NMR Spectroscopy in the Undergraduate Curriculum

Organizers: David Soulsby, Department of Chemistry, University of Redlands, 1200 East Colton Ave, P.O. Box 3080, Redlands, CA 92374, (909) 748-8546, Email: david_soulsby@redlands.edu; and Tony S. Wallner, Department of Physical Sciences, Barry University, 11300 NE 2nd Ave., Miami Shores, FL 33161, tel: (305) 899-3433, Email: twallner@mail.barry.edu

NMR is an increasingly important component of undergraduate education. As more programs obtain instruments, they work to provide the students with the highest quality experience and exposure to this modern instrumentation. This symposium is one way for faculty members to exchange ideas about best practices and successes for incorporating NMR into various aspects of the curriculum. This highly attended session elicited a great deal of conversation during the presentations which continued during the scheduled breaks. The session was divided into sessions dealing with NMR in all aspects of the undergraduate curriculum. The talks covered the range of using NMR for first year students through research and incorporating the instrument throughout the curriculum.

The first speaker, Michael Castellani from Marshall University, talked about using NMR as an introductory lab for high performing freshman. These students were recruited from the first semester course through a variety of methods. Once a cohort was identified, a separate lab section using both a 60 MHz and a 500 MHz instrument was created for these students. The course was divided into 25% known compound analysis, 25% unknown compound identification, and 50% for exams. The students performed an array of increasingly complex NMR experiments including DEPT, coupled 13C, NOE, and signal-to-noise measurement in addition to traditional 1H and 13C. The use of 13C as the introduction to NMR was utilized to allow the delay of discussions of coupling and to allow the instructor to show and discuss symmetry more easily. The students enjoyed the preparation the lab provided them for their subsequent organic course, the sense of accomplishment they obtained by determining unknown structures, and the fact that the lab was unique from other labs. But they were challenged by the fast pace of the curriculum.

Allen Schoffstall next discussed the use of guided inquiry experiments using NMR spectroscopy. The lab is a chemistry majors section using a 60 MHz instrument for 1H analysis. The use of inquiry based labs designed by instructor allows for inductive learning by the students as the conclusion is predetermined but unspecified. This provides students with an alternative to the more traditional expository lab where the learning is deductive. The labs use a combination of literature based experiments and modifications of traditional organic lab experiments to illustrate concepts. A lab based on tetraphenylethene allows the students to solve a puzzle of unanticipated products. The spectra are easily interpreted (both the intermediate di-chloro derivative and the final ketone product), the experiments work well but provide a somewhat lower yield. A second lab was discussed on the reduction of enones. Using sodium borohydride for the reduction of 2-cyclohexenone gives mainly the 1,4 addition product. The students are questioned about possible side reactions and challenged to determine the result. This models what scientists truly perform while doing research. The goal is to provide the exposure to students and potentially engage them in undergraduate research from this experience. Finally a Diels-Alder reaction of anthracene queried the students to determine on which ring the addition occurs. The inquiry based labs require a great deal of planning by the instructor but they are highly enjoyed by the students.

Paul Bonvallet presented a paper that presented the idea of using NMR early and often in the organic curriculum in order to form a basis for increased comprehension. Students are required to perform a year-long research project during their senior year. The goal of using NMR early and often is to better prepare the students to be successful and get the highest quality experience during their senior research. The early introduction of NMR allows for its use to illustrate resonance hybrids in the 1H and 13C spectra of dimethyl formamide. Two distinct methyl groups are seen in both spectra. At low temperatures (-80C), diagnostic splitting of iodocyclohexane is observed. The students use the observed splitting for axial and equatorial resonances to calculate equilibrium constants and Gibbs Free energy values. The effects of ring current on chemical shift are well illustrated with the spectrum of annulene. One set of protons are at 9 ppm and another set are at -2.3 ppm. This can be related to the illustration in the textbook for benzene. The acid/base character of the OH group in menthol is shown by shaking the compound with deuterium oxide using chloroform as a solvent. The disappearance of the OH resonance due to exchange with deuterium shows the lability of the proton. The benefits of this early introduction are that it reinforces the central importance of structure. It is new material for most students so it reinforces study skills and develops these skills in students. The shielding/deshielding trends are tied to identification of nucleophiles and electrophiles. NMR allows for prediction of reactivity, reinforces key concepts, reviews past material, and builds groundwork for future material.

After the break, J Thomas Ippoliti talked about overcoming problems incorporating NMR into the undergraduate curriculum. NMR is used in many courses starting with the organic chemistry sequence and continuing in several advanced courses. Students are taught the complete process of NMR analysis beginning with sample preparation, instrument operation, data processing and analysis. With the purchase of a new NMR spectrometer with an auto-sampler, the department has reduced the backup observed in previous labs. The department also obtained some free and purchased NMR software for data analysis to allow students to process and analyze their data. One early experiment requires the students to construct the overlay spectrum of p-methoxy benzaldehyde and p-isopropyl benzaldehyde. This helps introduce the processing software to the group. Another experiment uses NMR to determine the percent hydrogenation of eugenol (partial at 10 bar and total at 20 bar). The students observe the peak at approx 6 ppm decrease and eventually disappear. A plan for next year is to run this experiment at various values of pressure of H2 to follow the % conversion.

