Nmr spectroscopy.
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This document provides an introduction to nuclear magnetic resonance (NMR) spectroscopy. It discusses: 1) How NMR spectroscopy uses radio waves and magnetic fields to determine the structure of organic molecules. The two most common types are 1H NMR and 13C NMR. 2) In an NMR experiment, nuclei such as 1H and 13C can absorb energy and "flip" their spin when radio waves match their energy difference in magnetic fields. 3) NMR spectra provide information on the number, position, intensity, and splitting of peaks which reveal details about a molecule's carbon-hydrogen framework.
Nmr spectroscopy.
- 1. 1 THEORY OF NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY By Sayyad Ali Khan. Ph.D. Scholar Reg. # FA13-R60-002/ATD COMSATS Abbottabad Pakistan
- 2. 2 Nuclear Magnetic Resonance Spectroscopy • Nuclear magnetic resonance spectroscopy is a powerful analytical technique used to characterize organic molecules by identifying carbon-hydrogen frameworks within molecules. • Two common types of NMR spectroscopy are used to characterize organic structure: 1 H NMR is used to determine the type and number of H atoms in a molecule; 13 C NMR is used to determine the type of carbon atoms in the molecule. • The source of energy in NMR is radio waves which have long wavelengths, and thus low energy and frequency. • When low-energy radio waves interact with a molecule, they can change the nuclear spins of some elements, including 1 H and 13 C. Introduction to NMR Spectroscopy
- 3. 3 • When a charged particle such as a proton spins on its axis, it creates a magnetic field. Thus, the nucleus can be considered to be a tiny bar magnet. • Normally, these tiny bar magnets are randomly oriented in space. However, in the presence of a magnetic field B0, they are oriented with or against the applied M. field. More nuclei are oriented with the applied field because this arrangement is lower in energy. Introduction to NMR Spectroscopy
- 4. 4 • In a magnetic field, there are now two energy states for a proton: a lower energy state with the nucleus aligned in the same direction as B0, and a higher energy state in which the nucleus aligned against B0. • When an external energy source (hν) that matches the energy difference (∆E) between these two states is applied, energy is absorbed, causing the nucleus to “spin flip” from one orientation to another. • The energy difference between these two nuclear spin states corresponds to the low frequency RF region of the electromagnetic spectrum. Introduction to NMR Spectroscopy
- 5. 5 • Thus, two variables characterize NMR: an applied magnetic field B0, the strength of which is measured in tesla (T), and the frequency ν of radiation used for resonance, measured in hertz (Hz), or megahertz (MHz)—(1 MHz = 106 Hz). Introduction to NMR Spectroscopy Radiofrequency (RF) frequency ranges from 3 kilohertz (kHz) to 300 Megahertz (MHz), and microwave (MW) radiation are electromagnetic radiation from 300 MHz to 300 gigahertz (GHz).
- 6. 6 • The frequency of the light needed for resonance and the applied magnetic field strength are proportionally related: • NMR spectrometers are referred to as 300 MHz instruments, 500 MHz instruments, and so forth, depending on the frequency of the RF radiation used for resonance. • These spectrometers use very powerful magnets to create a small but measurable energy difference between two possible spin states. Introduction to NMR Spectroscopy
- 7. 7 Introduction to NMR Spectroscopy Schematic of an NMR spectrometer
- 8. 8 • Protons in different environments absorb at slightly different frequencies, so they are distinguishable by NMR. • The frequency at which a particular proton absorbs is determined by its electronic environment. • Modern NMR spectrometers use a constant magnetic field strength B0, and then a narrow range of frequencies is applied to achieve the resonance of all protons. • Only nuclei that contain odd mass numbers (such as 1 H, 13 C, 19 F and 31 P) or odd atomic numbers (such as 2 H and 14 N) give rise to NMR signals. Introduction to NMR Spectroscopy
- 9. 9 • An NMR spectrum is a plot of the intensity of a peak against its chemical shift, measured in parts per million (ppm). 1 H NMR—The Spectrum
- 10. 10 • NMR absorptions generally appear as sharp peaks. • Increasing chemical shift is plotted from right to left. • Most protons absorb between 0-10 ppm. • The terms “upfield” and “downfield” describe the relative location of peaks. Upfield means to the right. Downfield means to the left. • NMR absorptions are measured relative to the position of a reference peak at 0 ppm on the δ scale due to tetramethylsilane (TMS). TMS is a volatile inert compound that gives a single peak upfield from typical NMR absorptions and taken as “0”. 1 H NMR—The Spectrum
- 11. 11 • The chemical shift of the x axis gives the position of an NMR signal, measured in ppm, according to the following equation: 1 H NMR—The Spectrum • Four different features of a 1 H NMR spectrum provide information about a compound’s structure: a. Number of signals b. Position of signals c. Intensity of signals. d. Spin-spin splitting of signals.
- 12. 12 • The number of NMR signals equals the number of different types of protons in a compound. • Protons in different environments give different NMR signals. • Equivalent protons give the same NMR signal. 1 H NMR—Number of Signals • To determine equivalent protons in cycloalkanes and alkenes, one should always draw all bonds to hydrogen.
