Information about Chemical Shift
In nuclear magnetic resonance (NMR), the chemical shift describes the dependence of nuclear magnetic energy levels on the electronic environment in a molecule.[1][2][3] Chemical shifts are relevant in NMR spectroscopy techniques such proton NMR and carbon-13 NMR.
An atomic nucleus can have a magnetic moment (nuclear spin), which gives rise to different energy levels and resonance frequencies in a magnetic field. The total magnetic field experienced by a nucleus includes local magnetic fields induced by currents of electrons in the molecular orbitals (note that electrons have a magnetic moment themselves). The electron distribution of the same type of nucleus (e.g. 1H, 13C, 15N) usually varies according to the local geometry (binding partners, bond lengths, angles between bonds, ...), and with it the local magnetic field at each nucleus. This is reflected in the spin energy levels (and resonance frequencies). The variations of nuclear magnetic resonance frequencies of the same kind of nucleus, due to variations in the electron distribution, is called the chemical shift. The size of the chemical shift is given with respect to a reference frequency or reference sample (see also chemical shift referencing), usually a molecule with a barely distorted electron distribution.
The chemical shift is of great importance for NMR spectroscopy, a technique to explore molecular properties by looking at nuclear magnetic resonance phenomena.
where
is the actual strength of the magnet in units like teslas or gauss, and γ is the gyromagnetic ratio of the nucleus being tested which is in turn calculated from its magnetic moment μ and spin number I with the nuclear magneton μN and the Planck constant h:
Thus, the proton operating frequency for a 7 T magnet is calculated as:
Since the numerator is usually in hertz, and the denominator in megahertz, delta is expressed in ppm.
The detected frequencies (in Hz) for 1H, 13C, and 29Si nuclei are usually referenced against TMS (tetramethylsilane), which is assigned the chemical shift of zero. Other standard materials are used for setting the chemical shift for other nuclei.
Thus, an NMR signal at 300 Hz from TMS at an applied frequency of 300MHz has a chemical shift of:
Although the frequency depends on the applied field the chemical shift is independent of it. On the other hand the resolution of NMR will increase with applied magnetic field resulting in ever increasing chemical shift changes.
Nuclei are found to resonate in a wide range to the left (or more rare to the right) of the internal standard. When a signal is found with a higher chemical shift:
Electron density shields a nucleus from the external field. For example in proton NMR the electron-poor tropylium ion has its protons downfield at 9.17 ppm, those of the electron-rich cyclooctatetraenyl anion move upfield to 6.75 ppm and its dianion even more upfield to 5.56 ppm.
A nucleus in the vicinity of an electronegative atom experiences reduced electron density and the nucleus is therefore deshielded. In proton NMR of methyl halides (CH3X) the chemical shift of the methyl protons increase in the order I > Br > Cl > F from 2.16 ppm to 4.26 ppm reflecting this trend. In carbon NMR the chemical shift of the carbon nuclei increase in the same order from around -10 ppm to 70 ppm. Also when the electronegative atom is removed further away the effect diminishes until it can be observed no longer.
Anisotropic induced magnetic field effects are the result of a local induced magnetic field experienced by a nucleus resulting from circulating electrons that can either be paramagnetic when it is parallel to the applied field or diamagnetic when it is opposed to it. It is observed in alkenes where the double bond is oriented perpendicular to the external field with pi electrons likewise circulating at right angles. The induced magnetic field lines are parallel to the external field at the location of the alkene protons which therefore shift downfield to a 4.5 ppm - 7.5 ppm range. The three-dimensional space where a nucleus experiences diamagnetic shift is called the shielding zone with a cone-like shape aligned with the external field.
The protons in aromatic compounds are shifted downfield even further with a signal for benzene at 7.73 ppm as a consequence of a diamagnetic ring current.
Alkyne protons by contrast resonate at high field in a 2-3 ppm range. For alkynes the most effective orientation in the external field in parallel with electrons circulation around the triple bond. In this way the acetylenic protons are located in the cone-shaped shielding zone hence the upfield shift.
