Molecular Structures from 1H NMR Spectra: Education Aided by

Mar 1, 2007 - The article presents the way in which freeware Internet programs can be applied to teach 1H NMR spectroscopy. The computer programs ...
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Molecular Structures from 1H NMR Spectra: Education Aided by Internet Programs Barbara Debska* and Barbara Guzowska-Swider Department of Computer Chemistry, Rzeszów University of Technology, 35-041 Rzeszów, Poland; *[email protected]

There are a number of smaller documents and comprehensive electronic-books on spectroscopy available on the Internet that introduce the student to 1H NMR spectroscopy. A particularly good handbook for beginners is Applications of 1H-NMR Spectroscopy by P. Hallpap and H. Händel (the HTML version was created by R. Haeßner and St. Thomas) (2): the electronic-book is available in English, German, Spanish, and French. It is, in fact, a program that introduces some basic aspects of 1H NMR spectroscopy in an interactive way. The student reads chapters of the book and afterwards is able to verify his or her knowledge by taking online tests with right path of reasoning and consequently to reach the cor-

rect conclusions and solutions to the problems. After pointing the student to the appropriate sections of material, the program goes back to the question that the student was supposed to answer. This gives the student the opportunity to reevaluate the problem and to select the correct answer. Of course, the student can check the correct answer without these additional questions and explanations, but in this way, he or she forfeits the valuable chance of solving the problem alone. The program works as an instructor, who interacts with the student in a friendly manner, reassuring the student if he or she is unsuccessful and encouraging him or her to continue. In addition to gaining knowledge about 1H NMR by taking tests in electronic-books, the student can also solve spectral problems where questions and answers are presented either in a traditional manner (text, spectral curves) (2–4) or as interactive educational tutorials, such as 1H NMR Spectroscopy (5). 1H NMR Spectroscopy invites the student to predict 1H NMR spectra for chemical compounds selected from a list and to compare the predicted spectra with library spectra stored in the tutorial database. The student is supposed to predict a given spectrum by estimating chemical shifts, peak intensities, and multiplicities. The student can also choose an easier exercise option, where he or she is required to put a number of peaks that are already provided in correct positions on the spectrum (Figure 1). In the third option of the exercise, the student can assign each peak of the displayed spectrum to the right proton group of the molecule. To practice the modeling of 1H NMR spectra, free online Internet spectra collections can also be used. The student performs the modeling of the 1H NMR spectrum for a given chemical compound and compares the created spectrum with one from a database. The consistency between the created spectrum and the reference spectrum proves that the problem is solved correctly. Among the Internet resources, one can use libraries such as Organic Chemistry OnLine (6) (Figure 2) or SDBS Integrated Spectral Data Base System for Organic Compounds (7). The educational tutorial, Organic

Figure 1. Window for an exercise in 1H NMR Spectroscopy (5) where the student has to put peaks in correct positions.

Figure 2. 1H NMR spectrum, with the 13C NMR δ ppm values given on the structure, selected from the database of ref 6.

The progress in computer science and in Internet technology offers new educational opportunities and practical tools for instructors and students. Computer educational programs can be excellent teachers of theory and a good source of practice for a given domain. The programs are often used to present educational problems in a straightforward inductive manner, where the student is “told” what he or she is supposed to know. On the other hand, these programs often allow the student to acquire knowledge in a deductive way, where he or she is required to come to certain solutions and conclusions independently, by way of active problem solving. Irrespective of the way in which the knowledge is presented, it is an undeniably positive aspect of Internet teaching and learning that these resources can be used either in a regular class or by the student individually, without the help of the instructor. The Internet is a unique source of a broad range of information in various fields, including spectroscopy. A number of documents are available on the Internet that can be used for teaching spectroscopic theory and the interpretation of spectra (1). This article describes the application of freeware Internet documents to explore 1H NMR spectroscopy. Introducing 1H NMR with Internet Documents

