Examination of Bond Properties through Infrared Spectroscopy and

Laboratory Experiment pubs.acs.org/jchemeduc

Examination of Bond Properties through Infrared Spectroscopy and Molecular Modeling in the General Chemistry Laboratory Clifford M. Csizmar, Dee Ann Force, and Don L. Warner* Department of Chemistry and Biochemistry, Boise State University, Boise, Idaho 83725-1520, United States S Supporting Information *

ABSTRACT: A concerted effort has been made to increase the opportunities for undergraduate students to address scientific problems employing the processes used by practicing chemists. As part of this effort, an infrared (IR) spectroscopy and molecular modeling experiment was developed for the first-year general chemistry laboratory course. In the experiment, students explore the dynamic nature of the covalent bond in a hands-on, inquiry-based approach by experimenting with mass and spring systems, using molecular modeling software, and performing IR spectroscopy on a series of structurally related compounds. Students see the effect of bond order, atomic size, and molecular weight on bond strength, bond length, and vibrational frequency. As an added benefit, students are introduced to scientific instrumentation and tools that can then be expanded upon in later laboratory classes. KEYWORDS: First-Year Undergraduate/General, Laboratory Instruction, Physical Chemistry, Hands-On Learning/Manipulatives, Inquiry-Based/Discovery Learning, Covalent Bonding, IR Spectroscopy, Lewis Structures, Microscale Lab, Molecular Modeling

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properties (e.g., bond order, atom mass, and resonance) that, when brought together at the end of the laboratory session, expose the students to a more comprehensive view of bond properties. Thus, students come to realize the relationship between vibrational frequency and bond length, strength, and order: topics covered near the middle of a year-long general chemistry sequence.

ncreased instrumentation use is desired in general chemistry laboratory experiments. The impetus for the development of this type of experiment is due, in part, to the most recent ACS Guidelines for undergraduate chemistry programs, which state that a student graduating with an ACS-certified degree should be able to “define a problem clearly, develop testable hypotheses, design and execute experiments, analyze data, and draw appropriate conclusions”.1 Arguably, a practicing chemist must be intimately familiar with the use and application of scientific instrumentation to accomplish these things. Therefore, a new experiment has been developed that relies upon infrared (IR) spectroscopy to display the dynamic nature of the chemical bond. Students are guided toward discovering the relationship between bond strength, bond order, and vibrational frequency through a series of exercises that include observing masses oscillating on springs, modeling bond stretching computationally, and obtaining IR spectra of several related compounds. Although several previously published general chemistry experiments use IR spectroscopy as an educational component,2−18 only a few examine bond properties.14−18 Of these, one is somewhat similar to the laboratory described herein in that IR spectra are obtained and Hooke’s law is demonstrated via masses and springs.14 However, this experiment extends the learning as students build and conduct molecular modeling experiments on their own molecules in a computational chemistry program, animate the molecular vibrations of their compounds, and generate a theoretical IR spectrum for them. Thus, concepts are reinforced from both computational and experimental perspectives. Additionally, this experiment utilizes several compound sets with broad physical © 2012 American Chemical Society and Division of Chemical Education, Inc.

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ORGANIZATION OF LABORATORY SECTIONS The general chemistry laboratory sections consist of 24 students and meet for two hours and fifty minutes. For this experiment, each section is divided into eight groups of three students who work together for the entire experiment. The section is given access to at least one PerkinElmer IR spectrometer equipped with a diamond attenuated total reflectance (ATR) device (although availability of two spectrometers is preferable), eight spring and mass sets, and at least eight computers equipped with the Spartan computational chemistry software (Wavefunction, Inc.) used for the molecular modeling activities. A projector and laptop computer are placed in the laboratory to collect and display the students’ data.

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LABORATORY EXPERIMENT This experiment centers on the fundamental idea of bond strength and its relationship to bond length and bond order. During the experiment, students draw Lewis structures, operate an IR spectrometer, and computationally model each of their Published: January 13, 2012 379

dx.doi.org/10.1021/ed200100n | J. Chem. Educ. 2012, 89, 379−382

Journal of Chemical Education

Laboratory Experiment

Table 1. Example of Compound Sets and Student Data Entered into a Class Data Tablea

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A total of 8 sets of compounds were used in this experiment. The complete list of all compounds (and sets) is included in the Supporting Information. bRefers to the bond of interest for the given molecule. cStudents enter their data into these columns. dStudents are given the Lewis structures for these compounds to serve as a starting point for the remaining two compunds in each set. eStudents are given these values to serve as a frame of reference for the remainder of the compounds.

