Supporting the Teaching of Infrared Spectroscopy ... - ACS Publications

Laboratory Experiment pubs.acs.org/jchemeduc

Cite This: J. Chem. Educ. XXXX, XXX, XXX−XXX

Supporting the Teaching of Infrared Spectroscopy Concepts Using a Physical Model Lyniesha C. Wright* and Maria T. Oliver-Hoyo Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695, United States

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S Supporting Information *

ABSTRACT: A physical model has been designed to help students visualize concepts involved when solving infrared spectra. The physical model uses balls and springs to incorporate the harmonic oscillator model and Hooke’s law to study dynamic vibrations within diatomic molecules. Various concepts are addressed with the model to abstract principles about how bonds interact with infrared light as well as how reduced mass, bond order, electronegativity, bond dipole, and bond polarity influence peak position and peak intensity in a spectrum. The model has a corresponding activity developed to maximize applicable heuristics and deter the need for memorization based on functional groups or surface features. The model has been thoroughly tested in organic chemistry laboratories where students have successfully used the concepts to justify peak position and explain peak intensity in infrared spectra. KEYWORDS: Second-Year Undergraduate, Organic Chemistry, Hands-On Learning/Manipulatives, IR Spectroscopy, Laboratory Equipment/Apparatus

O

stretching peaks in IR spectra has made them attractive to use when studying molecular concepts. For example, students successfully predicted relative carbonyl stretching frequencies based on resonance, hydrogen bonding, and the inductive effect.12 Inquiry-guided instruction also aided in drawing resonance structures and predicting how resonance impacts the strength of the carbonyl bond.13 Instructors have also taken advantage of visualization tools such as Spartan, MOLVIB, and Gaussian 03W to depict animated vibrations for different types of bonds as a vehicle to reduce memorization.14−16 Using Gaussian 03W students were able to interactively compare the complementarities of IR and Raman spectra by viewing the corresponding bond oscillation of the molecule.15 The nature of IR spectroscopy lends itself to the incorporation of Hooke’s law in instructional resources. As molecules vibrate with the absorption of infrared light, the energy transmitted for a particular vibration is quantified when a change in dipole occurs. The wavelength that is transmitted is contingent upon the strength of the bond and the mass of the connected atoms. Hooke’s law as it relates to covalent bonds can be described with eq 1:

rganic chemists routinely select and utilize appropriate scientific instrumentation for characterization. Chemistry educators have explored the use of infrared spectroscopy in instructional laboratories to reflect the organic chemist’s research experience.1−3 The study of infrared spectroscopy in the undergraduate chemistry curriculum is traditionally structured for the identification of functional groups.2,4,5 Students are often introduced to infrared spectroscopy by first discussing wave theory and the electromagnetic spectrum, followed by analyzing spectra to compare to IR correlation tables. This process has led students to peak picking and memorizing wave numbers during structure elucidation.5 Memorization, without a focus on comparing and analyzing molecular structures, has prevented learners from constructing interconnected and long-term chemical knowledge.6 For example, studies have shown a prevalence of undergraduate students detaching bonding from its dynamic nature and seeing molecules as a collection of distinct atoms acting independently.4,7 Novice students are also likely to interpret a molecule by assuming additive properties of the atoms instead of emergent properties.8,9 Students tend to lack the ability to discern and identify domain-specific heuristics,6,10,11 which leads them to interpret “more peaks mean more fragments” or apply irrelevant heuristics like “the size of the atom is related to the size of the peak” when analyzing IR data.4 In order to make IR spectroscopy instruction more meaningful, inquiry-based approaches have been used to guide students to analyze spectra and find trends prior to referencing absorption tables.5,12 The prominence of carbonyl © XXXX American Chemical Society and Division of Chemical Education, Inc.