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Chemistry OnLine contains a small collection of 1H NMR spectra for various types of organic compounds. Additionally, 13 C NMR chemical shifts are displayed for each compound. The SDBS base was developed by the Japanese National Institute of Advanced Industrial Science and Technology. It is an integrated spectral database system for organic compounds, which includes six different types of spectra: 1H NMR, 1H decoupled 13C NMR, FT-IR, laser-Raman, EI mass, and ESR. The base contains ca. 14,000 1H NMR spectra stored as spectral curves and chemical shifts, which constitutes a large online collection of spectra for a variety of compound classes. To find a 1H NMR spectrum for a given compound in this database the student can use input information such as compound name, molecular formula, atom numbers, molecular weight, or CAS registry number of the compound. The SDBS managing program also enables the user to search for a particular 1H NMR spectrum by providing chemical shifts in δ ppm. This function of the program is particularly useful when a student is trying to determine the molecular structure from an 1H NMR spectrum of an unknown compound.

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Teaching students how to obtain structural information from spectroscopic data is an important aspect of spectroscopy education. The principal aim of such courses is to teach the student to solve simple structural problems using spectral techniques separately or in a combination. Usually, chemists make structural analyses using two methods. One method (library search, LS) is to search a library of standard spectra and to find the closest match to the spectrum analyzed. The other method (applying spectrum–structure correlations) is to find characteristic spectral features connected with some parts of a chemical molecule. A good educational program addressing structure elucidation should include both these methods of structure identification. Modern NMR spectroscopy, which has been expanded by the development of two-dimensional correlation spectroscopy, for example, COSY and NOESY, provides a third option. With sufficient data, one can apply deductive methods and derive the entire covalent structure without reliance on empirical correlations. One surely uses the values of the chemical shifts, but the connectivity information provided by methods such as COSY, NOESY, and HMQC has primacy in the new approach to NMR. Other methods, such

as IR spectroscopy, do not have this power. As our knowledge of unimolecular kinetics improves, mass spectrometry is also a tool that allows the user to determine the structure deductively. To determine the structure of an organic compound from its 1H NMR spectrum with the use of the spectrum– structure correlations method, the student should associate peaks with appropriate structural elements of the compound studied. A good tool for beginners in spectra interpretation ( 1H NMR, 13C NMR, IR, MS) is Organic Chemistry OnLine (6). The program contains an introduction to 1H NMR, 13C NMR, IR, MS spectroscopy, and a quiz. The most interesting part of the program contains exercises teaching students how to recognize structural elements of a compound by analyzing its 1H NMR spectrum (or 13C-NMR, IR, and MS spectrum). The 1H NMR practice section contains a display of successive problems, from very simple to those that are more complex. Although viewing the spectra in order is advised as it provides gradation of the problem, the student can also chose the problems to view as he or she prefers. Each problem presents the 1H NMR spectrum of an unknown compound and its formula (Figure 3). Using the spectroscopic and analytical data (molecular formula, degrees of unsaturation), the student should suggest a structure that is consistent with the data. The program can (optionally) provide the student with additional information that makes it easier to solve the problem. The option “Analyze” gives the student information about the calculation of degrees of unsaturation of the compound. By clicking on the peak, the student can view the structural fragment that causes this absorption and read additional explanation on how this correlation was deduced. The elucidation of all the peaks of the spectrum can be viewed by choosing the “Interpret” option. The “Structure” option presents the solution of the problem, that is, the appropriate structural formula. When the student learns to correctly solve the problems provided by the more beginner-oriented Organic Chemistry OnLine program, he or she can proceed with greater ease to spectra interpretation using Internet Web sites that are oriented towards more advanced students (8–13). One program is WebSpectra: Problems in NMR and IR Spectroscopy (8). This program is particularly recommended as it contains a wide range of problems on a number of levels of difficulty. All problems contain 1H NMR and 13C NMR spectra and, for some problems IR, DEPT NMR, or COSY NMR spectra are also available (Figure 4). The program is directed more

Figure 3. Example of a problem to solve from ref 6.

