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NMR-Challenge.com: Exploring the Most Common Mistakes in NMR Assignments

  • Zuzana Osifová
    Zuzana Osifová
    Institute of Organic Chemistry and Biochemistry, Czech Academy of Science, Flemingovo nám. 2, 160 00 Prague, Czech Republic
    Department of Organic Chemistry, Faculty of Science, Charles University, Hlavova 2030, Prague 128 00, Czech Republic
  • Ondřej Socha
    Ondřej Socha
    Institute of Organic Chemistry and Biochemistry, Czech Academy of Science, Flemingovo nám. 2, 160 00 Prague, Czech Republic
  • , and 
  • Martin Dračínský*
    Martin Dračínský
    Institute of Organic Chemistry and Biochemistry, Czech Academy of Science, Flemingovo nám. 2, 160 00 Prague, Czech Republic
    *E-mail: [email protected]
Cite this: J. Chem. Educ. 2024, 101, 6, 2561–2569
Publication Date (Web):May 14, 2024
https://doi.org/10.1021/acs.jchemed.4c00092

Copyright © 2024 The Authors. Published by American Chemical Society and Division of Chemical Education, Inc. This publication is licensed under

CC-BY 4.0.
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Abstract

NMR spectroscopy is the most powerful tool for determining the structures of chemicals. It is applied in a broad range of scientific disciplines, including physics, structural biology, material science, medicine, and chemistry. Interpreting NMR spectra is part of the core skill set mainly of organic chemists. In 2022 we introduced the educative website NMR-Challenge.com presenting 200 spectral tasks of various difficulties. The website collected more than 428,000 submitted solutions over 20 months and thus provides the largest data set of responses to NMR assignments. By analyzing the structures submitted by users of the website, we identified patterns of the most common mistakes, which might be useful in devising new teaching strategies. Here, we present three case studies of the most common sources of spectral misinterpretation–recognition of the structures of isomeric esters, determination of substitution positions in disubstituted benzenes, and recognition of intramolecular hydrogen bonding in 1H spectra–based on more than 28,000 answers collected at NMR-Challenge.com and recommend how to amend the frontal teaching about these problems.

This publication is licensed under

CC-BY 4.0.
  • cc licence
  • by licence

Introduction

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Nuclear magnetic resonance (NMR) spectroscopy is one of the most powerful tools for the determination of the structure of compounds and materials employed in a wide range of scientific disciplines. In chemistry, NMR is used as a nondestructive analytical method for the straightforward determination of the structure of synthetic or natural compounds. Many structural patterns can be recognized from 1H NMR spectra, which can be recorded within a few minutes. Additional experiments including 13C spectra and homo- and heteronuclear 2D correlations can be used for the determination of the structure of most organic compounds. This shows the enormous importance of NMR spectroscopy for synthetic chemists. Moreover, NMR spectroscopy is widely used in studies of intermolecular interactions, (1,2) reaction kinetics (3,4) or reaction mechanisms. (5,6) Not surprisingly, the interpretation of NMR spectra is part of the core knowledge of most chemists.
NMR spectroscopy is usually taught to undergraduate chemistry students from the very beginning of their Bachelor’s studies as a part of organic chemistry courses (7,8) or as a specialized subject. Although the interpretation of spectra can be learned only by practice, (9) there is a surprising lack of spectral tasks in textbooks (10−12) but online sources are emerging. (13−23) Moreover, students often do not receive any feedback, or the given structure can be determined only using a combination of techniques, such as mass spectrometry and UV–Vis or IR spectroscopy. (10,15,17) Available educational resources are therefore insufficient for most students and future organic chemists who need to recognize chemical structures independently and quickly during their lab day.
The solving of NMR spectral tasks increases students’ critical thinking abilities because they have to evaluate multiple aspects at the same time, such as the number, intensity and shape of signals, chemical shifts, and J-coupling values. (24,25) The determination of any structure from NMR spectra can be described as a two-step process consisting of the recognition of structural fragments and their connection to a molecule. Several studies aimed to characterize students’ approaches to NMR spectral interpretation. (26−30) Knowing both productive and unproductive strategies used when interpreting NMR spectra is enormously important for lecturers, who can use it to improve their classes. The studies were run mainly in small classes, and their results lack the statistical significance needed to draw general recommendations for NMR teachers.
In April 2022 we launched the educative website NMR-Challenge.com (31) presenting spectral tasks based on measurements of real samples. We enriched the website with an interactive structure drawing tool, allowing users to give immediate answers on both desktop computers and touch-screen devices (Figure 1). To keep both beginners and advanced NMR students motivated, we divided the assignments into two levels: The Basic level offers only 1D spectra (1H, 13C, 19F) whereas, at the Advanced level, 2D spectra (COSY, HSQC, HMBC) are needed for structure determination. Within these two levels, the tasks are further classified by difficulty as Easy, Moderate and Hard. The chemical formula is shown together with the spectra. By January 2024 (over 20 months), NMR-Challenge.com collected over 428,000 submitted solutions with the correct chemical formula, which made it the largest data set of responses in the field of NMR spectroscopy assignments worldwide.