ν=

1 2π

κ μ

(1)

Received: November 13, 2018 Revised: March 5, 2019

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DOI: 10.1021/acs.jchemed.8b00805 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Laboratory Experiment

Figure 1. Model includes (a) a triatomic arrangement to discuss stretching and bending, (b) magnets incorporated to increase mass and reflect dipole considerations, and (c) the system setup.

where ν is the vibrational frequency (s−1) directly related to wavenumber, μ is the atom’s reduced mass (kg), and κ is the force constant (N m−1). Instructional activities have been designed using this equation to calculate the force constant of carbon−hydrogen bonds, predict wavenumbers of carbon− deuterium or carbon−chlorine bonds,17,18 and compare force constants of several bonds.19 Hooke’s law has also been the instructional framework to examine bond properties by using various springs, masses, and timers to observe the relationships between mass, bond strength, and frequency. For example, in an activity designed by Csizmar and colleagues, students were assigned a trend (i.e., bond order) to observe how changes in covalent bonds influence the wavenumber required for vibration.14 Borgsmiller and colleagues designed a spring system using multiple springs to represent bond order15 while Anderson et al. overlaid spectra to compare how bond order and resonance influence peak position and how hydrogen bonding influences peak width.16 Missing in these instructional activities is a discussion of peak intensity in IR spectra. Only bonds with a change in dipole will give a peak, resulting in an intense peak for polar bonds. In addition, the abundance of a bond causes an increase in peak intensity because of the ability of those bonds to absorb more light. We describe herein a lab activity that allows students to explore the factors affecting peak position and intensity in an IR spectrum by using hollow plastic balls with holes (referred to throughout by a common brand name, wiffle balls) and springs. Magnets are incorporated to target electronic effects. Additionally, the oscillatory nature of vibrating atoms and bonds is incorporated as students manipulate the model, allowing the balls to experience the restorative force that is proportional to their displacement. Our physical model couples the harmonic oscillator model with Hooke’s law for students to study how IR spectroscopy is used to understand properties between diatomic atoms such as dipole, polarity, mass, and bond order. The use of the model and corresponding lab activity were evaluated with respect to the following questions:

2. What difficulties do students encounter when manipulating the model? 3. How well did the activity facilitate the incorporation of the principles gathered to analyze spectra? The activity was tested in three cycles; each cycle incorporated modifications to address findings to the above questions. We provide here the product that is currently used in our laboratories.

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MODEL CONSTRUCTION Hooke’s law and the simple harmonic oscillator model were used in the design where atoms and bonds are represented by wiffle balls (atoms) and springs (bonds). The harmonic oscillator model of molecular vibrations makes the assumptions that when a diatomic molecule vibrates two atoms move in phase with one another20 and the bond’s mass is neglected.21 The reduced mass in eq 1 has an inverse relationship with vibrational frequency and can be calculated using eq 2 below: μ=

MA × MB MA + MB

(2)

Here, MA and MB are the masses of balls A and B (atoms A and B), respectively. The physical model is constructed to explicitly demonstrate how various concepts influence peak position and intensity. Showing atoms as spheres is a common representation in chemistry, while using springs as bonds emphasizes dynamic motion and strength. The model is assembled by connecting wiffle balls to spring coils using small hooks. The hooks are large enough to allow for multiple coils to fit into the eye of the hook to demonstrate an uneven distribution of electron density. Yellow spheres are lighter than blue spheres for students to explore reduced mass effects. Magnets are hidden inside the spheres to increase mass and are also used to demonstrate attraction or repulsion of atoms as spheres have either a south pole or north pole (Figure 1b). The model uses a thin spring as a single bond and a thicker spring as a double bond. The model can be shown as triatomic, represented by three atoms connected in a bent formation by two springs (Figure 1a). The triatomic model is manipulated to show different

1. Can the activity be incorporated into an IR laboratory as a complementary module in the study of IR spectra? B




Figure 2. Manipulation of the diatomic model from a nonpolar bond to a polar bond (a) by increasing the number of coils in one sphere. The magnetic bar between the spheres attracts them to demonstrate how infrared light causes stretches and compressions with bond dipoles (b).

masses of both atoms through reduced mass. Students calculate the reduced mass of the spheres, stretch the spheres (as shown in Figure 1c), and record the number of vibrations. They repeat with heavier spheres and observe how reduced mass influences vibrational frequency. Students then label the stretching peaks of chloroform, predict the position of the carbon−deuterium stretch in a spectrum of chloroform-d, and are asked to justify all answers.

asymmetric, symmetric, or bending deformations. A diatomic model is used to demonstrate how mass, polarity, and bond strength are reflected on a spectrum. A magnetic bar is used to represent the electronic vector of infrared light. The bar has a north and south pole, labeled positive and negative, like the alternating charge of an electric vector. The entire system is comprised of the modified wiffle balls, springs, rod, and magnetic bar (Figure 1c).

