Infrared and Raman Spectroscopy: A Discovery-Based Activity for the

Article pubs.acs.org/jchemeduc

Infrared and Raman Spectroscopy: A Discovery-Based Activity for the General Chemistry Curriculum Karen L. Borgsmiller,* Dylan J. O’Connell, Kathryn M. Klauenberg, Peter M. Wilson, and Christopher J. Stromberg Department of Chemistry and Physics, Hood College, Frederick, Maryland 21701, United States S Supporting Information *

ABSTRACT: A discovery-based method is described for incorporating the concepts of IR and Raman spectroscopy into the general chemistry curriculum. Students use three sets of springs to model the properties of single, double, and triple covalent bonds. Then, Gaussian 03W molecular modeling software is used to illustrate the relationship between bond vibrations and both IR and Raman spectroscopy. Students view the characteristic vibrations of C−C, CC, and CC bonds in propane, propene, and propyne molecules. Finally, students collect IR and Raman spectra of hexane, 1-hexene, and 1-heptyne. This series of activities is completed in the first semester of the general chemistry sequence following the development of Lewis structures and before the discussion of electronegativity and bond polarity.

KEYWORDS: First-Year Undergraduate/General, Laboratory Instruction, Physical Chemistry, Analogies/Transfer, Computer-Based Learning, Inquiry-Based/Discovery Learning, IR Spectroscopy, Raman Spectroscopy

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published in this Journal promoting the use of springs and masses to simulate the oscillations present in chemical bonds.5−7 In the second activity, students used the software program Diatomic (version 1.02; 1991) as an aid to visualize molecular motion.8 Third, students used the software program IR Tutor (version 1.1; 1992−3) to learn about IR spectroscopy.9 Finally, students took an IR spectrum of an unknown sample and used IR Tutor to identify the compound from a list of three possible choices. Over the past two years, these activities have been modified. First, better spring-based models were created for simulating single, double, and triple bonds; second, updated software programs were incorporated to probe bond oscillation properties; and finally, a Raman spectroscopy component was added to complement the existing IR spectroscopy activity. The new activities are designed to help students better understand the nature of covalent bonds.

ixteen years ago, a commitment was made to teach the general chemistry sequence in a discovery-based style. This method of general chemistry instruction has a history that began in the early 1990s, when the American Chemical Society’s Task Force on Chemical Education challenged college chemistry instructors to investigate new methods for teaching general chemistry. Many colleges adopted this type of general chemistry curriculum.1−4 When the new science building at this college was being designed in the late 1990s, the floor plan was configured so that the chemistry classrooms would be located adjacent to the lab. This allows the general chemistry students to move freely between the laboratory and lecture space. All sections of general chemistry meet for 2 h 3 times a week and are limited to 24 students. Students work in lab groups of 2−3 students, and during most class periods, students spend time in both the classroom and in the laboratory. Most of the concepts taught in general chemistry are first introduced through a lab activity or other “hands-on” experience. Shortly after the discovery-based learning method was adopted, a series of four activities were included to help students understand the properties of covalent bonds and how these properties can be studied through the use of IR spectroscopy. The first activity was a laboratory exercise that used springs as models for covalent bonds and different sized masses to represent different atoms. Many articles have been © 2012 American Chemical Society and Division of Chemical Education, Inc.

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LEARNING OBJECTIVES There are five learning objectives with these activities. The goal is for students to recognize and understand Objective 1: That covalent bonds oscillate with discrete frequencies. Published: January 30, 2012 365

dx.doi.org/10.1021/ed2002835 | J. Chem. Educ. 2012, 89, 365−369

Journal of Chemical Education

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Objective 2: That the frequency of oscillation for a covalent bond increases as the bond strength increases. Objective 3: That the frequency of oscillation for a covalent bond decreases as the mass of the atoms associated with the bond increases. Objective 4: That the peaks in IR and Raman spectroscopy correspond to specific motions of covalent bonds. Objective 5: That Raman and IR spectroscopy are complementary analytical techniques. These activity learning objectives support two of the overall course objectives to learn laboratory techniques and to learn basic chemistry concepts.

