In the Laboratory
Quantitative Determination of the Rotameric Energy W Differences of 1,2-Dihaloethanes Using Raman Spectroscopy An Experimental Project for the Physical Chemistry Laboratory Mark D. Young, Natalia C. Borjemscaia, and Brian D. Wladkowski* Department of Chemistry, McDaniel College, Westminster, MD 21157; *
[email protected] Although the use of IR spectroscopy is now common in the undergraduate physical chemistry laboratory, Raman spectroscopy remains a novelty. This is true despite the complementary nature of these two techniques and the growing importance of Raman spectroscopy in many graduate programs and in industry. Most of the articles relating to Raman spectroscopy that have appeared in this Journal focus on overviews of the technique (1–5), symmetry implications (6, 7), or on related Raman techniques such as resonance Raman (8–10), solid-state Raman (11–13), or surface-enhanced Raman spectroscopy (14). The relative lack of interest in Raman spectroscopy at the undergraduate level is likely a result of several factors including: the historically high cost of Raman systems, a lack of expertise, especially when dealing with lasers, and possibly time constraints. Fortunately, with recent advances in detector technology and with the improvement in laser design, cost is less of a problem. Now, basic Raman spectrometers are well within the expense budgets of most small schools. As the use of Raman spectroscopy in many Arⴙ laser (488 nm)
mirror
power supply
lens
monochromator entrance slit
mirror
sample holder lens
PMT water bath
exit slit mirror
controller computer
Figure 1. Schematic of the laboratory setup.
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chemical fields continues to grow, and undergraduate institutions acquire the necessary Raman equipment, the need for simple projects applicable to undergraduates also increases. Ideally, projects utilizing Raman spectroscopy should be easy to implement, have relevance to basic principles, and be complementary to existing laboratory exercises. We recently introduced a physical chemistry laboratory project focusing on the rotameric stability of 1,2dihaloethanes (15). Specifically, the gauche, trans rotameric energy difference, ∆t-gE, was determined for 1,2-dichloro-, 1,2-dibromo-, and 1,2-diiodoethane in the gas phase, condensed phase, and in solvents of varying dielectric using IR spectroscopy. It was discovered that despite the intrinsic electronic stability of the trans rotamer, the gauche rotamer becomes preferentially stabilized in high dielectric solvents. Projects like this are important because they illustrate how vibrational spectroscopy can be used in a quantitative manner in exploring the conformational energies of intermediate size molecules. The results also allow students to see the importance of solvation in affecting the relative energies of closely spaced conformational states. Since the rotameric energy difference of these 1,2-dihaloethanes can be quantified on the basis of their relative vibrational intensities, Raman spectroscopy represents an excellent alternative experimental technique to explore these systems. In this article we present a complementary Raman spectroscopy experiment for the undergraduate laboratory, which can be used in conjunction with the IR spectroscopy experiment presented previously to study the rotameric stability of 1,2-dihaloethanes. Here, both 1,2-dichloroethane (DCE) and 1,2-dibromoethane (DBE) are studied using Raman spectroscopy to determine ∆t-gE the energy difference between the trans and gauche rotamers, for comparison with the IR data. Raman spectra for these compounds were collected in various environments, including the pure liquid, carbon tetrachloride, acetonitrile, and at various temperatures. Experimental Procedure The experimental Raman setup consisted of a 0.22 m double monochromator (Spex 1680) with a photomultiplier tube (PMT) detector (Hamamatsu R928). The PMT was mounted to the exit slit of the monochromator in a temperature-controlled housing allowing the temperature of the PMT to be lowered using chilled water. However, for the experiments presented here the dark current from the PMT was not a problem for room-temperature operation. The excitation source consisted of approximately 100 mW of the 488 nm line of an argon-ion laser (Spectra-Physics Stabalite 2016). The laser light was redirected upward through the sample cell such
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that the laser line was parallel with the entrance slit. Scattered light from the sample was then collected perpendicular to the laser line and focused onto the entrance slit. The entrance and exit slits were set to 150 µm. A standard 1-cm quartz fluorescence cell (Starna) was used for all room-temperature measurements. For temperature-variation experiments, a water-jacketed quartz cell was used (Hellma) to vary the temperature of the sample over a range from 15 C to 45 C. The layout of components is shown in Figure 1. The data were collected using SpectraMax/32 for Windows software (Galactic Industries Corp.). Raman spectra for solution-phase DCE samples were collected from 1400 to 1125 cm1, using a 0.02-nm step size, 3-s integration times, and 7 averaged scans. The neat samples used the same conditions but with a 0.8-s integration time, owing to the much stronger signal. The solution-phase DBE Raman spectra were collected from 725 to 500 cm1, using a 0.02-nm step size, 3-s integration times, and 7 averaged scans. Again, the neat DBE samples followed the same setup except for a 0.8-s integration time from the stronger signal received. The spectral range was chosen in each case to avoid solvent-peak interference. All solutions samples were approximately 0.5 M in their respective solvents. Hazards The dihaloethanes used in this experiment should be handled with caution. They are considered lachrymators, are cancer-suspect agents, are easily absorbed through the skin, and can burn the skin on prolonged contact. Therefore all samples should be prepared and the sample cells filled in a fume hood, and gloves should be worn at all times when handling these chemicals. The boiling points of DCE and DBE are 82 C and 131 C, respectively. To avoid damage to the temperature-variable Raman cell and possible explosion hazard, the temperature should not be raised above 60 C, and an air bubble should be left in the cell to allow for thermal expansion of the liquid sample. Laser radiation can cause severe damage to the optic nerve. Never look directly into the beam path. If available, laser goggles for the appropriate wavelength range should be worn.