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Peak Position: Polarity

ACTIVITY DESIGN The activity was designed for students to predict and justify the location and intensity of significant peaks on an IR spectrum without the aid of an IR absorption table, on the basis of the following principles: 1. Different types of vibrations require different amounts of energy. 2. Bonded atoms with smaller reduced masses vibrate at higher frequencies. 3. The stronger the bond, the greater the vibrational frequency required to stretch it. 4. The more polar the bond, the larger the vibrational frequency. 5. The greater the change in the dipole, the greater the peak intensity. For our laboratory setting, a 15 min prelab lecture includes a brief discussion of IR theory, a demonstration of different vibrational deformations and spectral axes, and a demonstration on how to assemble the model. Students then perform this activity as part of their IR laboratory where they also take the IR spectrum of an unknown. The activity has five sections, in which one section is used to discuss each molecular concept and its influence on peak intensity or position.

The transition from reduced mass to polarity elicits a discrepant event. Students first calculate reduced mass of carbon−hydrogen, oxygen−hydrogen, and nitrogen−hydrogen bonds. Following, they predict the order of the peaks only on the basis of reduced mass. Students then compare their predictions to the partial spectra of methyl cyclohexane, cyclohexylamine, and cyclohexanol and observe the opposite pattern. Given that a polar bond has an unevenly distributed electron density, students adjust the model (Figure 2a). The sphere on the right represents the most electronegative atom because it has the greatest electron density or number of coils. Upon stretching and releasing the model, students realize it takes more energy to vibrate a more polar bond, resulting in a higher wavenumber. Peak Position: Bond Order

Controlling mass and polarity, while varying spring strength, allows students to analyze the effect of bond order. The thin spring system is stretched, and when repeated with a thicker spring, students observe it takes more energy to vibrate a double bond than a single bond. The spectrum of pentane is overlaid and compared to the spectra of 1-pentene and 1pentyne. Students predict and justify the carbon−carbon change in bonding for each molecule. Peak Intensity: Abundance of Specific Bonds

Peak Position: Types of Vibrations

Students are guided to compare the carbon−hydrogen bonds in piperidine with the number of nitrogen−hydrogen bonds. The greater the abundance of a particular bond type, the more light is absorbed, and the more intense the peak.

Students use the triatomic model (Figure 1a) to replicate bending and asymmetric and symmetric stretching. Students feel the increased difficulty in doing an asymmetric stretch over a symmetric stretch and associate an asymmetric stretch with increased energy and wavenumber. They apply their observations to a spectrum of cyclohexane and label the carbon−hydrogen bending and stretching peaks.

Peak Intensity: Dipole

The magnets in the blue spheres (Figure 2b) have opposite poles attracting one another, therefore, resembling partial positive and negative atoms. A bar magnet represents the electric vector of light and is placed between the balls. For spheres with magnets, when the bar is placed between them, the spheres will either slightly stretch or compress. Students

Peak Position: Reduced Mass

Vibrations induced by IR light are at the molecular level, not the atomic level. Therefore, it is pertinent to compare the C




Figure 3. Percentage of students who successfully implemented the model according to each evaluation criterion.

Figure 4. On the left is an example of the instructions for observing reduced mass effects using the model. The right shows a student’s response and explanation about how reduced mass influences spectra. Spectra are obtained from the SDBS.22

To evaluate the feasibility of incorporating the activity into an existing IR organic laboratory, it was tested four times with over 100 first semester organic chemistry students. The students signed consent forms according to our IRB guidelines. The length of the activity was reduced by adjusting the sequence and number of examples students explored. Additionally, students take an IR spectrum of an unknown and use these principles to assist in their identification. The full activity can be found in the Supporting Information. The entire lab takes an average of 1 h 45 min.

observe an interaction only with a difference in charge present. The students look at the y-axis of the spectral data to further support the explanation about how a dipole influences peak intensity. Synthesis Questions

The activity finishes with two sets of synthesis questions requiring students to interpret spectra and predict spectral differences only on the basis of the structure. IR absorption tables are not available to answer these questions. Since the activity guides students to first focus on peak position and then to use abundance and dipole to justify their answers, it is expected students will use the appropriate concepts when answering these questions.

