In
nuclear magnetic resonanceNuclear magnetic resonance is a property that magnetic nuclei have in a magnetic field and applied electromagnetic pulse, which cause the nuclei to absorb energy from the EM pulse and radiate this energy back out...
(NMR), the
chemical shift describes the dependence of nuclear magnetic energy levels on the electronic environment in a
moleculeA molecule is defined as an electrically neutral group of at least two atoms in a definite arrangement held together by very strong chemical bonds. Molecules are distinguished from polyatomic ions in this strict sense...
. Chemical shifts are relevant in
NMR spectroscopyNuclear magnetic resonance spectroscopy, most commonly known as NMR spectroscopy, is the name given to a technique which exploits the magnetic properties of certain nuclei. This phenomenon and its origins are detailed in a separate section on nuclear magnetic resonance. The most important...
techniques such as
proton NMRProton NMR is the application of nuclear magnetic resonance in NMR spectroscopy with respect to hydrogen-1 nuclei within the molecules of a substance, in order to determine the structure of its molecules...
and
carbon-13 NMRCarbon-13 NMR is the application of nuclear magnetic resonance spectroscopy 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...
.
An atomic nucleus can have a magnetic moment (nuclear spin), which gives rise to different energy levels and
resonanceIn physics, resonance is the tendency of a system to oscillate at larger amplitude at some frequencies than at others. These are known as the system's resonant frequencies . At these frequencies, even small periodic driving forces can produce large amplitude vibrations, because the system...
frequencies in a
magnetic fieldMagnetic fields surround magnetic materials and electric currents and are detected by the force they exert on other magnetic materials and moving electric charges...
. 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 spectroscopyNuclear magnetic resonance spectroscopy, most commonly known as NMR spectroscopy, is the name given to a technique which exploits the magnetic properties of certain nuclei. This phenomenon and its origins are detailed in a separate section on nuclear magnetic resonance. The most important...
, a technique to explore molecular properties by looking at
nuclear magnetic resonanceNuclear magnetic resonance is a property that magnetic nuclei have in a magnetic field and applied electromagnetic pulse, which cause the nuclei to absorb energy from the EM pulse and radiate this energy back out...
phenomena.
Operating frequency
The operating frequency
Omega is the 24th and last letter of the Greek alphabet. In the Greek numeric system, it has a value of 800. The word literally means "great O" , as opposed to Omicron, which means "little O"...
of a magnet is calculated from the Larmor equation
where is the actual strength of the magnet in units like
teslasThe tesla is the SI derived unit of magnetic field B . One tesla is equal to one weber per square meter, and it was defined in 1960 in honor of the Serbian-American inventor, physicist, and electrical engineer Nikola Tesla...
or
gaussThe gauss, abbreviated as G, is the cgs unit of measurement of a magnetic field B , named after the German mathematician and physicist Carl Friedrich Gauss. One gauss is defined as one maxwell per square centimeter.-Unit name and convention:This unit is named after Carl Friedrich Gauss...
, and
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 . Letters that arose from Gamma include the Roman C and G and the Cyrillic letters Ge Г and Ghe Ґ.In Modern Greek, it...
is the gyromagnetic ratio of the nucleus being tested which is in turn calculated from its
magnetic momentThe magnetic moment of a system is a measure of the strength and the direction of its magnetism. More technically , the term magnetic moment of a system usually refers to its magnetic dipole moment, and quantifies the contribution...
- Language :* Mu , Μ or μ, a letter in the Greek alphabet* Mu , represented by the Japanese kana む or ム* 無, Mu , a Japanese and Korean word important in Zen practice* 木 , a Chinese character meaning tree...
and spin number with the
nuclear magnetonThe nuclear magneton , is a physical constant of magnetic moment, defined by:where: is the elementary charge, is the reduced Planck's constant, is the proton rest mass...
and the
Planck constantThe Planck constant , also called Planck's constant, is a physical constant used to describe the sizes of quanta in quantum mechanics. It is named after Max Planck, one of the founders of quantum theory...
h:
Thus, the proton operating frequency for a 1
TThe tesla is the SI derived unit of magnetic field B . One tesla is equal to one weber per square meter, and it was defined in 1960 in honor of the Serbian-American inventor, physicist, and electrical engineer Nikola Tesla...
magnet is calculated as:
Chemical shift referencing
Chemical shift
δDelta 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...
is usually expressed in parts per million (ppm) by
frequencyFrequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency....
, because it is calculated from:
Since the numerator is usually in
hertzThe hertz is a unit of frequency. It is defined as the number of complete cycles per second. It is the basic unit of frequency in the International System of Units , and is used worldwide in both general-purpose and scientific contexts...
, 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 (
tetramethylsilaneTetramethylsilane is the chemical compound with the formula Si4. It is the simplest tetraorganosilane. Like all silanes, the TMS framework is tetrahedral...
) or
DSSDSS is a chemical compound used in proton- and carbon-related NMR spectroscopy as a calibration standard, similar to tetramethylsilane , but with much higher water solubility...
, 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 lawLenz's law is an extension of the law of conservation of energy to the non-conservative forces in electromagnetic induction. It can be used to give the direction of the induced electromotive force and current resulting from electromagnetic induction...
and the nucleus is therefore said to be
shielded. Trends in chemical shift are explained based on the degree of shielding or deshielding.E.g. of deshielding effect: the hydrogen with a Electron withdrawing atom or group nearby.
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
Conversely a lower chemical shift is called a
diamagnetic shift, and is
upfield and more shielded.
Diamagnetic shielding
In real molecules protons are surrounded by a cloud of charge due to adjacent bonds and atoms. In an applied magnetic field (
B0) electrons circulate and produce an induced field (
Bi) which opposes the applied field. The effective field at the nucleus will be
B =
B0 −
Bi. The nucleus is said to be experiencing a diamagnetic shielding.