Figure 1

Figure 1. Scheme of using NMR-Challenge.com in three steps–analysis of spectra, drawing of a chemical structure, and receiving immediate feedback.

For each task, we collect all submitted structures with the correct chemical formula, data on the time users spend solving it, the number of attempts they take (users can submit an unlimited number of solutions for the same task), and the overall success rate. The website does not require any registration and does not collect any personal data of users, so it is not possible conduct geographic or demographic analyses. Nevertheless, there are many common mistakes found throughout the assignments that possibly originate from a misunderstanding of the basic principle of NMR spectral interpretation by students worldwide.
Here, we present the most common types of errors in structures submitted by users of the NMR-Challenge.com website, which can provide valuable insights for NMR spectroscopy lecturers. Furthermore, we provide tips on how to amend lectures to address the most common mistakes. We identified the main obstacles students face when attempting to recognize benzene ring substitutions from the splitting pattern of aromatic proton signals, when evaluating chemical shifts to properly connect structural fragments in isomeric esters, and when distinguishing the effect of an intramolecular hydrogen bond on the spectra. We examine the origins of these mistakes and recommend the inclusion of specific spectral tasks in the curricula of NMR frontal lectures.

Results

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NMR-Challenge.com: New features

There are currently 200 real NMR spectral tasks available on the NMR-Challenge.com Web site −148 of the Basic level, where only 1D (1H, 13C, 19F) spectra are available, and 52 tasks of the Advanced level, where two-dimensional correlation spectra (COSY, HSQC, HMBC) are needed for structure determination. Since the first report in the Journal of Chemical Education (31) in January 2023, we have added a total of 40 tasks −22 of the Basic level and 18 of the Advanced level. We have also included a new type of 2D correlation spectra: 1H,15N-HMBC. These spectra are measured at the natural abundance of the 15N isotope. Between the end of April 2022 and the end of December 2023, NMR-Challenge.com collected 428,029 submitted solutions of which 195,098 (46%) were correct. These days, the website, on average, collects more than 1,000 submissions per day (Figure 2). According to personal feedback given by lecturers, NMR-Challenge is used as an educational tool in NMR spectroscopy courses at several universities, such as the University of Sheffield, University of Münster, Charles University in Prague, or Iowa State University.

Figure 2

Figure 2. Usage of the NMR-Challenge.com website between May 2022 and December 2023 in terms of the number of submissions per day (only structures with correct chemical formula are counted). Note the lower usage during the summer and winter holiday seasons. The massive increase in submissions after October 21, 2022 was ignited by a Twitter announcement by the Institute of Organic Chemistry and Biochemistry of the Czech Academy of Sciences.

We regularly evaluate the success rate of all of the tasks to identify those whose difficulty classification should be adjusted. Moreover, we take note of the most common wrong answers and regularly publish short videos about the mistakes on our social media accounts @nmr_challenge on Instagram, X (Twitter), and YouTube. Through these accounts, we also publish posts about the basics of and fun facts about NMR spectroscopy with the aim of offering an educational and entertaining experience for students. Especially the educational videos posted on the Instagram account got quite popular in the NMR community, receiving the attention from 2,000–6,000 users each. The following sections describe three of the most common types of mistakes found in the submitted answers.