(2) What difficulties do students encounter when completing the activity?

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Model implementation was evaluated according to model usage, principles brought up by students, and spectral analysis. Model Usage pertains to the usability of the physical model when answering questions. Principles Obtained considers when students described the correct trend. Spectral Analysis accounts for when students accurately applied the principle to a

EVALUATION The student use of the model and the corresponding activity were evaluated to address the questions below. (1) Can the activity be incorporated into an IR laboratory as a complementary module in the study of IR spectra? D




weak for the carbon−bromine bond in 2-bromopentane and the carbon−nitrogen bond in ethanamine. The polarity principle was understood, but to those students neither a carbon−nitrogen nor a carbon−bromine bond is polar or has a dipole. Since students had to justify answers, the activity provides a way to identify and address concepts that students misunderstand. The second set of synthesis questions required students to accurately label three prominent peaks in a spectrum of ethyl propionate on the basis of the principles studied during the lab activity, and to justify the intensity of those peaks. Student performance is shown in Figure 6. Over 90% of the participants successfully identified the principles in the first four questions. However, applying the principles and justifying the intensity was more difficult for 50% of the students. As seen in Table 2, most students who incorrectly justified the intensity of carbon−hydrogen peaks based their justification on reduced mass. Errors occurred at very low percentages in both sets of synthesis questions except for the justification of intensity based on reduced mass, which spiked at 22.5%. Overall, we observed that students were successful at identifying the peaks even when IR absorption tables were not provided.

spectrum. The data from 40 participants enrolled in organic chemistry laboratories is summarized in Figure 3. Model Usage was evaluated on the basis of the students’ ability to independently complete the instructions involved when using the model and answering activity questions. As shown in Figure 3, almost 100% of students successfully manipulated the model when documenting their observations regarding reduced mass, bond order, and bond polarity. The use of the model was not as obvious for dipole effects on spectra. Some students had difficulty observing the subtle interactions with the magnetic bar. However, over 75% of students successfully used the model to discuss dipole effects. The other two evaluation criteria are interconnected. As an example, on the left of Figure 4 are the instructions for observing reduced mass effects. The image on the top right is a summary of the trend the students were expected to identify. This student successfully depicted the trend behind the principle (Principles). Then spectra were shown for students to label peaks. If students could accurately label specified peaks, then they were considered successful in applying it to spectra (Spectral Analysis). The bottom right shows a student’s response when applying the reduced mass effect to a spectrum of chloroform-d. If students completed the steps and obtained the targeted observations on each question, then the model was considered useful to explore that concept. The simple, yet powerful, model required no major adjustments. In every case, at least 90% of students grasped the principle when using the model, and over 80% used it correctly to analyze spectra. (3) How well did the activity facilitate the incorporation of the principles when analyzing IR spectra? The third evaluation criterion related to the incorporation of principles by students when analyzing spectra. This data was taken from the final synthesis questions as these required students to incorporate the concepts and analyze multiple variables when identifying and justifying peaks. The first set of synthesis questions provided students with values for position and intensity, and had them determine if the values were approximately correct or incorrect by focusing on molecular structure (Box 1).

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LIMITATIONS While the model assists students in visualizing why and how vibrations occur when interacting with light, it is limited to diatomic molecules. Therefore, some aspects of bending or asymmetric stretching interactions (i.e., the induced dipole only present in the CO2 asymmetric stretch) cannot be explored with this model. The model does not address how intermolecular forces affect peak width. However, these topics can be expanded upon in lecture having used the model as a foundation.

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HAZARDS There are no hazards associated with undertaking this laboratory activity as no chemicals are used in the completion of this lab.