Factors causing chemical shifts
Important factors influencing chemical shift are electron density,
electronegativityElectronegativity, symbol χ, is a chemical property that describes the ability of an atom to attract electrons towards itself in a covalent bond. An atom's electronegativity is affected by both its atomic weight and the distance that its valence electrons reside from the charged nucleus...
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 NMRProton NMR is the application of nuclear magnetic resonance in NMR spectroscopy with respect to hydrogen-1 nuclei within the molecules of a substance, in order to determine the structure of its molecules...
of methyl halides (CH
3X) 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
alkeneIn organic chemistry, an alkene, olefin, or olefine is an unsaturated chemical compound containing at least one carbon-to-carbon double bond...
s 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 to 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
benzeneBenzene, or benzol, is an organic chemical compound with the molecular formula C6H6. It is sometimes abbreviated Ph–H. Benzene is a colorless and highly flammable liquid with a sweet smell and a relatively high melting point...
at 7.73 ppm as a consequence of a diamagnetic ring current.
AlkyneAlkynes are hydrocarbons that have a triple bond between two carbon atoms, with the formula CnH2n-2. Alkynes are traditionally known as acetylenes, although the name acetylene also refers specifically to C2H2, known formally as ethyne using IUPAC...
protons by contrast resonate at high field in a 2–3 ppm range. For alkynes the most effective orientation is 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.
IsotopeIsotopes are different types of atoms of the same chemical element, each having a different number of neutrons. Correspondingly, isotopes differ in mass number but not in atomic number. The difference in the number of nucleons comes from a difference how many neutrons are in the atomic nucleus...
|
Occurrence
in nature
(%) |
spin In particle physics and quantum mechanics, spin is a fundamental characteristic property of elementary particles including the force carriers , composite particles , and atomic nuclei....
number l |
Magnetic momentThe magnetic moment of a system is a measure of the strength and the direction of its magnetism. More technically , the term magnetic moment of a system usually refers to its magnetic dipole moment, and quantifies the contribution...
μ
|
Electric quadrupole moment
(eThe elementary charge, usually denoted e, is the electric charge carried by a single proton, or equivalently, the negative of the electric charge carried by a single electron. This is a fundamental physical constant. To avoid confusion over its sign, e is sometimes called the "elementary positive...
×10−24 cm2) |
Operating frequency at 7 T The tesla is the SI derived unit of magnetic field B . One tesla is equal to one weber per square meter, and it was defined in 1960 in honor of the Serbian-American inventor, physicist, and electrical engineer Nikola Tesla...
(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 |
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 protein
Proteins are organic compounds made of amino acids arranged in a linear chain and folded into a globular form. The amino acids in a polymer chain are joined together by the peptide bonds between the carboxyl and amino groups of adjacent amino acid residues...
s and DNADeoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms and some viruses. The main role of DNA molecules is the long-term storage of information...
- 19F because of high relative sensitivity
- 31P because of frequent occurrence in organic compounds and moderate relative sensitivity
Other chemical shifts
The related
Knight shiftThe Knight shift is a shift in the nuclear magnetic resonance frequency of a paramagneticsubstance first published in 1949 by the American physicist Walter David Knight.The Knight shift is due to the conduction electrons in metals...
(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 spectroscopyX-ray photoelectron spectroscopy is a quantitative spectroscopic technique that measures the elemental composition, empirical formula, chemical state and electronic state of the elements that exist within a material...
as the shift in atomic core-level energy due to a specific chemical environment. The term is also used in Mössbauer spectroscopy, where similarly to NMR it refers to a shift in peak position due to the local chemical bonding environment. As is the case for NMR the chemical shift reflects the electron density at the atomic nucleus.
See also
- Carbon-13 NMR
Carbon-13 NMR is the application of nuclear magnetic resonance spectroscopy 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...
- MRI
- NMR spectroscopy
Nuclear magnetic resonance spectroscopy, most commonly known as NMR spectroscopy, is the name given to a technique which exploits the magnetic properties of certain nuclei. This phenomenon and its origins are detailed in a separate section on nuclear magnetic resonance. The most important...
- 2D-FT NMRI and Spectroscopy
2D-FT Nuclear magnetic resonance imaging , or Two-dimensional Fourier transform nuclear magnetic resonance imaging , is primarily a non-invasive imaging technique most commonly used in biomedical research and medical radiology/nuclear medicine/MRI to visualize structures and functions of the living...
- Nuclear magnetic resonance
Nuclear magnetic resonance is a property that magnetic nuclei have in a magnetic field and applied electromagnetic pulse, which cause the nuclei to absorb energy from the EM pulse and radiate this energy back out...
- Protein NMR
- Proton NMR
Proton NMR is the application of nuclear magnetic resonance in NMR spectroscopy with respect to hydrogen-1 nuclei within the molecules of a substance, in order to determine the structure of its molecules...
- Solid-state NMR
- Zeeman effect
The Zeeman effect is the splitting of a spectral line into several components in the presence of a static magnetic field. It is analogous to the Stark effect, the splitting of a spectral line into several components in the presence of an electric field...
External links
- Online tutorials (these generally involve combined use of IR, 1H NMR
Proton NMR is the application of nuclear magnetic resonance in NMR spectroscopy with respect to hydrogen-1 nuclei within the molecules of a substance, in order to determine the structure of its molecules...
, 13C NMR and mass spectrometryMass spectrometry is an analytical technique for the determination of the elemental composition of a sample or molecule. It is also used for elucidating the chemical structures of molecules, such as peptides and other chemical compounds...
)