Figure 4. Example of a problem to solve from ref 8.

Organic Structures from 1H NMR Spectra

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towards testing knowledge rather than teaching how to solve a particular problem. For each problem the WebSpectra program presents the solution of the problem, that is, the molecular structure of the compound. The line of reasoning, however, is not explained. It is advisable to work through the problems in the order they appear. However, the information about interpreting 13C NMR spectra is easily accessible and available on the Internet (14, 15). Another online workbook worth recommending for practising structure recognition is Organic Structure Elucidation (11). This workbook is for intermediate and advanced students. It contains exercises involving IR, 1H NMR, 13C NMR, and MS spectra, although the exercises do not contain any explanations or the key. This is particularly useful for instructors who prefer that their students use the document and work through the problems without having the option of an immediate answer. The instructor can obtain the answer sheet upon request, by writing to the email address provided. A Web page of the Widener University contains a sub page (12) that can be useful especially for advanced students practicing structure determination from spectral data registered from a variety of different NMR techniques (e.g., 1H NMR, 13C NMR, DEPT, APT, COSY, HETCOR). The option entitled “NMR experiment” displays a spectrum and the molecular structure of the compound and a brief description of the experiment. When a particular peak is selected with the cursor, appropriate atoms in the molecule are highlighted and a text providing the interpretation of the peak is displayed. The Web site (13) of the University of Colorado is another useful site providing access to an online tutorial entitled Spectroscopy Problem Tutorial. The tutorial teaches the student to solve simple structural problems using IR and NMR spectra. The Spectroscopy Problems sub page contains a collection of NMR and IR-related exercises with varying degree of difficulty. The option “Answer” gives the student information about the calculation of degrees of unsaturation of the compound, the structural fragments that cause the absorption peaks in analyzed spectra, additional explanation on how these correlations were deduced, and finally, the solution of the problem, that is the appropriate structural formula. When dealing with structure elucidation exercises, the students need to have access to information about correlations between structural elements and their spectral features. Such tabulated correlations containing lists of various structural elements and their spectral characteristics (e.g., NMR, IR) are available mainly in printed monographs (16–18), but they can also be found on the Internet (19–21). Some Web sites present the correlations in a static form: as tables, schematic diagrams, or chemical shift values marked on molecular structures (19, 20). Other Web sites, such as 1H-Wizard, IR-Wizard, MSWizard, 13C Database (21), approach the problem of presenting the correlations in a more interesting way. This Web site contains spectrum–structure correlations for 1H NMR, IR, and MS spectroscopies arranged as independent databases that can be searched for a given value of chemical shifts (1H NMR), infrared frequencies (IR), or m兾e values (MS). In this way, the student who tries to deduce the origin of a given peak in an 1 H NMR spectrum searches the base for the peak’s chemical 558

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Figure 5. 1H NMR correlation tables from ref 21.

shift and receives a list of structural fragments that absorb in this range of the spectrum (Figure 5). The student using the program is also able to look for appropriate nJH–H coupling constants, whose values are indispensable for spectrum interpretation. Apart from the above mentioned correlations, the Web site also contains other useful information, such as data about NMR solvents, schematic diagrams of chemical shifts, or a program for calculation of 13C-chemical shifts for some substituted aromatic and heteroaromatic rings. Some helpful facilities that can be used to teach intermediate and advanced students include computer programs working as virtual NMR spectrometers and are able to predict 1H NMR spectra for given spin systems. These include freeware Internet programs such as WinDNMR (22), PERCH (23), GAMMA (24, 25), or VSNMR (26, 27). Students who are already advanced to application of spectral methods for structure elucidation can practice the problems using the 2D 1H NMR techniques. The background information about the 2D 1H NMR theory can be found on numerous Internet pages (12, 28–34). One effective way of learning how to interpret 2D 1H NMR spectra is to start with a simulation of 2D spectra for a given spin system. This can be performed by using programs that work as virtual NMR spectrometers, such as some of the programs mentioned above (26, 27). The VSNMR software package enables computer simulation of multichannel, multidimensional NMR experiments on user-defined spin systems. The program can be obtained directly from its author (email request), who distributes a “bet-test version” of the package for testing and development. Also the GAMMA package (24) can only be obtained from its authors. The virtual spectrometer programs mentioned in this article are complex, which does not enable their more detailed presentation within the frame of this article. To determine the structure of an unknown compound from the 1H NMR spectrum, the student can also use the library search method. The application of this method requires access to a large spectra collection that can be searched through spectral parameters (chemical shifts). The SDBS database (7) is an excellent tool for such exercises. As input data, the student uses the chemical shifts and additional information about the compound, for example, its molecular formula.