1H, 13C, 15N, 19F and 31P are the five nuclei that have the greatest importance in NMR experiments:
The gauss, abbreviated as G, is the cgs unit of magnetic field (B), named after the German mathematician and physicist Carl Friedrich Gauss.
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An atomic nucleus can have a magnetic moment (nuclear spin), which gives rise to different energy levels and resonance frequencies in a magnetic field. The total magnetic field experienced by a nucleus includes local magnetic fields induced by currents of electrons in the molecular orbitals (note that electrons have a magnetic moment themselves). The electron distribution of the same type of nucleus (e.g. 1H, 13C, 15N) usually varies according to the local geometry (binding partners, bond lengths, angles between bonds, ...), and with it the local magnetic field at each nucleus. This is reflected in the spin energy levels (and resonance frequencies). The variations of nuclear magnetic resonance frequencies of the same kind of nucleus, due to variations in the electron distribution, is called the chemical shift. The size of the chemical shift is given with respect to a reference frequency or reference sample (see also chemical shift referencing), usually a molecule with a barely distorted electron distribution.
The chemical shift is of great importance for NMR spectroscopy, a technique to explore molecular properties by looking at nuclear magnetic resonance phenomena.
Operating frequency
The operating frequency ω of a magnet is calculated from the Larmor equation
where
is the actual strength of the magnet in units like teslas or gauss, and γ is the gyromagnetic ratio of the nucleus being tested which is in turn calculated from its magnetic moment μ and spin number I with the nuclear magneton μN and the Planck constant h:
Thus, the proton operating frequency for a 7 T magnet is calculated as:
Chemical shift referencing
Chemical shift δ is usually expressed in parts per million (ppm) by frequency, because it is calculated from:Since the numerator is usually in hertz, and the denominator in megahertz, delta is expressed in ppm.
The detected frequencies (in Hz) for 1H, 13C, and 29Si nuclei are usually referenced against TMS (tetramethylsilane), which is assigned the chemical shift of zero. Other standard materials are used for setting the chemical shift for other nuclei.
Thus, an NMR signal at 300 Hz from TMS at an applied frequency of 300MHz has a chemical shift of:
Although the frequency depends on the applied field the chemical shift is independent of it. On the other hand the resolution of NMR will increase with applied magnetic field resulting in ever increasing chemical shift changes.
The induced magnetic field
The electrons around a nucleus will circulate in a magnetic field and create a secondary induced magnetic field. This field opposes the applied field as stipulated by Lenz's law and the nucleus is therefore said to be shielded. Trends in chemical shift are explained based on the degree of shielding or deshielding.Nuclei are found to resonate in a wide range to the left (or more rare to the right) of the internal standard. When a signal is found with a higher chemical shift:
- the applied effective magnetic field is lower, if the resonance frequency is fixed, (as in old traditional CW spectrometers)
- the frequency is higher, when the applied magnetic field is static, (normal case in FT spectrometers)
- the nucleus is more deshielded
- the signal or shift is downfield or at low field or paramagnetic
Factors causing chemical shifts
Important factors influencing chemical shift are electron density, electronegativity of neighboring groups and anisotropic induced magnetic field effects.Electron density shields a nucleus from the external field. For example in proton NMR the electron-poor tropylium ion has its protons downfield at 9.17 ppm, those of the electron-rich cyclooctatetraenyl anion move upfield to 6.75 ppm and its dianion even more upfield to 5.56 ppm.
A nucleus in the vicinity of an electronegative atom experiences reduced electron density and the nucleus is therefore deshielded. In proton NMR of methyl halides (CH3X) the chemical shift of the methyl protons increase in the order I > Br > Cl > F from 2.16 ppm to 4.26 ppm reflecting this trend. In carbon NMR the chemical shift of the carbon nuclei increase in the same order from around -10 ppm to 70 ppm. Also when the electronegative atom is removed further away the effect diminishes until it can be observed no longer.