The next speaker, Eric Kantorowski, uses NMR for a chemistry majors “extra” organic lab taken after the usual two semester sequence. After performing the usual types of NMR experiments in the two semester sequence (1H,13C, DEPT), the students perform more advanced experiments (COSY, HETCOR) in this third lab course. The basis of the NMR experiments is unknowns. An experiment that uses NMR to analyze experimental data looks at the Grignard dehydration sequence of 1,1 diphenylpropan-1-ol . The NMR and IR of the alcohol product are obtained. The NMR shows overlap of the OH signal. The compound is then shaken with D2O. This allows for the separation of the OH which overlaps. The students then take the difference spectrum and the difference shows the peak for the OH that was masked by the overlap. A kinetic analysis of the alcohol dehydration with trichloroacetic acid in deuterated chloroform followed by NMR exposes the students to a quantitative kinetic analysis that is often lacking in organic labs. The experiment takes about 23 minutes to follow. The NMR spectra allow for the prediction/identification of intermediates which leads to a proposed mechanism for the reaction. A plot of loss of OH, appearance of alkene, and concentration of intermediate allows for reaction order analysis. Interestingly, the data do not match to either first or second order. A variety of parameters were modified and analyzed in this experiment such as rate dependence on trichloroacetic acid concentration, comparison of acid type on rate, and construction of a Hammett plot for the dehydration using trifluoroacetic acid in deuterated chloroform. More kinetic data are still needed to understand the dehydration of 1,1 diphenylpropan-1-ol.

The next talk shifted the focus to the use of NMR coupled with quantum mechanical calculations. James Foresman discussed the link between theory and experiment. This coupling can help predict and understand structure/reactivity relationships and pathways for syntheses. He used four different computational theories ; B3LYP, MP2, O3LYP, TPSSh. MP2 overestimates and B3LYP underestimates results. O3LYP and TPSSh are the best predictors of experimental data and are being used more often now in the class. TPSSh and B3LYP show very good correlation with experimental data for 127 data points. An example was shown using 2-nitroaniline and additivity patterns for comparison. He used hydrogen carbon correlation spectrum (HCCOSW ) to prove which carbon atoms correspond to which signals in the 13C spectrum. In 2,2,4 trimethyl 1-3 pentane diol, two methyls show one signal. It is possible to use gas phase calculations to show which two methyls are equivalent. Finally a proton coupled 13C NMR of 5-methyl-2-nitroaniline was given as an example to link experimental data to Gaussian calculations.

After the final break, the session presenter’s talks focused on use of NMR in advanced coursework and undergraduate research. Thomas Devore discussed the Shenandoah Valley Regional NMR Facility which houses 300, 400, and 600 MHz NMR spectrometers. The students use these instruments beginning in organic and continue to develop their skills in instrumental, physical chemistry, biochemistry, and research. The focus of this talk is on the use in physical chemistry. One experiment uses NMR to determine the enthalpy of vaporization for methanol and t-butanol. A large shift in the OH signal is observed when the spectrum of the neat liquid is compared to the vapor. The large shift is explained by the formation of dimer and trimer (due to hydrogen bonding) structures in the liquid phase. Essentially all monomer exists in the vapor phase. A calculation of the monomer/dimer equilibrium from the NMR data allows for a full set of thermodynamic calculations. By collecting the equilibrium constant values at various temperatures and plotting the data, the students can determine enthalpy and entropy for the process. The results show a strong correlation to the literature values. The students also compared calculated to measured chemical shifts using similar software (Gaussian 03) to the previous presenter.

The use of NMR in undergraduate research experiences was discussed by Patrick Desrochers. The students are exposed to traditional NMR experiments in the organic chemistry sequence. This prepares them for the advanced use needed in research. In this research project examining scorpionate chelates, students use 11BNMR to observe the downfield shift of the signal with each substitution of ligand. This shifting allows the students to follow the progress of the reactions. The reaction is continued until the NMR spectrum shows the correct substitution product. With the use of phosphine ligands, students use NMR to analyze fractions from column chromatography to test for separation efficiency using 31P NMR. Running the phosphine ligands at low temperature allows for the resolution of all three phosphine ligands, including the cis and trans isomers (this is not observable at room temperature). This shows the value of both multi-nuclear and variable temperature NMR in the structural analysis of a research based compound. These compounds also illustrate to students concepts about coupling. The spectra exhibit unique coupling patterns (C-P and C-Rh coupling constants for example) that match literature J-values.

The final presentation, by Thomas Wenzel, discussed the use of NMR throughout the curriculum as well as some information about funding possibilities for instruments. In the organic sequence, the emphasis is on spectral interpretation. Students must run NMR on all products obtained. This gives them multiple experiences with interpretation. In the advanced synthesis lab, students run NMR on every intermediate and final product. During the measurements lab (an integrated lab combining the classical analytical and physical chemistry sequence), the origin of the NMR signal, Boltzmann distribution, coupling and other aspects of NMR theory are presented. An advanced course that combines NMR and MS provides the students with extensive theoretical background and an emphasis on 1D and 2D NMR spectra. The overall pedagogical goal is to increase the complexity of NMR throughout the curriculum. In undergraduate research, NMR is used to follow and monitor the progress of the reactions being run by students. The observed changes in NMR signals illustrate the completion of the reactions. Finally, Tom discussed some options/funding sources for obtaining NMR spectrometers such as NSF MRI and NSF TUES programs. MRI puts a major emphasis on research and includes separate funds for primarily undergraduate institutions (PUI). The use of evidence based research outcomes will improve the proposal such as significant work being done, other grant support of the project and published outcomes. The proposer should also include curricular use of the instrument. TUES is primarily curriculum based but the proposer should also include research uses. The proposal should include sound pedagogy and plans for infusing NMR throughout the curriculum including an interesting suite of experiments/projects.

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