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MODELS FOR SIMULATING BONDS Springs have been used as models for covalent bonds for many years. Previously, three different springs were used to represent the single, double, and triple bond. The springs varied greatly in size and appearance. Unfortunately, the variety of springs used led to variations in data that obscured the overall point of the exercise. Thus, three spring systems were introduced to better represent single, double, and triple bonds. These spring systems were made using ductwork strapping (Home Depot) and Hooke’s Law Springs (Sargent-Welch: WL0569R). The ductwork strapping was coated with PlastiDip, a rubber coating material available at Home Depot to protect students from the sharp edges of the cut strapping. To simulate a single bond, a single spring is suspended between two pieces of strapping. A double bond uses two springs, and a triple bond has three springs. Photos of these systems are shown in Figure 1.

Figure 2. Testing the displacement of the spring systems.

Table 1. Data from the Springs Systems Displacement (std dev)/cm Number of Springs 1 (single bond) 2 (double bond) 3 (triple bond)

Frequency of Oscillation (std dev)/Hz

16.14 g Stopper

28.51 g Stopper

16.14 g Stopper

28.51 g Stopper

8.8 (0.37)

16.9 (0.21)

1.55 (0.074)

1.16 (0.020)

4.2 (0.39)

7.8 (0.26)

2.14 (0.032)

1.70 (0.065)

2.3 (0.26)

4.9 (0.25)

2.44 (0.073)

1.96 (0.070)

The new spring systems allow students to visualize how single, double, and triple bonds differ in their bond strengths. The displacement data clearly demonstrate that a single bond has a greater displacement than either the double or triple bond and that the double bond has a greater displacement than the triple bond. Students are readily able to interpret the data and recognize that single bonds are weaker than the double and triple bonds and that a double bond is weaker than a triple bond. The new spring systems also allow students to visualize the differences in bond frequencies between single and multiple bonds. When looking at the data for both stoppers, students see that the frequency of oscillation increases as the number of bonds increase. When this data is combined with the displacement data, students easily see that the frequency of oscillation increases as the bond strength increases (objective 2). Finally, the new spring systems allow students to probe the impact that atomic mass has on the frequency of oscillation. The data shows that, as mass increases, the frequency of oscillation decreases for all three types of bonds (objective 3). Once students have gathered the data from the laboratory activity, a series of software program activities are used to relate these data to oscillations across chemical bonds.

Figure 1. The spring systems that represent single, double, and triple bonds.

The lab procedure required each lab group to test the three systems, one system at a time. For testing, the spring system was attached with a clamp to a laboratory ring stand (Figure 2). Two masses were made from 2-hole rubber stoppers with a copper wire hook attached. The lighter mass was made using a #5 stopper that weighs about 16 g. The heavier mass was made using a #7 stopper that weighs about 28 g. Students began the experiment by weighing both stoppers. Then, they measured the difference between the unstressed spring length and the length of the spring when each mass has been suspended from the bottom (spring displacement). Finally, they measured the frequency of oscillation for both the heavy and the light mass on each system of springs. The frequency was determined using a stopwatch to measure the time it takes for the spring to oscillate 10 times. Sample data from four students and an instructor running the experiment three times each is summarized in Table 1. These data are typical of that obtained by general chemistry students during the lab.

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SOFTWARE PROGRAMS

Original Method

Previously, the software program Diatomic was used as an introduction to the three modes of molecular motion and the software program IR Tutor was used as a bridge between the 366



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formaldehyde with the CC stretch in propene. They see that the CO stretch occurs at a slightly higher frequency than the CC stretch. These results verify the relationship between atomic mass and frequency that students observed in the spring activity (objective 3).

springs activity and IR spectroscopy. IR Tutor allowed students to look at the IR spectra for various organic molecules. The interactive nature of IR Tutor allowed students to select a peak from an IR spectrum and the software showed an animation of the bond motion associated with that wavenumber (objective 1). Once again, students could see that higher atomic masses result in lower frequencies of oscillations and that multiple bonds have higher frequencies than single bonds (objectives 2 and 3). Students liked the visualizations in IR Tutor and they gained an appreciation for the different types of molecular motion. However, the program only has visualizations for a limited catalog of IR spectra.

IR and Raman Spectroscopy Similarities and Differences

While students are viewing the molecular simulations for each peak on the IR and Raman spectra, they are also asked to observe the differences between IR and Raman spectra. The objective is for the students to notice that the spectra are different, especially in the intensities of specific peaks. It is pointed out that IR absorption arises due to a change in the dipole moment of a molecule as it vibrates and that Raman is a scattering phenomena related to the molecular polarizability. The discussion leads to a short lecture highlighting the similarities and differences between IR and Raman spectroscopy. Students are told that, to understand all the vibrational motions a molecule can experience, both spectra should be analyzed. It is emphasized that the differences are important and often lead to more in-depth understanding about the molecules. For example, the CC peak is weak in IR and could be easily overlooked, but the presence of a carbon triple bond results in a strong peak in Raman. The importance of the complementary nature of these spectroscopic methods is highlighted (objective 5). A PowerPoint lecture about Raman spectroscopy is available to the instructors.

