Chemistry
Carbon -13 NMR
Carbon-13 NMR is a spectroscopic technique used to study the carbon atoms in a molecule. It provides information about the chemical environment and connectivity of carbon atoms in a compound. By measuring the chemical shifts of carbon-13 nuclei, this technique helps in determining the structure and composition of organic molecules.
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9 Key excerpts on "Carbon -13 NMR"
- Robert M. Silverstein, Francis X. Webster, David J. Kiemle, David L. Bryce(Authors)
- 2014(Publication Date)
- Wiley(Publisher)
CHAPTER 4 CARBON-13 NMR SPECTROSCOPY ∗ 4.1 INTRODUCTION Faced with a choice during the early development of nuclear magnetic resonance spectroscopy, most organic chemists would certainly have selected the carbon nucleus over the hydrogen nucleus for immediate investigation. After all, the carbon skeletons of rings and chains are central to organic chemistry. The problem, of course, is that the carbon skeleton consists almost completely of the 12 C nucleus, which is not accessible to NMR spectroscopy. The spectroscopist is left to cope with the very small amount of the 13 C nucleus. There are enough differences between 13 C and 1 H NMR to justify separate chapters on pedagogical grounds. With an understanding of the basic concepts of NMR in Chapter 3, mastery of 13 C spectroscopy will be rapid. The 12 C nucleus is not magnetically active (spin number, I , is zero), but the 13 C nucleus, like the 1 H nucleus, has a spin number of 1 2 . However, since the natural abundance of 13 C is only 1.1% and its magnetogyric ratio is only about a quarter that of 1 H, the overall sensitivity of 13 C compared with 1 H is about 1 5870 . Because of the low natural abundance of 13 C, the occurrence of adjacent 13 C atoms has a low probability; thus, we are free of the complication of 13 C⏤ 13 C coupling. 4.2 THEORY The theoretical background for NMR has already been presented in Chapter 3. Some of the principal aspects of 13 C NMR to consider that differ from 1 H NMR are as follows: • In the commonly used proton-decoupled 13 C spectrum (see Section 4.2.1), the peaks are singlets unless the molecule contains other magnetically active nuclei such as 2 H, 31 P, or 19 F. • The 13 C peaks are distributed over a larger chemical shift range in comparison with the proton range. • 13 C peak intensities do not correlate with the number of carbon atoms associated with a given peak in routine spectra, due to variable T 1 values and the NOE.
eBook - ePub- Preeti Gupta, S. S. Das, N. B. Singh(Authors)
- 2023(Publication Date)
- Jenny Stanford Publishing(Publisher)
and agriculture application. Furthermore, 13 C NMR provides a number of advantageous features over 1 H NMR for elucidating structures of organic and biological molecules. The fact that the backbones of molecules are identified with clarity using 13 C NMR rather than extracting information from the periphery offers the technique an obvious advantage over 1 H NMR. Furthermore, chemical shifts for 13 C for most of the organic compounds of our interest are about 200 ppm, compared to ca. 10–15 ppm for 1 H. Individual resonance peaks for each carbon atom may conveniently be assigned in compounds with molecular weights ranging from 200 to 400. In addition, in unenriched samples, the probability of two 13 C atoms occurring in the same molecule is low, and hence homonuclear, spin–spin coupling in carbon atoms is not encountered. Furthermore, the spin quantum number of the 12 C is zero, thus 13 C and 12 C do not allow heteronuclear spin coupling. Finally, there are effective ways to decouple the possible interaction of 13 C atoms with available protons. This usually gives rise to a single line in the spectrum for a certain kind of carbon. 13 C NMR spectroscopy allows the measurement of all nuclei in most organic compounds ranging from small to medium-sized. In addition, quadrupole and indirect spin–spin coupling constants have also been measurable. There have been significant developments in interpreting chemical shifts using theoretical treatments allowing accurate determination of structure to render the technique to a versatile tool. The use of 13 C as an isotopic tracer in reactions is one important application that might be exploited. By introducing a molecule enriched in 13 C, identification of different forms of carbon in a complex mixture has been possible by rigorous analysis of the chemical shifts and fine structures
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)
- 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.
