Chapter 1
Introduction to NMR Spectroscopy in the Undergraduate Curriculum Downloaded by 193.0.129.125 on October 3, 2016 | http://pubs.acs.org Publication Date (Web): September 15, 2016 | doi: 10.1021/bk-2016-1221.ch001
David Soulsby*,1 and Anton S. Wallner*,2 1Chemistry
Department, University of Redlands, 1200 E. Colton Ave., Redlands, California 92373, United States 2College of Arts and Sciences, Barry University, 11300 NE 2nd Ave., Miami Shores, Florida 33161, United States *E-mails:
[email protected] (D. Soulsby);
[email protected] (A. S. Wallner).
NMR spectroscopy is a powerful tool that is used in many disciplines and is centrally important to the undergraduate curriculum. In this chapter we provide a brief introduction to the history of NMR spectroscopy, describe the challenges associated with introducing NMR spectroscopy into the undergraduate curriculum, provide a summary of the many NMR experiments available on modern NMR spectrometers, and conclude with an overview of the resources contained within this volume.
Introduction The possibility of the generation of a signal from nuclei with spin perturbed by a magnetic field was first proposed in 1936 by Gorter (1–3). His attempts to observe signals for 7Li (in LiF crystals) and 1H (in potassium alum) were unsuccessful due to long relaxation times of the crystalline samples. In 1946, Bloch, Hanson and Packard (4) and Purcell, Torrey, and Pound (5) observed the first nuclear magnetic resonance (NMR) signals in water and paraffin respectively. From these beginnings, NMR has expanded greatly over the past 70 years. Early NMR technology used continuous wave methodology and permanent iron magnets. These instruments could easily detect 1H resonances and provided useful structural information for a variety of compounds. In 1966, Ernst and Andersen reported the use of a new Fourier Transform (FT) technique that © 2016 American Chemical Society
Soulsby et al.; NMR Spectroscopy in the Undergraduate Curriculum: First Year and Organic Chemistry Courses Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.
Downloaded by 193.0.129.125 on October 3, 2016 | http://pubs.acs.org Publication Date (Web): September 15, 2016 | doi: 10.1021/bk-2016-1221.ch001
improved sensitivity by a factor of ten or shortened the time for acquisition (for the same sensitivity) by a factor of 100 compared to the conventional sweep scan method (6). Around this same period, researchers were developing stable, useful superconducting materials (niobium-tin and niobium-titanium alloys) that could be used to create large magnetic fields at liquid helium, superconducting temperatures. These fields were often 10 times greater than the field achieved by permanent iron magnets; they produced magnetic fields with greater homogeneity, and were smaller in design (7, 8). The use of both superconducting magnet materials and FT-NMR produced a variety of new techniques and research projects in the area of NMR during the next several decades. Multi-nuclear NMR (most notably 13C), multi-dimensional NMR, and novel pulse sequences in these areas all developed during this period (see Volume 1 of this series for a review) (9). Additionally, cross-polarization-magic angle spinning (CP-MAS) solid state NMR (10–12) and magnetic resonance imaging (MRI) (13) developed and became more commonplace during the 1970’s and 1980’s. This expanded the application and usefulness of NMR to rigid solid materials as well as human and medical applications for non-invasive, non-destructive evaluation. Many of these applications became commonly available at research universities. With the expansion of multiple applications and potential uses of NMR and MRI, multiple organizations viewed the value of this technique as crucial knowledge for a well-trained scientist or clinician. Most notably, the American Chemical Society (ACS) Committee on Professional Training lists NMR spectroscopy as a requirement for ACS approved undergraduate programs (14) and a necessary instrumentation experience for trained undergraduate students in general. The challenge for many primarily undergraduate institutions is the purchase cost of these instruments, particularly high-field superconducting FT-NMR and the associated cost of cryogens and routine maintenance. For many years the National Science Foundation (NSF) supported the acquisition of both permanent and superconducting NMR instruments through its now obsolete Instrumentation and Laboratory Improvement (ILI), Course Curriculum and Laboratory Improvement (CCLI), and Transforming Undergraduate Education in Science, Technology, Engineering, and Mathematics (TUES) programs. Indeed, many institutions were successful in acquiring both superconducting and permanent magnet NMR instruments through these programs. Unfortunately, the only current general program available is the highly competitive Major Research Instrumentation (MRI) program which has a focus on research rather than curricular needs. Issues associated with funding and maintenance costs have led to a resurgence in the use of cheaper, permanent magnet NMR instruments for curricular use. In 1995, Anasazi Instruments, Incorporated developed a method to upgrade existing permanent magnets to FT capabilities at a reasonable cost. This upgrade combined with current software for data processing provides for high quality, well-resolved spectra without the cost and maintenance of a superconducting magnet. A variety of other companies have since entered the market of table top NMR (Magritek, Nanalysis Corporation, Oxford, Process NMR Associates and Thermo Scientific). These instruments combined with software packages 2
Soulsby et al.; NMR Spectroscopy in the Undergraduate Curriculum: First Year and Organic Chemistry Courses Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.
Downloaded by 193.0.129.125 on October 3, 2016 | http://pubs.acs.org Publication Date (Web): September 15, 2016 | doi: 10.1021/bk-2016-1221.ch001
including, MestReLab, ACORN, ACD Labs, Bruker TopSpin, Cambridge Soft – ChemDraw, Science Soft NMRanalyst, Nucleomatica iNMR, and ModGraph NMRPredict, allow for collection and analysis of high quality spectra for classroom and research use. Also, database collections are growing where the NMR and scientific community can post and store NMR spectra of compounds they have collected in their labs. Most notably, ChemSpider is a dynamic, growing repository of NMR spectra (15). Unlike most other instrumental methods, modern NMR spectroscopy continues to innovate at a fundamental level. A range of routine to advanced NMR experiments are available on many NMR spectrometers, with new pulse sequences published frequently. These NMR experiments allow for the acquisition of data that can reveal a range of connectivities. As part of this introductory chapter and as a guide to both the novice and more advanced NMR user, we provide an overview of many of the available techniques that can be routinely used in the laboratory as an aid to those looking to incorporate NMR spectroscopy into all levels of the undergraduate curriculum.
Basic 1D NMR Spectroscopy The high-abundance and high-sensitivity of the proton means that a 1H NMR spectrum is often one of the first spectra acquired (16). The 1D 1H NMR pulse sequence is found on all modern NMR spectrometers, though there are some interesting variations that will be introduced later. A 1H NMR spectrum provides functional group identification through chemical shift information, quantification of chemical shift equivalent protons, and connectivity information through homonuclear coupling. However, the relatively narrow spectral width of the 1H spectrum (0-12 ppm, with the bulk of the signals lying between 1-8 ppm) means that signal overlap can occur, particularly in complex molecules or mixtures. With low magnetic field strength instruments, analysis can be further complicated by the appearance of higher-order spectra that are present when the difference in chemical shifts is close or slightly greater in magnitude to the coupling constant (i.e., ΔJ≈Δυ or ΔJ>Δυ). First-order spectra, which are more straightforward to analyze, occur when ΔJ
