Communication pubs.acs.org/jchemeduc
NMR Interpretation: Getting from Spectrum to Structure Alison B. Flynn* Department of Chemistry, University of Ottawa, 10 Marie Curie, Ottawa, Ontario K1N 6N5, Canada S Supporting Information *
ABSTRACT: A tactile problem-solving strategy is described that helped students analyze spectral data, primarily from 1H NMR spectra, to determine the structures of unknown compounds. Two important elements in that process were (i) students organized their analysis in a table and (ii) they drew each molecular fragment, deduced from their analysis, on a separate sticky note. Possible combinations of the fragments were analyzed by arranging the sticky notes in different ways, comparing back to the given data to determine the most plausible structure of the unknown compound. As part of the structure determination lesson in a large class, a document camera served to project the sticky notes to the screen and student involvement was encouraged.
KEYWORDS: First-Year Undergraduate/General, Second-Year Undergraduate, Organic Chemistry, Hands-On Learning/Manipulatives, Problem Solving Decision Making, IR Spectroscopy, Molecular Properties/Structure, NMR Spectroscopy, Student-Centered Learning
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any resources exist for students to learn the basics of proton nuclear magnetic resonance (NMR) spectroscopy, including textbooks,1−10 Web sites,11−16 and online homework programs.17,18 When trying to determine the structure of an unknown compound, students generally become proficient at identifying molecular fragments. However, they often have difficulty assembling those fragments19 and few resources guide students through this final phase of problem-solving.1 The goal of this work was to help students develop an explicit problemsolving process (Figure 1).
the sticky notes in a way that respected the octet rule (e.g., Figure 5). The students could evaluate each possible structure by comparing back to the given data to decide whether it was plausible. Because there would only be two signals in the proton NMR spectrum of the structure in Figure 5A and not four as in the actual spectrum (Figure 2), this structure could be eliminated as a possibility. A subsequent attempt might generate the structure shown in Figure 5B, which fit all the given data and was thus the most likely structure of the unknown compound.
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EXECUTION IN LECTURE The process described above was easily demonstrated in a 420student lecture class using a document camera;23 alternatively, an overhead projector and pieces of acetate film or other methods could easily be used. The students contributed their own ideas and attempts for analysis by the full group. Students also voted on the various possible structures using a classroom response system.24 The example took 20 min of class time. A variation on the sticky note method was previously reported by McClusky, in which prefabricated “mosaic” pieces were connected to generate the final structure.25,26 There are many advantages of using sticky notes instead: 1. Students write the fragments on the sticky notes themselves, and so they can create any type of fragment, including unusual or complex fragments such as epoxides and other rings. 2. The cost is minimal. 3. Sticky notes are practical in large or small classes, at home, or on exams.
EXAMPLE OF THE METHOD In a large lecture class, students were asked to determine the structure of an unknown with molecular formula C6H10O3 using IR data (strong absorptions at 1723 and 1745 cm−1) and the proton NMR spectrum (Figure 2). The students were asked to organize their ideas in a table, such as the one shown in Figure 3. The students were encouraged to write all of their ideas in the “Comments/Ideas” column.21 This table helped students organize their analysis and made it much easier for the instructor to mark the students’ work. To help students assemble the fragments to form the final structure, they were encouraged to write each fragment that they had identified on a separate piece of paper or sticky note (Figure 4). They then compared their fragments to the molecular formula and to the degrees of unsaturation (DU) to identify any missing functional groups or atoms. In the current example, they would be able to identify a missing oxygen atom and add it as a fragment. Next, the students labeled the fragments so that they could easily be compared back to the spectrum, and then they arranged © XXXX American Chemical Society and Division of Chemical Education, Inc.
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dx.doi.org/10.1021/ed3000974 | J. Chem. Educ. XXXX, XXX, XXX−XXX
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Figure 4. Use of sticky notes to visualize all the molecular fragments.22.
Figure 5. Using sticky notes to assemble two possible structures, (A) and (B).
Figure 1. An analysis process for identifying an unknown compound (using 1H NMR data unless stated otherwise).
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DISCUSSION Students responded very positively to this strategy. Using the table (Figure 3), exam answers progressed from being disorganized messes to well-organized analyses. Many students brought sticky notes to class or to group study sessions. For exams, blank paper was provided and some students used their molecular models for the same purpose. From an instructor’s point of view, filling out the table in class was a way of keeping the analysis organized. Including the table on exams allowed students to organize their thoughts and made it easier to award marks for their problem-solving process. Being able to physically rearrange the molecular fragments using the sticky notes has made it easier to demonstrate and analyze various possible final structures. Future work will assess the impact that this method has had on students’ exam results by comparing the exam results on NMR questions before and after the method was introduced and also by determining how many students are using the method.
Figure 2. 1H NMR spectrum of an unknown compound with molecular formula C6H10O3.20.
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ASSOCIATED CONTENT
S Supporting Information *
An additional possible (incorrect) structure for the example described above, as well as an additional example of the use of the analysis table and sticky notes for a more complex example. This material is available via the Internet at http://pubs.acs.org.
Figure 3. Example of the analysis table used for structure determination. DU is degrees of unsaturation; IR is infrared.
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4. No specifically dedicated class time is required for students to learn to use sticky notes. 5. Sticky notes are usable with many presentation formats
AUTHOR INFORMATION
Corresponding Author
*E-mail: alison.fl
[email protected].
(overhead projector, document camera, tablet, flipchart,
Notes
The authors declare no competing financial interest.
etc.). B
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REFERENCES
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