Downloaded by NORTH CAROLINA STATE UNIV on March 20, 2013 | http://pubs.acs.org Publication Date (Web): March 19, 2013 | doi: 10.1021/bk-2013-1128.ch005
Chapter 5
NMR Spectroscopy in Nondeuterated Solvents (No-D NMR): Applications in the Undergraduate Organic Laboratory John E. Hanson* University of Puget Sound, Department of Chemistry, Tacoma, Washington 98416 *E-mail:
[email protected] Expensive deuterated solvents have traditionally been used for NMR spectroscopy in order to facilitate locking and shimming, as well as to suppress the large solvent signal that would otherwise occur in the proton NMR spectrum. Advances in NMR instrumentation now make the routine use of deuterated solvents unnecessary. The use of nondeuterated solvents for NMR spectroscopy (No-D NMR) results in significant savings, both of time and money. No-D NMR can be used for identifying unknown compounds, confirming the structure and purity of compounds isolated from a reaction, determining the ratio of products in a crude reaction mixture, and following reaction progress. Practical issues relating to the implementation of No-D NMR experiments, along with applications in the undergraduate organic laboratory, are discussed.
Introduction One of the first things that students are often taught about NMR spectroscopy is to prepare the sample by dissolving it in a deuterated solvent. “The solvent molecules should have all hydrogen atoms replaced with deuterium atoms (2H) for two reasons. First, if you are doing proton (1H) NMR, you do not want the solvent resonance to dominate your spectrum. Solvent © 2013 American Chemical Society In NMR Spectroscopy in the Undergraduate Curriculum; Soulsby, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.
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molecules typically outnumber solute molecules by 1000 to 1, so you would not really see your solute spectrum at all. Second, the spectrometer needs a deuterium (2H) signal to “lock” the magnetic field strength and keep it from changing with time. Because the NMR experiment usually adds together a number of FIDs (scans), if the field changes during the experiment the frequency changes with it and the NMR peaks will not add together correctly. The deuterium NMR signal is used to monitor “drift” of the field and to correct it… (1).” “If one uses the protonated solvent, then a ca. 0.5-1.0 ppm region will be obscured by the solvent peak….for proton FT studies, the deuterated solvent is vital, as the majority of pulse spectrometers use the solvent deuterium signal to stabilize and lock the spectrometer system. Also, the large signal of the protonated solvent in both 1H and 13C spectra causes digitization problems in FT spectrometers… (2)” In demanding applications, particularly those in which the amount of sample is very limited or the complexity of the sample is very high, the use of deuterated solvents is important for many of the reasons articulated above. But in more routine applications, such as those typically encountered in the undergraduate teaching laboratory, NMR experiments performed in nondeuterated solvents (No-D NMR) normally work well (Figure 1). We were inspired to implement No-D NMR experiments into our second-year undergraduate organic laboratory class by a series of papers published in 2004-2005 by Professor Thomas Hoye and co-workers demonstrating the usefulness of No-D NMR for a wide variety of applications (3–6). In early FT-NMR spectrometers, limitations on the analog-to-digital converters (ADC’s) that were used made it difficult to observe small sample signals in the presence of large solvent peaks. This is much less of an issue in modern instruments. In applications where relatively large amounts (>20 mg) of relatively simple compounds (MW
