Chemistry
Hydrogen -1 NMR
Hydrogen-1 NMR, also known as proton NMR, is a technique used to study the chemical environment of hydrogen atoms in a molecule. It relies on the magnetic properties of hydrogen nuclei and their interaction with an external magnetic field. By analyzing the resulting NMR spectrum, valuable information about the structure, connectivity, and dynamics of molecules can be obtained.
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12 Key excerpts on "Hydrogen -1 NMR"
eBook - PDF- David R. Klein(Author)
- 2021(Publication Date)
- Wiley(Publisher)
NMR spectroscopy involves the study of the interaction between electromagnetic radiation and the nuclei of atoms. A wide variety of nuclei can be studied using NMR spectroscopy, including 1 H, 13 C, 15 N, 19 F, and 31 P. In practice, 1 H NMR spectroscopy and 13 C NMR spectroscopy are used most often by organic chemists, because hydrogen and carbon are the primary constituents of organic compounds. Analysis of an NMR spectrum provides information about how the individual carbon and hydrogen atoms are connected to each other in a molecule. This information enables us to determine the carbon- hydrogen framework of a compound, much the way puzzle pieces can be assembled to form a picture. A nucleus with an odd number of protons and/or an odd number of neutrons possesses a quan- tum mechanical property called nuclear spin, and it can be probed by an NMR spectrometer. Con- sider the nucleus of a hydrogen atom, which consists of just one proton and therefore has a nuclear spin. Note that this property of spin does not refer to the actual rotation of the proton. Nevertheless, it is a useful analogy to consider. A spinning proton can be viewed as a rotating sphere of charge, which generates a magnetic field, called a magnetic moment. The magnetic moment of a spinning proton is similar to the magnetic field produced by a bar magnet (Figure 15.1). FIGURE 15.1 (a) The magnetic moment of a spinning proton. (b) The magnetic field of a bar magnet. Direction of rotation N S Axis of spin and of the magnetic moment Magnetic lines of force N S (a) (b) DO YOU REMEMBER? Before you go on, be sure you understand the following topics.
eBook - PDF- David R. Klein(Author)
- 2020(Publication Date)
- Wiley(Publisher)
In this chapter, we will see the role that diamagnetism plays in nuclear magnetic reso- nance (NMR) spectroscopy, which provides more structural information than any other form of spectroscopy. We will also learn how NMR spectroscopy is used as a power- ful tool for structure determination. 696 CHAPTER 16 Nuclear Magnetic Resonance Spectroscopy 16.1 INTRODUCTION TO NMR SPECTROSCOPY Nuclear magnetic resonance (NMR) spectroscopy is arguably the most powerful and broadly appli- cable technique for structure determination available to organic chemists. Indeed, the structure of a compound can often be determined using NMR spectroscopy alone, although in practice, structural determination is generally accomplished through a combination of techniques that includes NMR and IR spectroscopy and mass spectrometry. NMR spectroscopy involves the study of the interaction between electromagnetic radiation and the nuclei of atoms. A wide variety of nuclei can be studied using NMR spectroscopy, including 1 H, 13 C, 15 N, 19 F, and 31 P. In practice, 1 H NMR spectroscopy and 13 C NMR spectroscopy are used most often by organic chemists, because hydrogen and carbon are the primary constituents of organic compounds. Analysis of an NMR spectrum provides information about how the individual carbon and hydrogen atoms are connected to each other in a molecule. This information enables us to determine the carbon- hydrogen framework of a compound, much the way puzzle pieces can be assembled to form a picture. A nucleus with an odd number of protons and/or an odd number of neutrons possesses a quantum mechanical property called nuclear spin, and it can be probed by an NMR spectrometer. Consider the nucleus of a hydrogen atom, which consists of just one proton and therefore has a nuclear spin. Note that this property of spin does not refer to the actual rotation of the proton.
eBook - ePubSpin Resonance Spectroscopy
Principles and applications
- Chandran Karunakaran, CHANDRAN KARUNAKARAN(Authors)
- 2018(Publication Date)
- Elsevier(Publisher)