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CONCLUSION A lab activity was designed to guide students in the exploration of infrared spectroscopy without using IR absorption tables. The model was incorporated to promote a discussion of the molecular properties that manifested in IR spectroscopy by targeting how reduced mass, polarity, and bond order influence peak position and how bond abundance and dipole influence peak intensity. The model is constructed using readily available low cost materials and incorporates the harmonic oscillator model and Hooke’s law to study dynamic vibrations between atoms. The model was thoroughly tested, and students were able to manipulate the model as designed. This lab activity was evaluated to (a) identify students’ difficulties and (b) demonstrate students’ successful incorporation of the principles studied. Although students did not always use the principles correctly, over 90% of the students were able to abstract the principles under investigation (Figure 3, principles). In addition, over 68% of students were able to determine an approximate value for peak position and intensity and justify their answers on the basis of the concepts studied (Figure 5). Students struggled with the intensity of IR peaks, especially the intensity of carbon−hydrogen bonds, as only

Student performance is shown in Figure 5. Since students were prompted to justify their answers, a distinction was made between correct answers and justifications, correct answers without an explanation, correct answers with an incorrect justification, and incorrect answers. A majority of the students were able to correctly incorporate the principles and justify their responses as shown in Figure 5. The questions uncovered difficulties students had with understanding concepts, and these are summarized in Table 1. For example, a few students expected the intensity to be E




Figure 5. Results from the first set of synthesis questions.

Table 1. Errors Found in Synthesis Question 1 Question

Errors

Position 1

The lower the abundance of bonds, the lower the wavenumber Ignored reduced mass Does not consider a carbon−bromine bond polar Looked at nitrogen−hydrogen bond instead of carbon−nitrogen bond Ignored reduced mass Single bonds have weak intensities Single bonds have weak intensities Intense peaks are only a result of an abundance of peaks Single bonds have weak intensities Intense peaks are only a result of an abundance of peaks Carbon−bromine bond does not have a dipole Reduced mass influences intensity Carbon−nitrogen bond does not have a dipole Does not consider a carbon−nitrogen bond polar

Position 2

Position 3

Intensity 1

Intensity 2

Intensity 3

Table 2. Errors Found in Synthesis Question 2

Incorrect Responses,a% (N = 40)

Concept Bond types Reduced mass Bond order

2.5 7.5 2.5

Polarity

12.5

Intensity at 2985 cm−1

10.0 2.5 2.5 2.5


2.5 2.5 5.0 7.5 5.0


7.5

a

On the basis of the sample size, 2.5% represents one participant.

50% could correctly label peaks and justify intensity. This suggests that instruction should reinforce the concepts of abundance, polarity, and dipole and how they manifest in IR spectra for students to have a better understanding of peak

Incorrect Responses, % (N = 40)

Errors NA Can be used to assign every peak Double bond requires a lower energy wavenumber Bond order plays no roll More polar requires a lower energy wavenumber Ignored reduced mass

NA 5.0 5.0

C−H has a large dipole Reduced mass influences intensity Labeled it as C−C based on reduced mass Double bond requires a lower energy wavenumber Did not distinguish between C O and C−O Ignored reduced mass Bond order determines intensity Double bond requires a lower energy wavenumber Bond order determines intensity Labeled as C−C bond Ignored reduced mass

10.0 22.5 2.5

5.0 5.0 7.5

2.5 5.0 7.5 10.0 2.5 5.0 5.0 7.5

intensity. We have shown herein that this lab activity facilitates the study of IR spectroscopy without the reliance on memorizing values or using IR absorption tables.

Figure 6. Results from the second set of synthesis questions requiring students to accurately label the three prominent peaks on the basis of the principles studied in lab. F



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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.8b00805.

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IR spectroscopy laboratory materials (PDF, DOC)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Lyniesha C. Wright: 0000-0001-8126-0872 Maria T. Oliver-Hoyo: 0000-0003-3542-4930 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the organic chemistry lab director, Maria GallardoWilliams, for her assistance and feedback. We also thank the students who participated.

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Supporting the Teaching of Infrared Spectroscopy ... - ACS Publications

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