compounds. Also, they visualize Hooke’s law by observing the effect of spring strength and increasing mass on the spring’s vibrational frequency. Together, these activities illustrate, in a direct way, how a bond’s vibrational properties relate to its strength and length. Introductory Discussion and Lewis Structures

Following introductory remarks by the instructor, each student group is given a unique set of three compounds, each containing an obvious “trend” with regard to the type of bonding present in the molecule. Two example sets of compounds are shown in Table 1. Students draw a Lewis structure for each compound and identify a “bond of interest” (e.g., C−N, CN, CN for compound set 7) before beginning the next stage of the experiment. Hooke’s Law and Bond Vibrations

Figure 1. IR spectra from compound set 7 (stacked display), obtained on a PerkinElmer IR spectrometer with an ATR device. The indicated peaks correlate to the bond of interest for each specific molecule, and the trend is indicative of what students are expected to observe. In this set, the trend elucidates the effect of increasing bond order in the C− N bond(s). aDiisopropylethylamine.

Students hang three masses (50, 70, and 100 g) on each of three springs of increasing strength, which generates nine unique spring−mass combinations. They allow the mass to oscillate on the spring and observe the number of oscillations during a brief time interval. By comparing the various frequencies, students are able to qualitatively observe Hooke’s law and note the effects of both the spring constant (which is likened to bond strength) and mass (representative of molar mass) on oscillation.19

only the most significant changes among them. The process is illustrated using the three compounds and spectra in Figure 1. In this instance, students are informed that the band at 1207 cm−1 is due to the stretching of a C−N single bond in diisopropylethylamine (DIPEA). Using this as a frame of reference, students are easily able to determine that the most obvious new bands in the remaining two spectra are due to the C−N double and triple bonds of 2-methyl-1-pyrroline and acetonitrile, respectively. Once the proper correlations have been made, the absorption frequencies are recorded in the Class Data Table (see Table 1).

Infrared Spectroscopy

Students operate the IR spectrometer according to given parameters and operating procedures. The spectrometer is equipped with a diamond ATR device, allowing a large number of students to use the instrument in rapid succession. Students obtain an individual spectrum of each of their compounds, and then overlay them onto a single graph using the instrument’s software (Figure 1). They determine the position of the major bands using the “label peaks” function. After printing the spectra, they return to the laboratory bench to identify which peaks correspond to the bond of interest. These deductions can be made using their Lewis structures and the intuition gained during the spring and mass section. Additionally, students are told which one of their compounds to use as a frame of reference while examining the other spectra and to look for

Molecular Modeling

Students build a computational model of each of their compounds using Spartan, measure the length of their bond of interest, and generate a theoretical IR spectrum for their compound (see Figure 2 for an example). They then identify which peak in the spectrum corresponds to their bond of interest by using a feature of the software that animates a 380




students, thus providing them with a solid introduction to scientific tools and instrumentation that can then be expanded upon in their later courses. Overall, student responses to the experiment were generally favorable, with many expressing interest in using the IR spectrometer and the computational software. It was uncertain how students would react to the relatively complex molecular modeling exercises, and it is reassuring that this aspect of the experiment was viewed favorably. To this end, it was deemed that the material is appropriate for the general chemistry laboratory. Even though some students may be apprehensive about the content at first glance, many feel rewarded after discovering they are aptly able to successfully complete the exercises and draw meaningful conclusions from them. Students also showed appreciation for the class presentations, citing their utility in tying the individual laboratory exercises together and reinforcing the overall goals of the experiment. Additional discussion of student reaction to the experiment is included as part of the Supporting Information. Regarding the instrumentation, devices that can be incorporated into chemistry curriculum pose an additional benefit to departments considering the acquisition of new equipment. In this laboratory, the use of a spectrometer equipped with an ATR device allows throughput of many students in a rapid manner, permitting the incorporation of IR spectroscopy into general chemistry laboratories in a time slot of two hours and fifty minutes. This experiment also utilizes commercially available materials and chemicals, making it readily adoptable by the many programs that already possess an IR used by upper-division chemistry courses. Due to the success of the pilot project, we currently use this experiment as a part of our general chemistry curriculum.

Figure 2. Theoretical IR spectrum of DIPEA generated by the computational program Spartan. Notable is the carbon−nitrogen stretch at 1502 cm−1. Experimentally, this peak is located at 1207 cm−1, revealing reasonable agreement between the two methods.

specific molecular vibration and indicates its frequency on the spectrum. These values are then recorded in the Class Data Table (see Table 1). A tutorial is provided to orient them to the program and assist them in completing the exercise in a timely manner. Culminating Laboratory Activity: Student Presentations

Once all groups have filled in their portion of the Class Data Table, the document is projected onto a screen in the lab. A member of each group is chosen at random by the instructor to present their group’s data to the class. The presentations consist of students identifying how the differences among the bonds in their set bring about the trends in the experimental data that they observed. This allows all of the students to extrapolate their own findings to novel, yet conceptually related bond sets. An electronic copy of the class data is emailed to students at the end of the lab to aid in completion of a postlaboratory assignment that provides additional exposure to identifying and understanding the basis of the data trends observed during the experiments.