Improved Method

The introductory and background material available in IR Tutor continues to be used by instructors during their lectures. However, Gaussian 03W10 software was recently incorporated into the curriculum. Gaussian 03W is a molecular modeling software that can be used to predict the IR and Raman spectra for molecules built by the user. Use of this type of software in undergraduate courses has been frequently reported in this Journal.11−14 Two features in Gaussian 03W are exploited that are not available in IR Tutor. The first is the ability for an instructor to select and build the molecules that students evaluate. The second, and most important feature for this general chemistry activity, is the side-by-side comparison of IR and Raman spectra calculated by this software. The spectra generated by Gaussian 03W are interactive. Students can select a peak of interest and view an animation of the characteristic bond oscillation (objective 1). The students were not required to build the molecules or run the energy minimization routines. They were provided with the Gaussian 03W output files and used GaussView 4.1 to observe the predicted spectra. In the lab exercise, the students compared the IR and Raman spectra for two sets of molecules: (i) propane, propene, and propyne and (ii) formaldehyde and thioformaldehyde.

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IR AND RAMAN SPECTROMETERS Once the students complete the spring model lab activity and have investigated molecular motions using Gaussian 03W, they visit the instrument labs and are introduced to the two spectrometers. The advantages of each type of spectroscopy are highlighted, including that IR is a less expensive instrument available in all chemistry laboratories and that Raman does not experience the interference from water and carbon dioxide that troubles the IR spectroscopist. Sample preparation and spectra collection technique is demonstrated, and students collect spectra for hexane, 1-hexene, and 1-heptyne.

Propane, Propene, and Propyne

The student guidebook for this activity includes tables listing the peaks predicted by Gaussian 03W for each compound (Tables V1−V3 in the Supporting Information). Students are asked to look at the 19 peaks for propane, and using the animations, they are to determine which wavenumber is associated with the C−C stretches. The C−C stretches in propane are located at 878 cm−1 (symmetric) and 1084 cm−1 (asymmetric) (objectives 1 and 4). Once students complete the analysis of the propane spectra, they repeat the activity by looking for the CC stretch in propene and the CC stretch in propyne. These stretches are located at 1698 and 2206 cm−1, respectively. Looking at these spectra and the corresponding bond motions allows students to see the relationship between bond strength and oscillation frequencies; the stronger the bond, the higher the frequency of oscillation (objectives 1 and 2).

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ASSESSMENT RESULTS Students were tested on their knowledge of Raman by the use of a 10 question pre- and post-test. The questions on the preand post-tests were identical (Table 2). Data were collected from 83 students in five sections of the fall semester of general chemistry. Each section was taught by a different instructor. The pre-test was given before the beginning of the unit, and the post-test was given after the completion of the discussion of all activities. Scores on the pre- and post-tests did not influence student grades. The first four questions on the pre- and posttests were common across the chemistry curriculum in courses where students use the Raman spectrometer (organic, instrumental, biochemistry, and physical chemistry). The assessment data collected from the post-test is summarized in Tables 2 and 3. The student overall average was 0% on the pre-test (data not shown) and 32% on the post-test (Table 3). Section averages on the post-test ranged from 20% to 42% (Table 3). The average score on question 5, 48%, was the highest score overall (Table 2). The students may have scored highest on this question because this concept was explored in both the springs activity and the Gaussian 03W simulation.

Formaldehyde and Thioformaldehyde

Once the analysis of the propane, propene, and propyne system is complete, students are asked to look at the spectra for formaldehyde and thioformaldehyde and identify the wavenumber associated with the CO stretch in formaldehyde (1702 cm−1) and the CS stretch in thioformaldehyde (1014 cm−1). The CO stretch occurs at a higher frequency than the CS stretch. Students can also compare the CO stretch in 367


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Section

Average Post-Test Score (%)

Number of Students

1 2 3 4 Honors Overall

29 34 20 27 42 32

19 18 13 12 21 83

The difference in post-test averages between sections was significant. The Honors section had the highest overall average (42%), whereas the section taught by a first time adjunct instructor, section 3, had the lowest overall average (20%). There are weekly meetings of the general chemistry instruction team where the strategies for instruction are discussed. In preparation for this series of activities, instructors were given the solutions to the pre- and post-tests, a lesson on using the Gaussian 03W software, a copy of Gaussian 03W to install on their laptop computers, and a PowerPoint presentation to use when discussing IR and Raman spectroscopy. As the instructors become more familiar with Raman spectroscopy, it is anticipated that the student scores will improve. As more adjunct faculty are trained, the discussions about Raman will begin earlier in the semester so that they have more time to become familiar with the material.