Case Study 1: Recognition of Isomeric Esters

The molecule of an ester consists of two parts–an acyl group and an alkoxy group. Each part has an easily distinguishable fingerprint in both 1H and 13C NMR spectra. The signal of the carbon attached directly to the oxygen in the alkoxy group appears at about 50–80 ppm and its hydrogen atoms can be found in the range of 3–4 ppm. On the other hand, in the acyl group, the α-hydrogens can be found at 2–3 ppm. An experienced NMR user can therefore determine the signals of atoms in close proximity of the ester from the first look at the spectra.
Tasks involving isomeric compounds can be used as significant indicators of students’ knowledge. We obtained 18,595 responses to eight tasks concerning isomeric esters at all levels of difficulty. Although students performed quite well in recognizing individual fragments, they were more prompt to misunderstanding the connection between them. Below we analyze the most common mistakes in the examples of phenyl propionate (Basic Level, Task 25) and ethyl benzoate (Basic Level, Task 28), which are isomeric esters. Tasks 26, 29, 53, and 54 in the Basic level can serve as another source of tasks dedicated to the recognition of isomeric esters.
Task 25 (phenyl propionate) received 9,062 answers (as of January 2, 2024) with a success rate of 27%. Interestingly, the second most frequently submitted answer was the structure of ethyl benzoate (Figure 3). This incorrect structure was submitted in 24% of answers, so the ratio between correct and the most common incorrect answers is almost 1:1. In addition, 5% of the submissions ascribed the spectra to 4-ethylbenzoic acid. The first ten most common answers submitted in response to all assignments discussed in this paper are provided in the Supporting Information (SI).

Figure 3

Figure 3. Most common structures submitted as solutions to Tasks 25 and 28 of the Basic Level. Percentages and total numbers of each particular structure submissions. The correct solution is highlighted in green.

In task 28, ethyl benzoate was correctly recognized in 64% of the answers, and only 10% confused the structure with phenyl propionate. We hypothesize that the higher success rate for Task 28 is caused by its classification as Moderate. Tasks of the Moderate level are likely to be taken up by more experienced users. This tendency is observable also with Tasks 26 (Moderate) and 54 (Hard), which present the isomeric esters benzyl acetate and methyl 2-phenyl acetate. The success rate for Task 26 is 35% (3,078 answers) and that for Task 54 is 50% (1,137 answers). With both tasks, the two isomeric ester structures were confused in less than 15% of submissions. We can conclude that more difficult tasks tend to be taken up by fewer, more experienced users, which increases their success rate.
Many students probably start the structure elucidation by examining the 1H spectrum because it conveys more structural information due to the possibility of the quantitative integration of signals and their splitting caused by J-coupling. Students seem to understand the integration and J-coupling very well: The ethyl chain was correctly recognized in at least 72% and 83% of submissions for Task 25 and Task 28, respectively. Moreover, at least 60% of answers correctly contain a monosubstituted benzene ring in Task 25 (and 72% in Task 28). Although the users correctly recognized the ethyl chain of phenyl propionate, one-quarter of them stayed unaware of the low chemical shift of the CH2 group and attached it to an oxygen atom. For comparison, Figure 4 presents the 1H NMR spectra of both isomeric esters, phenyl propionate and ethyl benzoate.

Figure 4

Figure 4. Part of the 1H NMR spectra of isomeric phenyl propionate and ethyl benzoate with highlighted functional fragments and their usual chemical shift ranges. Full spectra are shown in SI.

Although 13C spectra do not include as much structural information as 1H spectra, it is easier for beginners to analyze them first. In proton-decoupled spectra, all carbon signals appear as singlets, and their signals span a larger chemical shift range; therefore, overlaps of signals are uncommon. In 13C spectra of the discussed esters (phenyl propionate and ethyl benzoate), we can consider four regions, as shown in Figure 5 – carboxylic acids and their derivatives (ester group), aromatic, alkoxy and aliphatic.

Figure 5

Figure 5. 13C NMR spectra of isomeric phenyl propionate and ethyl benzoate with highlighted functional groups and their usual chemical shift ranges.

The chemical shifts of the signals of methyl, aromatic CH and carbonyl carbons are almost the same in both structures (Figure 5). Isomeric esters are identifiable based on the signal produced by the C–O connection. In phenyl propionate, the phenyl is directly attached to an oxygen atom, resulting in a chemical shift of the quaternary aromatic carbon of 151 ppm, which is 20 ppm greater than the signal of the quaternary carbon in ethyl benzoate possessing a C–C connection. Similarly, the ethyl chain is attached to the carbonyl group in phenyl propionate, and its CH2 signal is found at 28 ppm. On the other hand, the CH2 signal shows up at about 61 ppm in ethyl benzoate with a CH2–O connection.
The success rate of the recognition of the individual structural fragments of phenyl propionate is shown in Figure 6. The monosubstituted phenyl ring was recognized in 60% of the answers, but the phenoxy connection was identified in only 28% of them. This connection is reflected by the high chemical shift of 150 ppm of aromatic quaternary carbon. Moreover, the ethyl chain was correctly recognized in 72% of answers, but only 42% contained the correct formula CH3–CH2–C. The ester group was recognized in 55% of answers, but the structure was correctly identified as phenyl propionate in only 27% of cases.