1 H Nuclear Magnetic Resonance2.1.1. What Can Be Obtained From a 1 H Nuclear Magnetic Resonance Spectrum?
Analysis and interpretation of nuclear magnetic resonance (NMR) spectrum provided the following useful informations to understand the structure of a molecule and compound [1 –3 ].1. The number of peaks (in low-resolution NMR without spin–spin splitting) tells us about how many different kinds or types of proton present in the molecule. 2. The chemical shift values, i.e., the positions of the peaks tell us about the electronic environment of each kind of proton and the functional group. 3. The relative intensities of the peaks (or area of the peaks) tell us about the number of protons of each kind, which are present. The area of the peak is obtained by integration.4. The spin–spin splitting of a peak into several peaks (in high-resolution NMR) identifies the environment of a proton with respect to other nearby protons in the molecule. In other words, it tells us about adjacent functional groups and therefore connectivity.For example, CH3 protons connected to CH2 protons show triplet signal.2.2. Assignment of 1 H Nuclear Magnetic Resonance
The different positions of NMR absorptions are described as chemical shifts (δ). A chemical shift is defined as the difference in parts per million (ppm) between the resonance frequency of the observed proton and that of the tetramethylsilane (TMS) hydrogens. TMS is the most common reference compound used in NMR. It is set at δ = 0 ppm.Chemical shift , ppm δ =( Frequency of signal − frequency of reference )×10 6Spectrometer frequency( in Hz )The assignment of chemical shift values of each peak to protons in the functional group of molecules is given in Table 2.1 .The proton chemical shifts exhibit a range from − 20 (for metal hydrides viz ., [H-Rh(CN)5 ]3- ) to 40 ppm (for naked protons). But the majority of the organic compounds appear in the range 0–10
- Kenneth Williamson, Katherine Masters(Authors)
- 2016(Publication Date)
- Cengage Learning EMEA(Publisher)
These small varia-tions, called chemical shifts, are plotted versus signal intensity to produce the NMR spectrum. The interpretation of these signals and other spectral features such as splitting patterns and peak areas, as described in the following sections, facilitates organic structure elucidation. 1 H NMR: Determination of the number, kind, and relative locations of hydrogen atoms (protons) in a molecule. Chemical shift, δ (ppm) Nuclear Magnetic Resonance Spectroscopy CHAPTER 12 PRE-LAB EXERCISE: Outline the preliminary solubility experiments you would carry out using inexpensive solvents before preparing a solution of an unknown compound for nuclear magnetic resonance (NMR) spectros-copy using expensive deuterated solvents. 240 Copyright 2017 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 241 Chapter 12 ■ Nuclear Magnetic Resonance Spectroscopy I N T E R P R E T A T I O N O F 1 H N M R S P E C T R A There are two approaches to interpreting proton NMR spectra as follows: 1. The structure from the spectrum approach is the strategy of using the informa-tion in the NMR spectrum to draw the structure of the molecule based on ref-erence tables and rules. This approach is used if the compound’s structure is unknown. In this case, the NMR spectrum alone is often insufficient to “solve” the complete structure and must be combined with knowledge about the com-pound’s source (synthetic reaction or natural product) and complementary spectral data (infrared, ultraviolet, and/or mass spectrometric).
eBook - PDFModelling 1H NMR Spectra of Organic Compounds
Theory, Applications and NMR Prediction Software
- Raymond J. Abraham, Mehdi Mobli(Authors)
- 2008(Publication Date)
- Wiley(Publisher)
This was first observed by Knight 3 in metals and metal salts and later by Dickinson 4 for the 19 F nuclei in fluorocompounds. Also Proctor and Yu 5 observed two signals in the 14 N spectrum of ammonium nitrate. They attributed this unexpected result to some ‘nasty chemical effect’. Thus the phenomenon of nuclear chemical shifts was discovered. Further advances in magnet resolution allowed the historic experiment of Arnold, Dharmatti and Packard, 6 when they resolved the three types of hydrogen atoms in ethanol (Figure 1.1), the first example of 1 H chemical shifts and this illustrated the immense potential of 1 H NMR in structural organic chemistry. Since this original discovery 1 H NMR spectroscopy is now widely used in all scientific disciplines from physics to medicine and is now even part of the high school syllabus. It is also the most common and powerful analytical tool of the research scientist. The detection of the hydrogen atom 1 H resonances in a molecule was possible since this isotope has a spin of 1 / 2 , is magnetically active, has a high natural abundance and is present in most organic compounds. The other nucleus of general interest for the organic chemist, the carbon 12 C isotope, has zero spin and therefore no magnetic moment. The 13 C nucleus has spin 1 / 2 but has a natural abundance of only ca. 1 %. For this reason it took another two decades for the first 13 C NMR spectrum with acceptable quality to be produced. Modelling 1 H NMR Spectra of Organic Compounds: Theory, Applications and NMR Prediction Software Raymond Abraham and Mehdi Mobli c 2008 John Wiley & Sons, Ltd 2 Modelling 1 H NMR Spectra of Organic Compounds Figure 1.1 The first NMR spectrum of ethanol (from Arnold, Dharmatti and Packard, 6 reproduced by permission of the American Institute of Physics).