Anisotropic induced magnetic field effects are the result of a local induced magnetic field experienced by a nucleus resulting from circulating electrons that can either be paramagnetic when it is parallel to the applied field or diamagnetic when it is opposed to it. It is observed in alkenes where the double bond is oriented perpendicular to the external field with pi electrons likewise circulating at right angles. The induced magnetic field lines are parallel to the external field at the location of the alkene protons which therefore shift downfield to a 4.5 ppm - 7.5 ppm range. The three-dimensional space where a nucleus experiences diamagnetic shift is called the shielding zone with a cone-like shape aligned with the external field.
The protons in aromatic compounds are shifted downfield even further with a signal for benzene at 7.73 ppm as a consequence of a diamagnetic ring current.
Alkyne protons by contrast resonate at high field in a 2-3 ppm range. For alkynes the most effective orientation in the external field in parallel with electrons circulation around the triple bond. In this way the acetylenic protons are located in the cone-shaped shielding zone hence the upfield shift.
Magnetic properties of most common nuclei
1H and 13C aren't the only nuclei susceptible to NMR experiments. A number of different nuclei can also be detected, although the use of such techniques is generally rare due to small relative sensitivities in NMR experiments (compared to 1H) of the nuclei in question, the other factor for rare use being their slender representation in nature/organic compounds.| Isotope | Occurrence in nature (%) |
spin number l | Magnetic moment μ[4] |
Electric quadrupole moment (e×10-24 cm2) |
Frequency at 7 T (MHz) |
Relative sensitivity | |
|---|---|---|---|---|---|---|---|
| 1H | 99.984 | 1/2 | 2.79628 | 300.13 | 1 | ||
| 2H | 0.016 | 1 | 0.85739 | 2.8 x 10-3 | 46.07 | 0.0964 | |
| 10B | 18.8 | 3 | 1.8005 | 7.4 x 10-2 | 32.25 | 0.0199 | |
| 11B | 81.2 | 3/2 | 2.6880 | 2.6 x 10-2 | 96.29 | 0.165 | |
| 12C | 98.9 | 0 | |||||
| 13C | 1.1 | 1/2 | 0.70220 | 75.47 | 0.0159 | ||
| 14N | 99.64 | 1 | 0.40358 | 7.1 x 10-2 | 21.68 | 0.00101 | |
| 15N | 0.37 | 1/2 | −0.28304 | 30.41 | 0.00104 | ||
| 16O | 99.76 | 0 | |||||
| 17O | 0.0317 | 5/2 | −1.8930 | −4.0 x 10-3 | 40.69 | 0.0291 | |
| 19F | 100 | 1/2 | 2.6273 | 282.40 | 0.834 | ||
| 28Si | 92.28 | 0 | |||||
| 29Si | 4.70 | 1/2 | −0.55548 | 59.63 | 0.0785 | ||
| 31P | 100 | 1/2 | 1.1205 | 121.49 | 0.0664 | ||
| 35Cl | 75.4 | 3/2 | 0.92091 | −7.9 x 10-2 | 29.41 | 0.0047 | |
| 37Cl | 24.6 | 3/2 | 0.68330 | −6.2 x 10-2 | 24.48 | 0.0027 | |
| Magnetic properties of common nuclei[5] | |||||||
1H, 13C, 15N, 19F and 31P are the five nuclei that have the greatest importance in NMR experiments:
- 1H because of high sensitivity and vast occurrence in organic compounds
- 13C because of being the key component of all organic compounds despite occurring at a low abundance (1.1%) compared to the major isotope of carbon 12C, which has a spin of 0 and therefore is NMR inactive.
- 15N because of being a key component of important biomolecules such as proteins and DNA
- 19F because of high relative sensitivity
- 31P because of frequent occurrence in organic compounds and moderate relative sensitivity
Other chemical shifts
The related Knight shift (first reported in 1949) is observed with pure metals. The NMR chemical shift in its present day meaning first appeared in journals in 1950. Chemical shifts with a different meaning appear in X-ray photoelectron spectroscopy as the shift in atomic core-level energy due to a specific chemical environment. The term is also used in Mössbauer spectroscopy [6]See also
- Carbon-13 NMR
- MRI
- NMR spectroscopy
- Nuclear magnetic resonance
- Protein NMR
- Proton NMR
- Solid-state NMR
- Zeeman effect
External links
- www.chem.wisc.edu
- BioMagResBank
- wwwchem.csustan.edu
- Proton chemical shifts
- Carbon chemical shifts
- Online tutorials[7]
- Problem set 1, advanced[8]
- Problem set 2, moderate
- Problem set 3, for beginners[9]
- Problem set 4, moderate, German language (don't let that scare you away!)