Figure 6

Figure 6. Proportions of correctly and incorrectly recognized molecular fragments of phenyl-propionate.

In summary, students seem to be well oriented in the recognition of individual fragments of the molecule (a monosubstituted benzene ring, an ester group, and an ethyl chain), but they tend to overlook the chemical shift values, which are crucial for the proper connection of these fragments into the correct molecular structure. In frontal lessons, it may help to focus on the spectral tasks of isomeric structures and to determine structural fragments and their connections. We also recommend comparing spectra of esters with spectra of their starting materials–an alcohol and a carboxylic acid. Additional recommendations for NMR teachers are shown in the SI.

Case Study 2: Recognition of Benzene Substitution in Disubstituted Benzenes

Benzene substitution patterns can be, in many cases, determined based on J-splitting in 1H NMR spectra. At NMR-Challenge.com, there are 27 tasks of the Basic level (9, 22, 24, 30, 31, 44, 46, 56, 55, 62, 63, 64, 65, 83, 84, 85, 89, 90, 104, 105, 108, 112, 113, 137, 144, 145, and 147) and 8 tasks of the Advanced level (10, 11, 13, 14, 15, 17, 18, and 19) focusing on disubstituted benzene rings.
We analyzed a series of tasks containing three isomers of tolyl acetates (Figure 7). Tolyl acetates occur in the Moderate (para) and Hard (ortho) sections of the Basic level, and meta-tolyl acetate is included in the Advanced level. Students seem to be well oriented in para-substitution because they correctly recognized p-tolyl acetate in 41% of submissions (835 out of 2,035); however, the para-substitution pattern of the benzene ring was recognized in as many as 73% of submissions. In the case of ortho-substitution, students correctly recognized the substitution pattern of the benzene ring in 66% of answers, although only 33% (513) matched the correct structure. The success rate of recognizing the meta substitution pattern in m-tolyl acetate was even lower (55%), with less than 30% (285 of 937) of fully correct answers (Figure 7).

Figure 7

Figure 7. Structures of tolyl acetates, the success rate, and the total number of answers. Graphics describing the percentage of correctly recognized benzene substitution: wrongly recognized substitution (orange), correct answer (green) and wrong answer but correctly recognized substitution (blue).

The high success rate of recognizing para-disubstituted structures may stem from its symmetry leading to a reduced number of proton and carbon signals and a characteristic splitting pattern in 1H spectra (Figure 8). In para disubstituted benzenes, such as p-tolyl acetate, the spin system is AA′BB′. The hydrogen atoms in ortho positions to a substituent are chemically equivalent, but they are not magnetically equivalent; therefore, their signals should be described as multiplets.

Figure 8

Figure 8. Characteristic fingerprints in the high-resolution 1H NMR spectra of tolyl acetates.

By contrast, in cases of ortho- and meta-disubstituted benzenes, it is useful to approach spectra while bearing in mind the n + 1 rule and to recognize singlet-, doublet-, and triplet-like signals. As depicted in Figure 8, o-tolyl acetate exhibits two doublet-like and two triplet-like signals arising from the J-couplings. Similarly, in meta substituted benzenes, we can observe two doublet-like signal, one singlet-like signal and one triplet-like signal (Figure 8).
We also analyzed the results of tasks involving ortho- and para- dimethylbenzenes (xylenes). In the case of p-xylene, students recognized the substitution pattern correctly in 78% of answers, but in that of o-xylene, they assigned the structure correctly in only 52% of submissions. It is interesting to compare the proton spectra of o-tolyl acetate and o-xylene available at NMR-Challenge.com: The multiplet analysis of aromatic signals cannot be used in the case of o-xylene because of signal overlap but this region is well resolved in case of o-tolyl acetate (see Figures 8 and 9), the success rate of o-tolyl acetate is higher (66% to 52%), which indicates that the highly resolved multiplets observed with o-tolyl acetate increase the success rate.

Figure 9

Figure 9. Aromatic part of 1H and 13C spectra of p- and o- xylenes and the proportions of correctly and incorrectly recognized benzene substitutions.

