In the Laboratory
Structure and Nuclear Magnetic Resonance
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An Experiment for the General Chemistry Laboratory Rosa M. Dávila* and R. K. Widener Physical Science Department, College of Southern Idaho, Twin Falls, ID 83301; *
[email protected] Nuclear magnetic resonance has made enormous contributions to the development of modern chemistry. However, most students enrolled in general chemistry never become familiar with it. It has been argued that early exposure to advanced instrumentation can greatly enhance first-year chemistry instruction (1a). Curricula are being developed that include more organic chemistry concepts and modern instrumentation in lecture and laboratory beginning with the first semester of chemistry (1b, 1c). Although some progress has been made in incorporating NMR throughout the chemistry curriculum (2), NMR is still far from being integrated into traditional first-year course work (3). Molecular structure is a major topic in general chemistry. Most experiments involving molecular structure applications are limited to the use of molecular model sets or programs. Only a few NMR experiments have been designed to complement the study of molecular structure at the freshman level (4, 5). Since isomerism is not stressed at this level, students may erroneously nurture the concept that a molecular formula corresponds to one specific substance. We have addressed these issues in a nontraditional laboratory exercise entitled “Structure and Nuclear Magnetic Resonance”. This exercise, which is part of a project to integrate NMR across the chemistry curriculum, utilizes an Anasazi EFT-60 nuclear magnetic resonance spectrometer. With this instrument students can learn to obtain both 1H and 13C spectra in a short period of time with minimal supervision. The lab exercise described here is part of a chemistry course designed for science and engineering majors. Its main goals are to expose students to NMR as a valuable technique in chemistry and to help them develop problem-solving skills as they work on simple structure-determination problems. Through this experiment students are introduced to the ideas of functional groups and isomerism, and they apply what they have learned in lecture about molecular structure and bonding in a concrete and innovative way. The Experiment Before coming to lab, students receive a handout containing background information for the experiment, procedure, and prelab and postlab questions.W In a prelab assignment students visit a Web site developed by the instructor that displays and explains the major components of the EFT-60 instrument (http://www.csi.edu/ip/physci/faculty/rex/NMRMain.HTM). This gives them a concrete image of what they will use in lab. Students turn in answers to prelab questions before lab discussion. The experiment is divided into two lab sessions. On the first day students are introduced to basic NMR theory and the second day they work on data collection and analysis. They have already studied quantum numbers in lecture, so
it is relatively easy to incorporate the phenomena of nuclear spin and magnetic resonance. We then introduce the concept of functional groups and show how they can be used as identifying tags for molecules. Functional groups are limited to simple oxygen-containing groups as in alcohols, ketones, aldehydes, and carboxylic acids. The compounds chosen have very specific signatures in 1H and 13C, which make them easy to identify. At this point we move on to the interpretation of NMR spectra. Students are given sample spectra of progressively more complex molecules. We present the ideas of distinct chemical environments and chemical shift followed by a discussion of proton–proton coupling (n + 1 rule) and integration. We also provide a table of typical 1H and 13C chemical shifts for the oxygen-containing functional groups discussed earlier. The discussion describes how integration is used to count the different types of protons and how coupling shows interactions between protons. This information is valuable in reinforcing the concepts of molecular structure and narrows down the identification of unknowns later in the experiment. So far, the NMR applications we have found in the literature at this level are limited to identifying only the different proton environments (4 ).
Isomer Exercise During the first day of the experiment students are divided into small groups. Each group uses a molecular model set to construct the three possible isomers of C3H8O. Students apply what they learned in lecture about Lewis structures, the octet rule, etc. to build the models. The three possible isomers of C3H8O are 1-propanol, 2-propanol, and ethyl methyl ether. Once they have built the models, students create a table showing the number of distinct carbon and proton environments they expect to observe in the 1H and 13C spectra of these compounds. They are then given two sets of 1H and 13 C spectra. Their goal is to identify which set belongs to which isomer. Figures 1 and 2 show sample 1H spectra for 1-propanol and 2-propanol, respectively, recorded from the Anasazi EFT-60 instrument. Through molecular modeling students make concrete connections between structure and the data observed in the NMR spectra. This exercise prepares them for the second-day activities: obtaining the 1H and 13C spectra of an unknown sample and identifying this unknown. Use of the Instrument The use of the Anasazi EFT-60 instrument is relatively easy. While the EFT-60 provides an option for automatic processing of the data, we choose to engage the students in manual data handling. They process their data immediately after running the spectrum. After short training, they are able to phase their spectra, fit the baseline, and integrate their 1H
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In the Laboratory
Data Analysis Questions The following questions will help you analyze your data. Please be aware that for aldehydes, ketones and carboxylic acids an extra carbon is attached as part of the functional group. 1. How many distinct hydrogen environments can you identify in the 1H NMR? 2. How many different types of carbons can you identify in the 13C NMR? 3. Are there any functional groups that might be identified by their distinct chemical shift? Figure 1. 1H NMR spectrum of 1-propanol (60 MHz).