- Problem set 5, the best!
- Combined solutions to problem set 5 (answers 1-32)
- Combined solutions to problem set 5 (answers 33-64)
References
1. ^ Spectrometric Identification of organic Compounds Silverstein, Bassler, Morrill 4th Ed. ISBN 047109700
2. ^ Organic Spectroscopy William Kemp 3th Ed. ISBN 0333417674
3. ^ Basic 1H - 13C-NMR spectroscopy Metin Balei ISBN 0444518118
4. ^ In units of the Nuclear magneton
5. ^ CRC Handbook of Chemistry and Physics 65Th Ed
6. ^ A Short History of Three Chemical Shifts Shin-ichi Nagaoka Vol. 84 No. 5 May 2007 Journal of Chemical Education 801
7. ^ Since the easiest way to obtain knowledge is by practice - that is, solving problems, a few external links are shown underneath. The problems are made of different combined spectra (IR, 1H-NMR, 13C-NMR etc.), most of the links contain solutions to problems. The last URL is the best, with the solutions being on two separate pages.
8. ^ In order to solve this problems, you must have some knowledge about IR spectroscopy and mass spectroscopy. Basic principles of spin-spin coupling are also required. You will find all information about the latter on this excellent link.
9. ^ For Problem set 3, you will need Adobe Reader
2. ^ Organic Spectroscopy William Kemp 3th Ed. ISBN 0333417674
3. ^ Basic 1H - 13C-NMR spectroscopy Metin Balei ISBN 0444518118
4. ^ In units of the Nuclear magneton
5. ^ CRC Handbook of Chemistry and Physics 65Th Ed
6. ^ A Short History of Three Chemical Shifts Shin-ichi Nagaoka Vol. 84 No. 5 May 2007 Journal of Chemical Education 801
7. ^ Since the easiest way to obtain knowledge is by practice - that is, solving problems, a few external links are shown underneath. The problems are made of different combined spectra (IR, 1H-NMR, 13C-NMR etc.), most of the links contain solutions to problems. The last URL is the best, with the solutions being on two separate pages.
8. ^ In order to solve this problems, you must have some knowledge about IR spectroscopy and mass spectroscopy. Basic principles of spin-spin coupling are also required. You will find all information about the latter on this excellent link.
9. ^ For Problem set 3, you will need Adobe Reader
Nuclear magnetic resonance (NMR) is a physical phenomenon based upon the quantum mechanical magnetic properties of an atom's nucleus. NMR also commonly refers to a family of scientific methods that exploit nuclear magnetic resonance to study molecules.
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molecule is defined as a sufficiently stable electrically neutral group of at least two atoms in a definite arrangement held together by strong chemical bonds.[1][2] In organic chemistry and biochemistry, the term molecule
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Nuclear magnetic resonance spectroscopy most commonly known as NMR spectroscopy is the name given to the technique which exploits the magnetic properties of certain nuclei. This phenomenon and its origins are detailed in a separate section on Nuclear magnetic resonance.
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Proton NMR (also Hydrogen-1 NMR, or 1HNMR) is the application of nuclear magnetic resonance in NMR spectroscopy with respect to hydrogen.
Simple NMR spectra are recorded in solution, and solvent protons must not be allowed to interfere.
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Simple NMR spectra are recorded in solution, and solvent protons must not be allowed to interfere.
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Carbon-13 NMR is the application of nuclear magnetic resonance with respect to carbon. It is analogous to proton NMR and allows the identification of carbon atoms in an organic molecule just as proton NMR identifies hydrogen atoms.
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magnetic field is a field that permeates space and which exerts a magnetic force on moving electric charges and magnetic dipoles. Magnetic fields surround electric currents, magnetic dipoles, and changing electric fields.