- 13. 13 • In comparing two H atoms on a ring or double bond, two protons are equivalent only if they are cis (or trans) to the same groups. 1 H NMR—Number of Signals
- 14. 14 • In the vicinity of the nucleus, the magnetic field generated by the circulating electron decreases the external magnetic field that the proton “feels”. • Since the electron experiences a lower magnetic field strength, it needs a lower frequency to achieve resonance. Lower frequency is to the right in an NMR spectrum, toward a lower chemical shift, so shielding shifts the absorption upfield. 1 H NMR—Position of Signals
- 15. 15 • The less shielded the nucleus becomes, the more of the applied magnetic field (B0) it feels. • This deshielded nucleus experiences a higher magnetic field strength, i.e. it needs a higher frequency to achieve resonance. • Higher frequency is to the left in an NMR spectrum, toward higher chemical shift—so deshielding shifts an absorption downfield. • Protons near electronegative atoms are deshielded, so they absorb downfield. 1 H NMR—Position of Signals
- 16. 16 1 H NMR—Position of Signals
- 17. 17 • The chemical shift of a C—H bond increases with increasing alkyl substitution. 1 H NMR—Chemical Shift Values
- 18. 18 • In a magnetic field, the six π electrons in benzene circulate around the ring. • The magnetic field induced by these moving electrons reinforces the applied magnetic field in the vicinity of the protons. • The protons thus feel a stronger magnetic field and a higher frequency is needed for resonance. Thus they are deshielded and absorb downfield. 1 H NMR—Chemical Shift
- 19. 19 1 H NMR—Chemical Shift Values
- 20. 20 1 H NMR—Chemical Shift Values) Figure 14.5 Regions in the1 H NMR spectrum
- 21. 21 • Consider the spectrum below: 1 H NMR—Spin-Spin Splitting
- 22. 22 • Spin-spin splitting occurs only between nonequivalent protons on the same carbon or adjacent carbons. 1 H NMR—Spin-Spin Splitting • When placed in an applied electric field, (B0), the adjacent proton (—CH2Br) can be aligned with (↑) or against (↓) B0. • Thus, the absorbing CH2 protons feel two slightly different magnetic fields. • Since the absorbing protons feel two different magnetic fields, they absorb at two different frequencies in the NMR spectrum, thus splitting a single absorption into a doublet.
- 23. 23 1 H NMR—Spin-Spin Splitting The frequency difference, measured in Hz between two peaks of the doublet is called the coupling constant, J.
- 24. 24 1 H NMR—Spin-Spin Splitting consider how a triplet arises: • When placed in an applied magnetic field (B0), the adjacent protons Ha and Hb can each be aligned with (↑) or against (↓) B0. • Thus, the absorbing proton feels three slightly different magnetic fields.
- 26. 26 1 H NMR—Spin-Spin Splitting Three general rules describe the splitting patterns commonly seen in the 1 H NMR spectra of organic compounds. 1. Equivalent protons do not split each other’s signals. 2. A set of ‘n’ non-equivalent protons splits the signal of a nearby proton into n + 1 peaks. 3. Splitting is observed for nonequivalent protons on the same carbon or adjacent carbons. If Ha and Hb are not equivalent, splitting is observed when:
- 27. 27
- 28. 28 13 C NMR 13 C Spectra are easier to analyze than 1 H spectra because the signals are not split. Each type of carbon atom appears as a single peak.
- 29. 29 13 C NMR • The lack of splitting in a 13 C spectrum is a consequence of the low natural abundance of 13 C. • As splitting occurs when two NMR active nuclei—like two protons are close to each other. Because of the low natural abundance of 13 C nuclei (1.1% of 12 C ), the chance of two 13 C nuclei being bonded to each other is very small (0.01%), and so no carbon-carbon splitting is observed. • A 13 C NMR signal can also be split by nearby protons. This 1 H-13 C splitting is usually eliminated from the spectrum by using an instrumental technique so that every peak in a 13 C NMR spectrum appears as a singlet. • The two features of a 13 C NMR spectrum that provide the most structural information are the number of signals observed and the chemical shifts of those signals.
- 30. 30 13 C NMR—Number of Signals • The number of signals in a 13 C spectrum gives the number of different types of carbon atoms in a molecule. • Because 13 C NMR signals are not split, the number of signals equals the number of lines in the 13 C spectrum. • In contrast to the 1 H NMR situation, peak intensity is not proportional to the number of absorbing carbons, so 13 C NMR signals are not integrated.
- 31. 31 13 C NMR—Position of Signals • In contrast to the small range of chemical shifts in 1 H NMR (1-10 ppm usually), 13 C NMR absorptions occur over a much broader range (0-220 ppm). • The chemical shifts of carbon atoms in 13 C NMR depend on the same effects as the chemical shifts of protons in 1 H NMR.
- 32. 32 13 C NMR—Number of Signals
- 33. 33