The substitution of dimethylbenzenes can be determined solely from 13C spectra. Due to the symmetry of the molecule, p-xylene shows only two signals in the 13C aromatic region, o-xylene exhibits three signals (Figure 9) and m-xylene four (not yet included in NMR-Challenge.com).
We can conclude that students are well educated in recognizing para-substituted benzenes. On the other hand, they are not used to characterizing the substitution of aromatic rings using high-resolution proton spectra; for example, the rate with which benzene substitutions were recognized successfully was 66% in the case of o-tolyl acetate and 55% in m-tolyl acetate. The lower success rate in the recognition of o-xylene is caused by the overlapping aromatic signals in the 1H spectrum. The data clearly show how students benefit from high resolution spectra of aromatic regions. Regardlesss of the resolution of proton spectra, users do not fully utilize the potential of 13C spectra even though homodisubstituted benzenes can be easily distinguished by the number of 13C signals. Additional recommendations for NMR teachers on how to train students in correct recognition of benzene substitution are shown in the SI.

Case Study 3: Intramolecular Interaction

Intramolecular interactions, such as the formation of a hydrogen bond between an exchangeable hydrogen atom (OH, SH and NH2 groups) and a hydrogen bond acceptor (O, S or N atom), are characterized by a chemical shift change of the involved hydrogen atom.
We analyzed responses to tasks involving hydroxyphenones (Figure 10). In 2-hydroxy acetophenone, there is a strong intramolecular hydrogen bond between the acidic phenol hydrogen and the carbonyl group, leading to a chemical shift of the hydroxyl hydrogen atom of 12 ppm. However, no such intramolecular interaction is possible in 4-hydroxy acetophenone and the chemical shift of OH is substantially smaller.

Figure 10

Figure 10. Spectra of the isomeric hydroxyacetophenones. Note the large chemical shift change of the phenolic proton involved in the intramolecular hydrogen bond.

Task 43, included in the Hard section of the Basic level, is 2-hydroxy propiophenone (Figure 11). The success rate of this task is poor (16%). The most common answer (26%) is 2-ethylbenzoic acid. The ortho-substitution of the benzene ring was correctly recognized in 52% of submitted solutions, which corresponds to the range of 52–66% seen with o-xylene and o-tolyl acetate. More than half of all submissions (57%) incorrectly assign the structure to a carboxylic acid, even though there is a signal at 207 ppm in the 13C spectrum, which clearly indicates the presence of a ketone. Students probably tend to assign the structure preferentially based on 1H spectra, but they misinterpret the signals at 12 ppm in 1H and 162 ppm in 13C as a fingerprint of a carboxylic acid (Figure 11). Similar situation occurs also in other tasks including intramolecular hydrogen bonding between a phenolic hydrogen and a carbonyl group, e.g., tasks 144 (Basic) and 5 (Advanced).

Figure 11

Figure 11. A) Correct answer to Task 43, B) NMR spectra presented at NMR-Challenge.com, and C) an analysis of responses to this task.

We recommend that frontal teachers of NMR spectroscopy focus on intramolecular interactions besides the usual chemical shift ranges. The signal of a proton involved in an intramolecular hydrogen bond is usually sharper than that of a carboxylic acid that is usually involved in intermolecular hydrogen bonding (dimerization); (32) see, for example, the carboxylic proton in 4-ethylbenzoic acid (Figure S1). Therefore, the shape of the signal can also help interpret the structures correctly. Additional recommendations for NMR teachers are shown in the SI.

Conclusion

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NMR spectral interpretation is one of the core skills of all chemists across the chemical disciplines. Although the only way to become good at NMR spectral interpretation is through practice, the number of freely accessible spectral problems is limited, and the current state of knowledge of students is unknown. We therefore introduced the educative webpage NMR-Challenge.com containing more than 500 real NMR spectra of 200 organic compounds. This webpage receives worldwide attention and has reached more than 428,000 submitted solutions over a mere 20 months. According to our knowledge, this makes our database the largest set of responses to NMR assignments.
We analyzed the success rates of all 200 tasks and identified the patterns of the most common mistakes. In this article, we discuss spectral problems concerning the recognition of isomeric esters, substitutions on the benzene ring, and intramolecular hydrogen bonding. Our data reveal that students do not fully utilize the potential of chemical shift values when interpreting NMR spectra. They also often do not take into account intramolecular interactions affecting chemical shifts. On the other hand, they are well oriented in J-coupling splitting in aliphatic chains. Nevertheless, they also show little experience with the multiplicity of aromatic hydrogen atom signals in disubstituted benzenes.
We recommend including more tasks involving isomeric compounds in the curricula of introductory NMR courses and, when it comes to isomeric esters, discussing the spectra of the parent compounds–the alcohol and acid–to encourage students to think critically about these tasks. Examining tasks involving intramolecular hydrogen bonds can increase the awareness of this phenomenon. Spectra of hydroxyacetophenones can be used as model problems.
Students show good orientation in the recognition of para-disubstitution of the benzene ring; however, they frequently fail to recognize ortho- and meta-disubstituted structures. The data suggest that students do not concentrate on the multiplicity of aromatic hydrogen signals.
On average, NMR-Challenge.com collects more than 1,000 new submissions per day, and the number of spectral tasks it offers will be increased continuously. Besides being an educational resource, it can serve as a general indicator of the current state of knowledge attained by students of NMR spectroscopy.