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Literature Cited

Figure 6. The spectrum found in the SDBS base (7) as identical to the experimental one.

The program retrieves reference spectra identical or similar to the analyzed spectrum and displays their SDBS numbers and names of the compounds. By clicking on a particular SDBS number the student can access the spectral patterns related to this number (Figure 6) and compare them with the pattern of the analyzed spectrum. The student checks all the SDBS numbers and the patterns behind them, which allows him or her to pick the closest match to the spectrum being analyzing. Conclusions The Internet is a rich source of freeware documents and programs that can be useful for students and instructors of spectroscopy courses at various levels. These documents, being accessible on the Web without time and location restriction, are an excellent learning aid for students who wish to acquire and practice their knowledge of spectra prediction for a given structure or structure elucidation from spectral data. Beginners in NMR spectroscopy interpretation can use programs that are in many aspects very much like an instructor, leading students through spectral problems step-by-step, asking questions and providing answers and explanations. Intermediate and advanced students can use programs that provide problem-solving exercises without additional explanations or answer sheets. In this case, however, it is essential for the students to consult their instructors. Another usage of the Internet lies is as a good online source of spectral databases, thus providing a useful reference tool for students working on spectral problems. These educational documents and spectral databases can be accessed by the students individually, but they can also be an integral part of group lessons. Non-English speaking students may require additional language assistance from their instructors, especially when dealing with tutorials and programs containing extensive explanations. The educational documents and programs available on the Internet enable spectroscopy instructors to develop more interesting, up-to-date teaching curricula. The educational programs discussed in this article are used during a spectroscopy course, making the educational process more attractive for the students and facilitating the development of more independent and reflective learners. www.JCE.DivCHED.org

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26. Nicholas, P.; Fushman, D.; Ruchinsky, V.; Cowburn, D. Virtual NMR Spectrometer. http://www.vsnmr.org/ (accessed Nov 2006). 27. Nicholas, P.; Fushman, D.; Ruchinsky, V.; Cowburn, D. J. Magn. Reson. 2000, 145, 262–275. 28. Hornak, Josef P. The Basics of NMR. http://www.cis.rit.edu/ htbooks/nmr/ (accessed Nov 2006). 29. Bria, Marc; Watkin, Pierre; Planche, Yves. 2D NMR Spectroscopy. http://www.univ-lille1.fr/lcom/RMN2D/resum2D_us/ index1.htm (accessed Nov 2006). 30. Hunt, Ian. Chapter 13: Spectroscopy—2D-NMR Spectra. http://www.chem.ucalgary.ca/courses/351/Carey5th/Ch13/ch132dnmr-1.html#cosy (accessed Nov 2006). 31. Kuo-Bin, Li. Introduction to 2D NMR Spectroscopy. http://web.bii.astar.edu.sg/~kuobin/thesis/html/chapter2/node2.html (accessed). 32. JEOL FT NMR Spectra. http://www.jeol.co.jp/english/technical/ nm-data/nm-data-j.html (accessed). 33. Shokhirev Nikolai V. NMR Tutorials. http://www.shokhirev. com/nikolai/abc/nmrtut/NMRtut2.html (accessed Nov 2006). 34. Acorn NMR Inc. Technical NMR Tidbits. http://www. acornnmr.com/nmr_topics.htm (accessed Nov 2006).

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