eBook - ePub- Thomas J. Bruno, James W. Robinson, George M. Frame II, Eileen M. Skelly Frame(Authors)
- 2023(Publication Date)
- CRC Press(Publisher)
13 C, hence the name “incredible.” COSY: Correlated spectroscopy Homonuclear 2D experiment; plot of chemical shift versus chemical shift identifies spin-coupled resonances. Many variations permit measurement of J coupling constants, long-range connectivity, suppression, and enhancement of selected resonances. HETCOR: Heteronuclear chemical shift correlated experiment Heteronuclear 2D experiment; usually to connect 1 H resonances with 13 C resonances or 1 H–X, where X is another NMR-active nucleus. Plot is 13 C chemical shift (or X chemical shift) versus 1 H chemical shift. NOESY: Nuclear Overhauser effect spectroscopy Identifies dipolar-coupled nuclei within certain distances (e.g., within 0.4 nm for first-order coupling) and identifies connectivities through cross-relaxation.Source: Modified from Petersheim, M., Nuclear magnetic resonance, in Ewing, G.A. ed., Analytical Instrumentation Handbook, 2nd edn., Marcel Dekker Inc., New York, 1997. With permission.6.6.3 Interpretation of Proton Spectra
NMR absorption spectra are characterized by the chemical shift of peaks and spin–spin splitting of peaks. Recall that the chemical shift is caused by the drifting, not orbiting or spinning, of nearby electrons under the influence of the applied magnetic field. It is therefore a constant depending on the applied field (i.e., if the field is constant, the chemical shift is constant). The chemical shift therefore identifies the functional group, such as methyl, methylene, aldehydic H, aromatics, and so on (see Table 6.3 ). All proton spectra shown have TMS as the reference, with the TMS absorbance set at 0.0 ppm. The student should note that all of the 300 MHz proton NMR spectra provided by Aldrich Chemical Company, Inc., also include the 75 MHz 13 C spectrum at the top. 13 C NMR spectra are discussed in Section 6.6.4.Spin–spin splitting is caused by adjacent nuclei and is transmitted through the bonds. It is independent of the applied field. The multiplicity is therefore a function of the number of equivalent 1 H nuclei in the adjacent functional groups. Numerically, it is equal to (2nI + 1), where n is the number of equivalent 1 H and I is the spin number (in this case, I = 1/2). For two adjacent groups, the number is (2nI + 1) (2n′I′ + 1) where n and n′ are the numbers of 1 H nuclei in each separate group and I and I
- Haleem J. Issaq, Timothy D. Veenstra(Authors)
- 2019(Publication Date)
- Academic Press(Publisher)
Chapter 7Current NMR strategies for biomarker discovery
Que N. Van Laboratory of Proteomics and Analytical Technologies, Advanced Technology Program, SAIC-Frederick, Inc., Frederick National Laboratory for Cancer Research, Frederick, MD, United StatesAbstract
The viability of nuclear magnetic resonance (NMR) spectroscopy as a tool for biomarker discovery has increased over the years as improvements in instrumentation that address NMR's inherent low sensitivity and bioinformatics resources for NMR data handling and analysis have matured. In this chapter, current NMR strategies for metabolic profiling are surveyed. The main focus is analysis of small molecules in biological samples using high-resolution liquid-state techniques. Analysis of cell and tissue via ex vivo and in vivo methods is also discussed. The methodologies covered include one- and two-dimensional experiments, as well as sample preparation and data treatment for statistical analysis.Keywords
Nuclear magnetic resonance spectroscopy; NMR; MRS; HR-MAS; Biomarker; Metabolomics; Metabolic profiling; BiofluidsOutline- Introduction: Why NMR?