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Nuclear magnetic resonance spectroscopy most commonly known as NMR spectroscopy is the name given to the technique which exploits the magnetic properties of certain nuclei. This phenomenon and its origins are detailed in a separate section on Nuclear magnetic resonance.
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Nuclear magnetic resonance (NMR) is a physical phenomenon based upon the quantum mechanical magnetic properties of an atom's nucleus. NMR also commonly refers to a family of scientific methods that exploit nuclear magnetic resonance to study molecules.
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OMEGA is the premier counter-terrorism unit of Latvia.
Founded in 1992, OMEGA cooperates with many other counter-terrorism units over the world. Its equipment and weaponry includes the Heckler & Koch MP5, the Steyr AUG and the Makarov PM.
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Founded in 1992, OMEGA cooperates with many other counter-terrorism units over the world. Its equipment and weaponry includes the Heckler & Koch MP5, the Steyr AUG and the Makarov PM.
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In physics, Larmor precession (named after Joseph Larmor) refers to the precession of the magnetic moments of electrons, atomic nuclei, and atoms about an external magnetic field.
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tesla (symbol T) is the SI derived unit of magnetic field. The tesla is equal to one weber per square metre and was defined in 1960[1] in honor of inventor, scientist and electrical engineer Nikola Tesla.
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This article refers to the unit gauss; for other meanings see Gauss (disambiguation).
The gauss, abbreviated as G, is the cgs unit of magnetic field (B), named after the German mathematician and physicist Carl Friedrich Gauss.
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Gamma (uppercase Γ, lowercase γ) is the third letter of the Greek alphabet. In the system of Greek numerals it has a value of 3. It was derived from the Phoenician letter Gimel .
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In physics, the gyromagnetic ratio (also sometimes known as the magnetogyric ratio in other disciplines) of a particle or system is the ratio of its magnetic dipole moment to its angular momentum.
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In physics, the magnetic moment or magnetic dipole moment is a measure of the strength of a magnetic source. In the simplest case of a current loop, the magnetic moment is defined as:
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Mu can be:
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- Mu (letter), M or μ, a letter in the Greek alphabet pronounced "mew" (/mju/). μ is used as a symbol for:
- The SI prefix micro
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In atomic physics, the spin quantum number is a quantum number that parametrizes the intrinsic angular momentum (or spin angular momentum, or simply spin) of a given particle.
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The nuclear magneton (symbol ), is a physical constant of magnetic moment, defined by:
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- is the elementary charge,
- is the reduced Planck's constant,
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Planck constant (denoted ) is a physical constant that is used to describe the sizes of quanta. It plays a central role in the theory of quantum mechanics, and is named after Max Planck, one of the founders of quantum theory.
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Tesla may refer to:
In science:
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In science:
- Nikola Tesla, a physicist, inventor, and electrical engineer
- List of Tesla patents
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Delta (uppercase Δ, lowercase δ) is the fourth letter of the Greek alphabet. In the system of Greek numerals it has a value of 4. It was derived from the Phoenician letter Dalet .
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hertz (symbol: Hz) is the SI unit of frequency. Its base unit is cycle/s or s-1 (also called inverse seconds, reciprocal seconds). In English, hertz is used as both singular and plural.
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Tetramethylsilane is the chemical compound with the formula Si(CH3)4 or SiMe4 (Me = CH3). Commonly abbreviated TMS, it is the simplest tetraorganosilane.
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Lenz's law (pronounced (IPA) [ˈlɛntsəz lɔ]) gives the direction of the induced electromotive force (emf) and current resulting from electromagnetic induction. Heinrich Lenz formulated it in 1834.
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Electronegativity, symbol χ, is a chemical property which describes the power of an atom (or, more rarely, a functional group) to attract electrons towards itself.[1] First proposed by Linus Pauling in 1932 as a development of valence bond theory,[2]
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In organic chemistry, the tropylium ion is an aromatic species with a formula of [C7H7]+. Its name derives from the molecule tropane (itself named for the molecule atropine).
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