The peaks are labeled with t or g to designate the trans or gauche rotamer, respectively. The areas of the peaks can be related to the energy difference because the relative population of each rotamer can be expressed as a ratio of the Boltz-
X
X H
X
H
H
H
H
H
X
gauche
trans
H H
X: halogen atom Figure 2. Stable conformations of 1,2-dihaloethanes.
Figure 3. Comparison of Raman and IR spectra of neat DCE. The peaks are labeled with t or g to designate the trans or gauche rotamer, respectively.
Data Analysis At or near room temperature, 1,2-dihaloethanes undergo rapid rotation about the CC bond in establishing equilibrium between two rotameric states: the trans state, with the halogens 180 apart, and the gauche state, with the halogens either 60 or 300 apart. These rotamers are shown in Figure 2. In the gas phase, the trans rotamer is intrinsically more stable owing to the fact that steric hindrance from the adjacent halogen atoms is minimized. However, because the gauche rotamer exhibits a nonzero dipole moment, it is preferentially stabilized in high dielectric environments through dipole interaction with the solvent. Therefore, in polar solvents these compounds show an increasing preference for the gauche rotamer. Because the vibrational modes in each rotamer exhibit slightly different force constants, the relative energy difference between them can be determined from the relative population of each rotamer, through measurement of the peak intensities spectroscopically. Figures 3 and 4 show the Raman and IR spectra of DCE and DBE, respectively.
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Figure 4. Comparison of Raman and IR spectra of neat DBE. The peaks are labeled with t or g to designate the trans or gauche rotamer, respectively.
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mann distribution in the form ∆t − g E g Nt = t exp − Ng gg RT
(1)
where Nt and Ng are the populations of the trans and gauche rotamers, respectively, gt and gg are the degeneracy factor of each rotamer, ∆t-gE is the energy difference between the two rotamers, R is the gas constant in appropriate units, and T is the temperature. For Raman spectroscopy, the integrated intensity of scattered light for a given normal mode over a selected wavelength range is given by (2)
Ai = k i c i
where Ai is the total peak area, ci is the concentration of species, and ki is a proportionality constant relating to the scattering efficiency of each particular normal mode. Since Nt兾Ng = [t]兾[g], the two equations above can be combined to produce ln
At Ag
= ln
kt kg
+ ln
gt gg
−
∆t − g E RT
(3)
Thus, if the areas of trans and gauche peaks are measured over a range of temperatures, a plot of ln(At兾Ag ) versus T 1 would allow the energy difference between the two rotamers to be determined from the slope of the equation of the bestfit line. The y intercept of such a plot would include the natural logs of the proportionality constants and the degeneracy factors. In the case of DCE and DBE, the degeneracy factor ( gt兾gg ) is equal to 0.5, since there are two gauche rotamers (60 and 300), while there is only one trans rotamer (180).
2.5
DBE neat y 452.356x 0.607
Results and Discussion The plots of ln(At兾Ag ) versus T 1 for both neat DCE and DBE samples are shown in Figure 5. The temperaturedependent DBE data follow the trend found in previous IR work (15), with the relative population of the gauche rotamer increasing as the temperature is raised. This indicates that for DBE in the liquid phase, the trans rotamer is more stable than the gauche rotamer, which is consistent with the intrinsic gas-phase expectation based on steric hindrance. In the case of DCE, however, the temperature-dependent data follow the reverse trend; the gauche rotamer population decreases relative to the trans rotamer population as the temperature is raised. There are two factors contributing to this behavior. Steric repulsion is intrinsically less for DCE than DBE, owing to the lower steric repulsion between the smaller chlorine atoms. As a result, the gauche and trans rotamers in DCE are closer in energy and thus ∆t-gE will be smaller in magnitude. Because of this, DCE will exhibit a larger relative population of the gauche rotamer than DBE, which will in turn cause bulk DCE to have a higher dielectric. Therefore, when DCE is “dissolved” in DCE, a more polar solvent, the polar gauche rotamer will be stabilized by an additional amount. A linear fit to the data in Figure 5 in combination with eq 3 also allows for the determination of the proportionality constants (kt兾kg ) for both DCE and DBE liquids. Assuming that the ratio of the proportionality constants is independent of the environment, the rotameric energy difference, ∆t-g E can be determined for DCE and DBE in the different solvents at a single temperature. The experimental values of ∆t-g E for DCE and DBE using both Raman and IR spectroscopy are shown in Table 1. The maximum variance in the two methods of data collection occurs for DCE in acetonitrile, where the difference between the Raman and IR is 1.64 kJ兾mol. The variation in the results between the two techniques is small and likely a
1.5
ln
At Ag
2.0
It should be noted that the choice of the trans and gauche peaks used in the data analysis is not important, as long as the same peaks are used throughout. Changes in intensity will be corrected for in the proportionality term, kt兾kg. Peaks chosen in this case were those that are easily resolved and do not overlap solvent peaks.