- Advancements in NMR hardware
- Sample preparation for NMR analysis
- Biological fluids without macromolecules
- Biological fluids with macromolecules
- Cells and tissue extracts
- Intact tissue for HR-MAS
- Internal and external chemical shift standards
- One-dimensional NMR methods: 1 H, 13 C, 31 P
- 1 H
- 13 C
- 31 P
- 2D methods
- Homonuclear 2D
- Heteronuclear 2D: 1 H-13 C HSQC
- Targeted metabolic profiling
- Targeted analysis: Stable isotope tagging
- Targeted analysis: Metabolite specific
- Flux analysis using 13 C labeling
- High-resolution magic angle spinning (HR-MAS) NMR spectroscopy
- Magnetic resonance spectroscopy (MRS)
- NMR data processing and preparation for statistical analysis
- Data postprocessing
- Spectral alignment
eBook - ePub- S. A. Richards, J. C. Hollerton(Authors)
- 2022(Publication Date)
- Wiley(Publisher)
9 Carbon-13 NMR Spectroscopy9.1 General Principles and 1-D 13 C
13 C NMR gives us another vast area of opportunity for structural elucidation and is incredibly useful in many cases where compounds contain relatively few protons, or where those that are available are not particularly diagnostic with respect to the proposed structures. Before we delve into any detail, there are certain general observations which we need to make regarding 13 C NMR and the fundamental differences that exist between it, and 1H NMR.For a start, we must be mindful of the fact that 13 C is only present as 1.1% of the total carbon content of any organic compound. This, in combination with an inherently less sensitive nucleus, means that signal-to-noise issues will always be a major consideration in the acquisition of 13 C spectra – particularly 1-D 13 C spectra which we will restrict the discussion to for the moment. (Note that the overall sensitivity of 13 C, probe issues aside, is only about 0.28% that of proton because the nucleus resonates at a far lower frequency – in a 400 MHz instrument, 13 C nuclei resonate at around 100 MHz.) So it takes a great deal longer to acquire 13 C spectra than it does proton spectra. More material is obviously an advantage but can in no way make up for a 350-fold inherent signal-to-noise deficiency!Another important aspect of 13 C NMR is that the signals are never normally integrated. The reason for this is that some carbon signals have quite long relaxation times. In order to make NMR signals quantitative, acquisition must allow for a relaxation delay (delay period between acquisition pulses) of at least five times the duration of the slowest relaxing nuclei in the compound being considered. With relaxation times of the order of 10–20 seconds, it is therefore obvious why we cannot obtain quantitative 13 C data. The inherent insensitivity of the 13 C nucleus often demands thousands of scans to achieve acceptable signal/noise so we can ill afford 100 second relaxation delays between pulses! The only thing that we can say is that methine, methylene and methyl carbons generally
eBook - PDF- David R. Klein(Author)
- 2016(Publication Date)
- Wiley(Publisher)
This effect is quite strong, producing a signal at −1 ppm (even further upfield than TMS). All π bonds exhibit a similar anisotropic effect. That is, π electrons circulate under the influence of an external magnetic field, generating a local magnetic field. For each type of π bond, the pre- cise location of the nearby protons determines their chemical shift. For example, aldehydic protons produce characteristic signals at approximately 10 ppm. Table 15.2 summarizes important chemical shifts. It would be wise to become familiar with these numbers, as they will be required in order to interpret 1 H NMR spectra. TYPE OF PROTON CHEMICAL SHIFT (δ) Methyl R CH 3 ∼ 0.9 Methylene CH 2 ∼ 1.2 Methine CH ∼ 1.7 Allylic H ∼ 2 Alkynyl R H ∼ 2.5 Aromatic methyl CH 3 ∼ 2.5 TYPE OF PROTON CHEMICAL SHIFT (δ) Alkyl halide C X R H R 2–4 Alcohol O R H 2–5 Vinylic H 4.5–6.5 Aryl H 6.5–8 Aldehyde R H O ∼ 10 Carboxylic acid R O O H ∼ 12 TABLE 15.2 CHEMICAL SHIFTS FOR PROTONS IN DIFFERENT ELECTRONIC ENVIRONMENTS CONCEPTUAL CHECKPOINT 15.10 For each of the following compounds, identify the expected chemical shift for each type of proton: (a) H O H (b) OH (c) (d) O 15.6 Integration 667 15.6 Integration In the previous section, we learned about the first characteristic of every signal, chemical shift. In this section, we will explore the second characteristic, integration, or the area under each signal. This value indicates the number of protons giving rise to the signal. After acquiring a spectrum, the computer calculates the area under each signal and then displays this area as a numerical value placed either above or below the signal. 3.0 2.8 2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.6 ppm 0.8 Proton NMR Integration Values 27.0 40.2 28.4 42.2 These numbers only have meaning when compared to each other.