- S. A. Richards, J. C. Hollerton(Authors)
- 2022(Publication Date)
- Wiley(Publisher)
Essential Practical NMR for Organic Chemistry, Second Edition. S.A. Richards and J.C. Hollerton. © 2023 John Wiley & Sons Ltd. Published 2023 by John Wiley & Sons Ltd. 9.1 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 acquisi- tion 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 com- pound being considered. With relaxation times of the order of 10–20 seconds, it is therefore obvious why we cannot obtain quantitative 13 C data.
eBook - PDF- Stefan Bienz, Laurent Bigler, Thomas Fox(Authors)
- 2021(Publication Date)
- Thieme(Publisher)
199 Nuclear Magnetic Resonance Spectroscopy 3.4 13 C NMR Spectroscopy Table 3.51 13 C, 31 P coupling constants of selected compounds (in Hz) 200 3 Nuclear Magnetic Resonance Spectroscopy Table 3.52 1 J ( 13 C, 13 C) coupling constants for carbon atoms with dif-ferent hybridization 345 3.4.8 Special Techniques Since the very beginning, in the middle of the 20th century, NMR spectroscopy has developed terrifically. Many methods qualified originally as special measurements are nowadays routine. Sections 3.4.8 and 3.4.9 contain a survey of 1D and 2D techniques. Applications for structure determinations are in the foreground, whereas pulse sequences and other measurement details are only mentioned briefly. The following methods are discussed here: ▪ Enhancement of the measurement frequency/field strength. ▪ Application of special probeheads for the measurement of small quantities of samples or hardly soluble compounds. ▪ Spin decoupling: heteronuclear double resonance. ▪ J -Modulated spin-echo (attached proton test, APT). ▪ Spectrum integration. ▪ Use of lanthanide shift reagents. ▪ Specific isotope labeling. ▪ NOE measurements. ▪ Polarization transfer. The majority of these methods have already been described for 1 H NMR spectroscopic applications (section 3.3.8, p. 145). Two-dimensional techniques are discussed in Section 3.4.9. Enhancement of the Measurement Frequency/Field Strength The sensitivity of the measurement scales with B 0 3/2 . When the frequency for 13 C is enhanced from 100 to 150 MHz (by going from a 400- to a 600-MHz spectrometer), the sen-sitivity increases by a factor of 1.5 1.5 ≈ 1.8. A questionable signal, which just disappears in the electronic noise, can be unambiguously identified by this procedure. Moreover, the enhancement of the strength B 0 of the magnetic field leads to a linear increase of ∆ ν of neighboring signals and thus to an improvement of the resolution .
eBook - PDF- Donald Pavia, Gary Lampman, George Kriz, James Vyvyan, Donald Pavia, Gary Lampman, George Kriz, James Vyvyan(Authors)
- 2014(Publication Date)
- Cengage Learning EMEA(Publisher)
Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 308 Nuclear Magnetic Resonance Spectroscopy • Part Two: Carbon-13 Spectra The results of DEPT experiments may be used from time to time in this textbook to help you solve assigned exercises. In an effort to save space, most often only the results of the DEPT experi-ment, rather than the complete spectrum, will be provided. 6.11 SOME SAMPLE SPECTRA—EQUIVALENT CARBONS Equivalent 13 C atoms appear at the same chemical shift value. Figure 6.10 shows the proton-decoupled carbon spectrum for 2,2-dimethylbutane. The three methyl groups at the left side of the molecule are equivalent by symmetry. Although this compound has a total of six carbons, there are only four peaks in the 13 C NMR spec-trum. The 13 C atoms that are equivalent appear at the same chemical shift. The single methyl carbon a appears at highest field (9 ppm), while the three equivalent methyl carbons b appear at 29 ppm. The quaternary carbon c gives rise to the small peak at 30 ppm, and the methylene carbon d appears at 37 ppm. The relative sizes of the peaks are related, in part, to the number of each type of carbon atom present in the molecule. For example, notice in Figure 6.10 that the peak at 29 ppm ( b ) is much larger than the others. This peak is generated by three carbons. The quaternary carbon at 30 ppm ( c ) is very weak. Since no hydrogens are attached to this carbon, there is very little NOE enhancement. Without attached hydrogen atoms, relaxation times are also longer than for other car-bon atoms. Quaternary carbons, those with no hydrogens attached, frequently appear as weak peaks in proton-decoupled 13 C NMR spectra (see Sections 6.5 and 6.7). Figure 6.11 is a proton-decoupled 13 C spectrum of cyclohexanol. This compound has a plane of symmetry passing through its hydroxyl group, and it shows only four carbon resonances.