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ASSOCIATED CONTENT

S Supporting Information *

HAZARDS The students handle small quantities of a variety of common organic compounds when operating the IR spectrometers, thus gloves should be worn. Some of the compounds are flammable, are mild irritants, or are mildly toxic if ingested. As such, care should be taken when handling these chemicals and open flames should be prohibited. When possible, manipulation of chemicals should be conducted in a fume hood.

Laboratory procedures, instructor notes, compound set handout sheets, spectra, and student survey data. 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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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support of the NSF (DUE #0737128) for the acquisition of the IR spectrometer. The contributions of Morgan Davis, Christopher Siepert, and Autumn White are immensely appreciated.

RESULTS Students generally obtain IR stretching frequencies that are consistent with literature expectations, as indicated in Table 1 where student results for two of the eight compound sets are shown. It is evident, however, that there is some discrepancy between the theoretical frequencies produced by the computational software and the experimental frequencies obtained by the IR spectrometer. Although this offset is not atypical for IR frequencies calculated using the Spartan program,20 it is important to note that the overall trends are the same for both data sets. Guidance and direction from the instructor on this point may be needed for some students to make the connection. The Spartan-generated bond lengths are also in agreement with the expected trends.

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REFERENCES

(1) American Chemical Society Committee on Professional Training. Development of Student Skills in a Chemistry Curriculum. http:// portal.acs.org/portal/PublicWebSite/about/governance/committees/ training/acsapproved/degreeprogram/CNBP_025490 (accessed November 28, 2010). (2) Amey, R. L. J. Chem. Educ. 1992, 69, A148. (3) Garizi, N.; Macias, A.; Furch, T.; Fan, R.; Wagenknecht, P.; Singmaster, K. A. J. Chem. Educ. 2001, 78, 1665. (4) Mabbott, G. A. J. Chem. Educ. 1995, 72, 471. (5) Byrd, H.; O’Donnell, S. E. J. Chem. Educ. 2003, 80, 174. (6) Shalhoub, G. M. J. Chem. Educ. 1980, 57, 525. (7) Szalay, P. S. J. Chem. Educ. 2008, 85, 285. (8) Arrington, C. A.; Hill, J. B.; Radfar, R.; Whisnant, D. M.; Bass, C. G. J. Chem. Educ. 2008, 85, 288.

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DISCUSSION AND CONCLUSION This laboratory experiment successfully integrates infrared spectroscopy and molecular modeling into a general chemistry course in a manner that is both informative and interesting to 381




(9) Ackermann, M. N. J. Chem. Educ. 1970, 47, 69. (10) Baer, C.; Cornely, K. J. Chem. Educ. 1999, 76, 89. (11) Hess, K. R.; Smith, W. D.; Thomsen, M. W.; Yoder, C. H. J. Chem. Educ. 1995, 72, 655. (12) Hill, M. A. J. Chem. Educ. 2001, 78, 26. (13) Werner, T. C.; Hull, L. A. J. Chem. Educ. 1993, 70, 936. (14) Anderson, J. S.; Hayes, D. M.; Werner, T. C. J. Chem. Educ. 1995, 72, 653. (15) Burke, J. T. J. Chem. Educ. 1997, 74, 1213. (16) Huer, W. B.; Koubek, E. J. Chem. Educ. 1997, 74, 313. (17) Swartz, J. E.; Schladetzky, K. J. Chem. Educ. 1996, 73, 188. (18) Wiskamp, V.; Fichtner, W.; Kramb, V.; Nintschew, A.; Schneider, J. S. J. Chem. Educ. 1995, 72, 952. (19) (a) At this stage, students could be asked to calculate the spring force constant, but this was not included to focus on the IR and molecular modeling aspects of the experiment. (b) Likewise, although it is more appropriate to use a two-body reduced mass rather than atomic mass when discussing Hooke’s law, this approximation greatly simplifies the overall discussion. Instructors may choose to acknowledge the simplification to more astute students. (20) Hehre, W. J. Chapter 7: Vibrational Frequencies and Thermodynamics Quantities. A Guide to Molecular Mechanics and Quantum Mechanical Calculations; Wavefunction, Inc.: Irvine, CA, 2003; pp 253−269.

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Examination of Bond Properties through Infrared Spectroscopy and

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