- Haleem J. Issaq(Author)
- 2013(Publication Date)
- Academic Press(Publisher)
Metabolomics Database of Linkoping (MDL) 1 H, 13 C, 15 N, 31 P http://www.liu.se/hu/mdl/main NMRShiftDB 1 H, 13 C, 15 N, 31 P http://nmrshiftdb.nmr.uni-koeln.de Spectral Database for Organic Compounds (SDBS) 1 H, 13 C http://riodb01.ibase.aist.go.jp/sdbs/cgi-bin/cre_index.cgi Purdue Isotope Enhanced NMR (PIE-NMR) Metabolite Database 13 C and 15 N tagged metabolites http://www.chem.purdue.edu/raftery/pie-nmr/pie-nmr.html∗ Some databases contain additional information not listed under the “NMR Data” heading.Future Directions and Conclusion
The work flow from biomarker discovery of biological specimens using high-resolution liquid-state spectroscopy, confirmed ex vivo in whole cell and intact tissue using HR-MAS and then validated using in vivo MRS demonstrates the uniqueness and strength of NMR spectroscopy for translation of basic research to clinical applications. The work by Andronesi and colleagues demonstrated how magnetic resonance imaging technology already routine in the clinic can be used for noninvasive in vivo molecular biomarker detection.139“NMR spectroscopy is a low-sensitivity technique” is a familiar refrain among literature reviews and editorials comparing available instrumentation platforms for metabolic profiling and biomarker discovery. One could argue that the NMR spectrometer actually has very high detection sensitivity considering the fact that it is detecting a negligible fraction of the nuclei in the sample. The NMR signal is directly proportional to the population difference between the two energy states for spin-½ nuclei. The Boltzmann distribution gives the relative populations between the low and high energy levels, and the population difference is extremely small. At 500 MHz, only about 1 out of every 10,000 nuclei contributes to the detected NMR signal.
- Nelu Grinberg, Sonia Rodriguez, Nelu Grinberg, Sonia Rodriguez(Authors)
- 2019(Publication Date)
- CRC Press(Publisher)
The goal of this section is to introduce the reader to analytical techniques that enable the characterization and study of molecular structure using NMR spectroscopy. In particular the application of modern NMR spectroscopic techniques to the structural elucidation of small organic molecules will be covered. The practical aspects of key NMR experiments will be explored. Topics covered will include the homonuclear and heteronuclear correlation experiments, and use of integration, chemical shift, and coupling constants in a structural example. Analysis of basic and complex molecules with overlapping signals will be discussed.For most structure elucidation problems encountered in organic chemistry there are seven basic experiments commonly applied. These include one-dimensional proton and one-dimensional carbon-13 experiments. In addition, two-dimensional heteronuclear 1 H,1 H COSY, 1 H,1 H TOCSY, I H,I H NOESY, or I H,l H ROESY, and heteronuclear 1 H,13 C HSQC, 1 H,13 C HMQC, and 1 H,13 C HMBC experiments are required.For NMR structural characterization, 2D homonuclear correlation experiments that connect signals through chemical bonds include correlation spectroscopy (COSY) [45 ] and total correlation spectroscopy (TOCSY) [46 ]. The two-dimensional spectra are typically displayed as a 2D contour plot.In the COSY spectrum the 1D proton spectrum is traced on the diagonal of the plot. Peaks that are not on the diagonal are correlation peaks that are primarily a result of 3 J-coupling. By tracing a rectangle using the diagonal and cross peaks as vertices it is possible to determine which protons are coupled to each other. Because COSY experiments require phase cycling to remove unwanted signals, this can make experiments lengthy. Gradient-selected COSY (gCOSY), which utilizes pulsed field gradients to destroy unwanted z-magnetization, allows spectra to be acquired with one scan in as little as 5 minutes.The 1 H-1
- (Author)
- 2014(Publication Date)
- Elsevier(Publisher)
The structural assignment of a new natural product molecule is not only to establish the 3D structure of a compound, but potentially to provide the basis for research in a multitude of disciplines, ultimately generating new therapeutic agents and/or new understanding of disease biology. The development of modern spectroscopic techniques has transformed the structure assignment process, which previously was essentially based on chemical degradation or derivatization followed by partial or total synthesis. Notably, it was only in the specialization era of the spectroscopic structural assignment of natural products that the field of marine natural products chemistry took shape.Today the processes of marine and terrestrial natural product isolation and structural determination are frequently streamlined and expeditious due to the spectacular advances in chromatographic and spectroscopic technologies as well as chemical synthesis.The NMR spectroscopy is a powerful tool in structure elucidation because the properties it displays can be related to the molecular structure. The chemical environment of a particular nucleus is associated with the chemical shift (δ , ppm), and the area of a resonance, usually presented as its relative integral, is related to the number of nuclei giving rise to the NMR signal. The interactions between individual nuclei, mediated by electrons in a chemical bond, determine the coupling constant (J , Hz). In this chapter we will present the techniques commonly used, basic concepts, and how they are useful for chemists in the structural elucidation of mainly bioactive marine natural products. Its complex planar structure is determined by 1 H and 13 C NMR analysis strongly supported by other 1D (DEPT) and 2D (COSY, TOCSY, HSQC/HMQC, HMBC) NMR techniques. The stereochemistry is generally based on NOE experiments (NOE difference, NOESY, and ROESY), 1 H–1 H and 1 H–13
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