eBook - PDFChemistry & Physics of Carbon
Volume 31
- Ljubisa R. Radovic(Author)
- 2012(Publication Date)
- CRC Press(Publisher)
5, pp. 2909–2914. Chichester: Wiley, 1996. 25. VanderHart, D.L., Earl, W.L., and Garroway, A.N. Resolution in 13 C NMR of organic solids using high-power proton decoupling and magic-angle sample spin-ning. J. Magn. Reson . 1981;44:361–401. 26. Freitas, J.C.C., Emmerich, F.G., Cernicchiaro, G.R.C., Sampaio, L.C., and Bonagamba, T.J. Magnetic susceptibility effects on 13 C MAS NMR spectra of carbon materials and graphite. Solid State Nucl. Magn. Reson . 2001;20:61–73. 27. Preston, C.M. Carbon-13 solid state NMR of soil organic matter—Using the tech-nique effectively. Can. J. Soil Sci . 2001;81:255–270. 28. Abelmann, K., Totsche, K.U., Knicker, H., and Kögel-Knabner, I. CP dynamics of heterogeneous organic material: characterization of molecular domains in coals. Solid State Nucl. Magn. Reson . 2004;25:252–256. 29. Bartuska, V.J., Maciel, G.E., Schaefer, J., and Stejskal, E.O. Prospects for carbon-13 nuclear magnetic resonance of solid fossil-fuel materials. Fuel 1977;56:354–358. 30. Retcofsky, H.L., and VanderHart, D.L. 13 C— 1 H cross-polarization nuclear magnetic resonance spectra of macerals from coal. Fuel 1978;57:421–423. 31. VanderHart, D.L., and Retcofsky, H.L. Estimation of coal aromaticities by proton-decoupled carbon-13 magnetic resonance spectra of whole coals. Fuel 1976;55:202–204. 32. Kidena, K., Murata, S., and Nomura, M. Studies on the chemical structural change during carbonization process. Energy Fuels 1996;10(3):672–678. 33. NMR Wiki—Open NMR project. Available from: http://nmrwiki.org [Accessed 20 January 2010]. 34. Reich, H.J. C-13 NMR chemical shifts. Available from http://www.chem.wisc.edu/ areas/reich/handouts/nmr-c13/cdata.htm [Accessed 20 January 2010]. 35. Snape, C.E., Ladner, W.R., and Bartle, K.D. Survey of carbon-13 chemical shifts in aromatic hydrocarbons and its applications to coal-derived materials. Anal. Chem. 1979;51:2189–2198. 36. Wehrli, F.W., Marchand, A.P., and Wehrli, S.
- Wallace Brey(Author)
- 2012(Publication Date)
- Academic Press(Publisher)
These relations are sometimes expressed in terms of co, the angu-lar frequency in radians/sec, which is equal to 27TV. 13-4 SPECTRA OF OTHER NUCLEI NMR spectra of any of a variety of nuclei that have magnetic moments can in principle be obtained. The spectra of different nuclei cannot be confused, because the chemical shift range of any one nucleus is very small compared to the difference between the resonance frequencies of two nuclides. Spectra are most easily obtained for the two nuclei *H and 1 9 F because of their high natural abundance and large magne-togyric ratio. Fluorine has a wide range of chemical shifts. Figure 13-23 shows typical resonance positions of some functional groups and molecules containing fluorine. 1 9 F spectra have been used as indicators in various 13-4 SPECTRA OF OTHER NUCLEI 521 biological systems. Where it can be introduced by a chemical reaction, there is no interference with its resonance by a large number of other peaks as is true for a hydrogen absorption in an organic molecule. By suitable reactions CF 3 CO groups can be placed in macromole-cules or in molecules involved in biological reactions, and the CF 3 group used as a probe. For example, certain amino acids in an enzyme can be selectively trifluoroacetylated, and the presence or absence of a chemical shift change when an inhibitor or substrate molecule is bound may indicate whether that particular amino acid is involved in the binding process. Conversely, if an inhibitor or substrate molecule is labeled, its binding state can be ascertained by following spectral changes. In hemoglobin and related compounds, changes in the CF 3 resonance of attached CF 3 CO groups have been used as indicators of conformational changes occurring on the uptake of oxygen. Carbon-13 spectroscopy has recently been developed into a valu-able method for structural determinations of organic compounds and natural products.
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