Table 1. ∆t-gE Values for DCE and DBE in Various Solventsa 1.0
Phase
DCE neat y 156.452x 1.535
Gas Phase
3.1
3.2
3.3
1 T Figure 5. Plot of ln(At/Ag) versus T DBE.
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3.4
3.5
(10ⴚ3 Kⴚ1) 1
for neat samples of DCE and
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DBE
Raman
IR
Raman
IR
0
—
3.15
—
5.76
b
2.24
0.58
2.08
5.71 4.67
Neat (DCE/DBE)
10.42/ 4.96
1.31
0.36
3.60 2.40
0.5 M CH3CN
36.64
2.42
0.78
2.45 1.94
0.5 M CCl4
0.5
DCE
Dielectric Constant
a
All values are given in units of kJ/mol.
b
Complementary IR data is included for comparison (15).
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result of a breakdown in the assumptions in the data analysis or perhaps a result of the differences in the inherent uncertainty in the measurement using the two techniques. The experimental data showcase several trends. First, the value of ∆t-g E is larger (more negative) for DBE. This is due to the larger steric repulsion of the bromine atoms compared to the chlorine atoms on DCE. Secondly, as the environment becomes more polar (higher dielectric constant), the value of ∆t-g E becomes more positive, showing a shift in preference towards gauche. This shift towards gauche is clearly seen in Figure 6, which shows the Raman spectra of DBE in three different environments. As the solvent polarity is increased from carbon tetrachloride to the neat sample to acetonitrile, the trans peak (near 675 cm1) becomes smaller compared to the gauche peak (near 560 cm1). Also seen in this figure are the small shifts in scattering frequency owing to solvent interactions. Based on both sets of data, it appears that the point at which the two rotamers in DCE become equal in energy (i.e., ∆t-g E = 0) occurs very close to the environment created in the neat sample.
Figure 6. Comparison of DBE Raman spectra in different solvents.
Conclusions and Future Work
Literature Cited
The use of Raman spectroscopy in studying some physical properties of 1,2-dihaloethanes provides an excellent opportunity for students to apply spectroscopic methods to quantitative analysis of intermediate-sized molecules. Through this procedure students can study solvent effects, force constants, and the effect of symmetry on IR and Raman spectroscopy. Modifications to this experiment may include the use of additional solvents, such as methanol or diethyl ether, to achieve a more complete picture of solvent effects on these systems. 1,2-Diiodoethane could also be used in this lab for a more complete comparison to the IR method. While it would be impractical and extremely difficult to obtain the neat liquid spectra (mp 82 C), energy values could still be obtained, since the ratio of proportionality constants can be found from any plot of ln(At兾Ag ) versus T 1.
1. Hoskins, L. C. J. Chem. Educ. 1975, 52, 568–572. 2. Steehler, J. K. J. Chem. Educ. 1990, 67, A65–A71. 3. Fitzwater, D. A.; Thomasson, K. A.; Glinski, R. J. J. Chem. Educ. 1995, 72, 187–189. 4. Grant, C. A.; Hardwick, J. L. J. Chem. Educ. 1997, 74, 318– 321. 5. McClain, B. L.; Clark, S. M.; Gabriel, R. L.; Ben-Amotz, D. J. Chem. Educ. 2000, 77, 654–660. 6. Comstock, M. G.; Gray, J. A. J. Chem. Educ. 1999, 76, 1272– 1275. 7. Nakamoto, K.; McKinney, M. A. J. Chem. Educ. 2000, 77, 775–780. 8. Hoskins, L. C. J. Chem. Educ. 1984, 61, 460–462. 9. Strommen, D. P.; Nakamoto, K. J. Chem. Educ. 1977, 54, 474–478. 10. Shaw, C. F. J. Chem. Educ. 1981, 58, 343–348. 11. Aponick, A.; Marchozzi, E.; Johnston, C.; Wigal, C. T. J. Chem. Educ. 1998, 75, 465–466. 12. Kettle, S. F. A.; Diana, E.; Rossetti, R.; Stanghellini, P. L. J. Chem. Educ. 1998, 75, 1333–1338. 13. Nissum, M.; Shabanova, E.; Nielsen, O. F. J. Chem. Educ. 2000, 77, 633–636. 14. Weaver, G. C.; Norrod, K. J. Chem. Educ. 1998, 75, 621– 624. 15. Wladkowski, B. D.; Broadwater, S. J. J. Chem. Educ. 2002, 79, 230–233.
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Supplemental Material
Instructions for the students and notes for the instructor are available in this issue of JCE Online. Acknowledgments This work was supported by grants from Research Corporation (CC4662) and the NSF-RUI program (CHE9983321).
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