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SUMMARY The entire discussion of covalent bond properties and spectroscopy requires a total of 4 h of class and lab time. The new spring models, the updated computer software programs, and the addition of Raman spectroscopy have enhanced our general chemistry curriculum. Students now have better tools for visualizing bond vibrations. The discussions are intended to be introductory, as students who continue as chemistry majors revisit these topics many times throughout the curriculum. At the end of the IR and Raman lab activities, the students show gains in understanding the basic concepts outlined as the 5 objectives at the beginning of this paper.

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ASSOCIATED CONTENT

S Supporting Information *

Student handout. This material is available via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

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These data are the sum of all sections.

ACKNOWLEDGMENTS We would like to thank the National Science Foundation (CCLI #0632829) for the purchase of the Raman spectrometer, and Life Technologies with their support of the Summer Research Institute at Hood College. We would also like to thank Terry Sumpter, an adjunct instructor at Hood College, for his willingness to be the first instructor to adopt the new springs activity with his class.

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REFERENCES

(1) Ricci, R. W.; Ditzler, M. A. J. Chem. Educ. 1991, 68, 228−231. (2) Ditzler, M. A.; Ricci, R. W. J. Chem. Educ. 1994, 71, 685−688. (3) Spencer, J. N. J. Chem. Educ. 1999, 76, 566−569.

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4 5 5 5 2 4 5 5 2 4 Briefly describe what molecular properties are measured using Raman spectroscopy. How does Raman spectroscopy relate to other types of spectroscopy (particularly IR, fluorescence, UV−vis spectroscopies)? List three advantages of Raman spectroscopy compared to other types of spectroscopy that measure the same molecular properties. List three disadvantages of Raman spectroscopy compared to other types of spectroscopy that measure the same molecular properties How does the frequency of the Raman peak associated with the C−C stretch compare to the frequency of the Raman peak for the CC stretch? What is the unit of frequency used in Raman spectroscopy? Are all the peaks observed in the Raman spectra found in the IR spectra? Are all the peaks observed in the IR spectra found in the Raman spectra? What information can Raman spectroscopy give you about bond strength? Why does propane have a more complicated spectrum than propyne? 1 2 3 4 5 6 7 8 9 10

Objective

38 12 6 30 48 45 37 30 35 34

Table 3. Average Post-Test Score by Section

Question Question Number

Table 2. Questions Used in the Tests and Post-Test Question Scores

Post-Test Scorea (%)


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(4) Farrell, J. J.; Moog, R. S.; Spencer, J. N. J. Chem. Educ. 1999, 76, 570−574. (5) Anderson, J. S.; Hayes, D. M.; Werner, T. C. J. Chem. Educ. 1995, 72, 653−655. (6) Burke, J. T. J. Chem. Educ. 1997, 74, 1213. (7) Heuer, W. B.; Koubek, E. J. Chem. Educ. 1997, 74, 313−315. (8) Reid, B. P.; Host, B. Diatomic Molecular Mechanics and Motion; Trinity Software: Campton, NH, 1991. (9) Abrams, C. IR Tutor; Wiley: New York,NY, 1993. (10) Frisch, M. J.; et al. Gaussian 03, Revision E.01; Gaussian, Inc.: Wallingford, CT, 2004. (11) Pearson, J. K. J. Chem. Educ. 2007, 84, 1323−1325. (12) McClain, B. L.; Clark, S. M.; Gabriel, R. L.; Ben-Amotz, D. J. Chem. Educ. 2000, 77, 654−660. (13) Lahti, P. M.; Motyka, E. J.; Lancashire, R. J. J. Chem. Educ. 2000, 77, 649−653. (14) Feller, S. E.; Dallinger, R. F.; McKinney, P. C. J. Chem. Educ. 2004, 81, 283−287.

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Infrared and Raman Spectroscopy: A Discovery-Based Activity for the

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