Supporting Information

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The Supporting Information is available at https://pubs.acs.org/doi/10.1021/acs.jchemed.4c00092.

  • NMR spectra of 4-ethyl benzoic acid, phenyl propionate and ethyl benzoate; the most common structures submitted to the discussed tasks; additional recommendations for NMR teachers (PDF)

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Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html.

Author Information

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  • Corresponding Author
  • Authors
    • Zuzana Osifová - Institute of Organic Chemistry and Biochemistry, Czech Academy of Science, Flemingovo nám. 2, 160 00 Prague, Czech RepublicDepartment of Organic Chemistry, Faculty of Science, Charles University, Hlavova 2030, Prague 128 00, Czech Republic
    • Ondřej Socha - Institute of Organic Chemistry and Biochemistry, Czech Academy of Science, Flemingovo nám. 2, 160 00 Prague, Czech RepublicOrcidhttps://orcid.org/0000-0002-7218-9119
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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This work was supported by the Czech Science Foundation, grant number 22-15374S.

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    Socha, O.; Osifová, Z.; Dračínský, M. NMR-Challenge.com: An Interactive Website with Exercises in Solving Structures from NMR Spectra. J. Chem. Educ. 2023, 100 (2), 962968,  DOI: 10.1021/acs.jchemed.2c01067
  30. 32
    Socha, O.; Dračínský, M. Dimerization of Acetic Acid in the Gas Phase-NMR Experiments and Quantum-Chemical Calculations. Molecules 2020, 25 (9), 2150,  DOI: 10.3390/molecules25092150

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  • Abstract

    Figure 1

    Figure 1. Scheme of using NMR-Challenge.com in three steps–analysis of spectra, drawing of a chemical structure, and receiving immediate feedback.

    Figure 2

    Figure 2. Usage of the NMR-Challenge.com website between May 2022 and December 2023 in terms of the number of submissions per day (only structures with correct chemical formula are counted). Note the lower usage during the summer and winter holiday seasons. The massive increase in submissions after October 21, 2022 was ignited by a Twitter announcement by the Institute of Organic Chemistry and Biochemistry of the Czech Academy of Sciences.

    Figure 3

    Figure 3. Most common structures submitted as solutions to Tasks 25 and 28 of the Basic Level. Percentages and total numbers of each particular structure submissions. The correct solution is highlighted in green.

    Figure 4

    Figure 4. Part of the 1H NMR spectra of isomeric phenyl propionate and ethyl benzoate with highlighted functional fragments and their usual chemical shift ranges. Full spectra are shown in SI.

    Figure 5

    Figure 5. 13C NMR spectra of isomeric phenyl propionate and ethyl benzoate with highlighted functional groups and their usual chemical shift ranges.

    Figure 6

    Figure 6. Proportions of correctly and incorrectly recognized molecular fragments of phenyl-propionate.

    Figure 7

    Figure 7. Structures of tolyl acetates, the success rate, and the total number of answers. Graphics describing the percentage of correctly recognized benzene substitution: wrongly recognized substitution (orange), correct answer (green) and wrong answer but correctly recognized substitution (blue).

    Figure 8

    Figure 8. Characteristic fingerprints in the high-resolution 1H NMR spectra of tolyl acetates.

    Figure 9

    Figure 9. Aromatic part of 1H and 13C spectra of p- and o- xylenes and the proportions of correctly and incorrectly recognized benzene substitutions.

    Figure 10

    Figure 10. Spectra of the isomeric hydroxyacetophenones. Note the large chemical shift change of the phenolic proton involved in the intramolecular hydrogen bond.

    Figure 11

    Figure 11. A) Correct answer to Task 43, B) NMR spectra presented at NMR-Challenge.com, and C) an analysis of responses to this task.

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  • Supporting Information

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    • NMR spectra of 4-ethyl benzoic acid, phenyl propionate and ethyl benzoate; the most common structures submitted to the discussed tasks; additional recommendations for NMR teachers (PDF)


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