i o n (Incidentally, .~ part of their enthusiasm may stem from relief-particularly for the Herman Hackinlabs-there is no way the instrument can turn against them in a dry lab experiment.) For the past four years Occidental students in Physical Chemistry interested in Raman spectroscopy have been able to choose a spectral analysis experiment which we have developed titled "Raman Spectra of ZXYJ Compounds." At this point in time (third term) the students have had enough group theory (Cotton's book3 and a new set of lecture notes by Prof. Harry Gray are used a s texts) to handle the assignment. With very few exceptions our students have successfully completed the entire experiment, including t h e extra credit problem (vide infra). The Experiment I. Reading Assignment 1) Tobias, R. S., "Raman Spectroscopy in Inorganic Chemistry," Part I,, J. CHEM. EDUC., 44, 2 (1967); Part 2, 44, 70 (1967). 2) Hendra, P. T., and Stratton, P. M., "Laser-Raman Spectroscopy," Chem. Rev., 69,325 (1969). 3) Cotton, F. A., "Chemical Applications of Group Theory," 2nd Ed., Wiley-Interscience. New York, 1971, p. 295-308, 316321. Additional Theory-Polarized Roman Spectra. A polaroid film is oriented so that its electric vector is either parallel or perpendieular to the electric vector of the source light (in this case, a 99.9% polarized laser beam). The intensity of a given peak will then he proportional to its peak height minus the base line height. This gives two values of intensities, IlandI, ;their ratio PO*, =
IJI,
is then the observed depolarization ratio. For our particular instrumental set up we theoretically should obtain the following values
Type of Vibration Nan-symmetric Totally symmetric il. Preliminary work You will be given Raman spectra of two molecules having the ZXY3 formula. In order to elucidate the structures of these compounds it will be necessary to predict the Raman spectra of all the possible stable ZXY, spacial configurations. For each ZXYs configuration, determine 1) The symmetry types of all the normal modes of vibration. 2) The form of vihration-stretching or bending-of each of these
modes. 3) The number of Raman-active modes. 4) The expected depolarizatian ratio of each Raman-active mode. 111. The Spectra The spectra provided are Alow resolution spectrum of unknown A. High resolution scans of the three lowest energy bands of unknmvn A. A low resolution spectrum of unknown B. High resolution scans of the three lowest energy bands of unknown B. 1) What is the structure of compound A? Compound B? State the reasons for your conclusions.
2) Determine the combinations of internal coordinates representing each Raman-active normal mode for the structures found above. 3) Assign symmetry types, stretching or bending labels, and combinations of internal coordinates to each band of spectra 1 and 3. 4) What is compound A? Compound B? How did you arrive at these conclusions? If you are not sure, state your analysis, as far as it goes, of compounds A and B. Extra credit problem 5) One of the bands of unknown A is split into a number of parts. What explanation can you offer for this splitting? From your theory, predict the relative intensity of each of the parts of the split band. Why does the intensity decrease as the energy decreases instead of vice versa? Would you expect a similar effect in spectrum 4? Explain. Your written report must include your preliminary work (all other stable ZXYI configurations-Part 11) as well as the spectra analysis (Part 111). 'To whom correspondence should he addressed. Our approach is hardly Editorial suooort for the "drv . oripinal. . l a b app&h to the instrument gap appeared in J. CHEM EDUC., 42, 63 (1965). Many excellent experiments have followed, e.g., Boer, F. P. and Jordan, T. H., J. CHEM. EDUC., 42, 76 119fi51. R.., J. CHEM. 43. 2 119fifi1 K ,--~ ,Little. , -~ -~ -~ EDIJC.. -- ~ ..,. , .Knhlmann~.,--. F., and Braun, C. L., J. CHEM. EDUC., 46, 750(1969). Cotton, F. A., "Chemical Applications of Group Theory," 2nd Ed., Wiley-Interscience, New Yark, 1971. ~
In practice, due to a loss of laser beam orientation during the multiple internal reflections occurring in the capillary sample container, the valuesusually run about
Volume 51. Number 4 , April 1974
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I WAVE NUMBER (cm-') Figure 1. Spectra of Unknown A. Upper spectrum: I,I. Lower spectrum: II. Insets: high resolution scans of the three lowest energy bands: S2, 668 cm-'; Sh 366cm-'; S6, 262cmmm.
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WAVE NUMBER (cm? Figure 2. Spectra of Unknown B. Upper spectrum: I),. Lower spectrum: I I . insets: high resolution scans of the three lowest energy bands: S2. 539cm-';Sr 222cm-':S6, 154cm-'.
The Spectra
The polarized Raman spectra of unknown A and B are shown in Figures 1 and 2. The spectra were obtained from the neat liquid using a Cary 81 laser Raman spectrometer. A Spectra-physics 125 He-Ne laser was the light source, and 50 mW of power a t 632.8 nm were delivered to the sample cell, a l / m m i.d. glass capillary tube 5 cm in length. Spectral slit widths for survey runs and high resolution scans were 5 cm-1 and 0.5 cm-', respectively. The observed band energies . and depolarization ratios are summarized in Table 1. It is our feeling that the students would benefit significantly from using reproductions of the spectra as close to the originals in appearance as possible. To this end we have made arrangements to provide large scale copies of the spectra free of charge to those i n t e r e ~ t e d . ~ The Analysis
The most obvious arrangements of four atoms about a central atom lead to the square planar and tetrahedral structures having Cz, and Ca symmetry, respectively. If we also consider the possibility of valence shell nonbonding pairs, other structures, such as the #-trigonal bipyramid having C, symmetry, are plausible.* For completeness we must also include structures having C1 symmetry. In each of the above cases the molecule will have 3N 6 = 9 vibrational modes. The group theoretical methods for determining the symmetry type and form of vibration of each of these normal modes have been adequately discussed e l ~ e w h e r eand ~ , ~ will not be repeated here. The results for the four symmetries of interest are summarized in Table 2. It is important for the student to realize that the results listed in Table 2 depend only on the symmetry Table 1. Observed Bands and Assignments
(em-I)
pob.
3019 1216 761 668 366 262