4. Is there any evidence of coupling between hydrogens? In other words, is there any evidence of hydrogens on one environment “seeing” another hydrogen in another environment? 5. What information can you obtain from the integrals in your 1H NMR spectrum?
Conclusions
Figure 2. 1H NMR spectrum of 2-propanol (60 MHz).
spectra. Supplemental written instructions are provided. Neat unknown samples are prepared in advance to guarantee efficient data collection. During the first lab session, while the rest of the class is working on the isomer exercise, small groups of students are instructed in the operation of the instrument. The instructor shows the components of the instrument, demonstrates proper sample handling and insertion, and runs a sample spectrum following the same steps given in the lab handout. This familiarization process enables the students to obtain spectra on their unknowns faster and more smoothly on the second day of the experiment.
Data Analysis To aid students in the correct identification of their unknowns we provide a set of questions to guide them through their data analysis. These questions are shown in the box. Since unknowns are limited to methyl and ethyl compounds containing oxygen functional groups, most students are able to identify their unknowns without difficulty. Hazards Samples used in this experiment all contain relatively volatile organic compounds. Sample tubes are capped and should remain capped throughout the experiment. Vapors of organic solvents can be dangerous upon prolonged exposure.
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The Structure and Nuclear Magnetic Resonance experiment described here meets the objectives we set out to achieve. Students are exposed to a valuable research tool early in their academic careers. The simplicity and efficiency of the EFT-60 NMR instrument permit meaningful hands-on experience for all students in the class. Direct connections are made between structural concepts learned in lecture and actual structure characterization by NMR. Qualitative evidence of student success in achieving these objectives was gathered through lab reports, final lab exam NMR questions, and successful unknown identification. One unanticipated finding is that the brief introduction to simple organic nomenclature needed for this exercise translates well into the regular lecture. The instructor can refer to compounds and identify them by their functional groups. Even with traditional stoichiometry problems from the textbook, students can associate compounds used as examples with those studied in the lab. A survey was created to receive student feedback.W Questions were geared toward showing which stages of the experiment and instrument usage were easier or more difficult. Over 90% of the students found the experiment interesting and easy to understand, and 89% found it easy to obtain the spectra. Students thought that the discussion on NMR theory was the hardest part of the experiment and using the instrument was the easiest. Phasing and integration were the most difficult steps in processing the spectra. New NMR experiments are being developed for the second semester of the freshman chemistry laboratory, which is primarily inorganic qualitative analysis. The experiments will use chelating ligands that provide different coordination geometries when complexed with appropriate transition metal ions. The goal is to have students distinguish the different coordination geometries using NMR. Since these coordination complexes are expected to have different colors, it should be possible to apply other techniques such as column chromatography and UV–vis spectroscopy in conjunction with the NMR experiment.
Journal of Chemical Education • Vol. 79 No. 8 August 2002 • JChemEd.chem.wisc.edu
In the Laboratory
Literature Cited
Acknowledgments We thank The Camille and Henr y Dreyfus Foundation, Inc., Special Grant Program in the Chemical Sciences for a generous award (Enhancing the Chemical Sciences at the Two-year College: Integrating NMR Across the Curriculum SG-00-64) and the College of Southern Idaho and the CSI Foundation, Inc., for additional funding. W
Supplemental Material
A student handout and post-experiment survey are available in this issue of JCE Online.
1. (a) Steehler, J. K. J. Chem. Educ. 1998, 75, 274–275. (b) Rettich, T. R.; Bailey, D. N.; Frank, F. J.; Frick, J. A. J. Chem. Educ. 1996, 73, 638. (c) Rettich, T. R. J. Chem. Educ. 1995, 72, 535. 2. Davis, D. S.; Moore, D. E. J. Chem. Educ. 1999, 76, 1617–1618. 3. Brief exposure to NMR applications are now included in some first-year texts. Ebbing, D. D.; Gammon, S. D. General Chemistry, 6th ed.; Houghton Mifflin: Boston, 1999; pp 313–314. Brown, T. V.; Lemay, H. E. Jr.; Bursten, B. E. Chemistry: The Central Science, 8th ed.; Prentice Hall: Upper Saddle River, NJ, 2000; p 210. 4. Baer, C.; Cornely, K. J. Chem. Educ. 1999, 76, 89–90. 5. Parmentier L. E.; Lisensky, G. C.; Spencer, B. J. Chem. Educ. 1998, 75, 470–471.
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