NationMaster - Encyclopedia: 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. Pacific Northwest National Laboratorys high magnetic field (800 MHz, 18. ... In science, a molecule is a group of atoms in a definite arrangement held together by chemical bonds. ... It has been suggested that NMR Data Processing be merged into this article or section. ... Pacific Northwest National Laboratorys high magnetic field (800 MHz, 18. ... Pacific Northwest National Laboratorys high magnetic field (800 MHz, 18. ...

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. In physics, spin refers to the angular momentum intrinsic to a body, as opposed to orbital angular momentum, which is generated by the motion of its center of mass about an external point. ... This article is about resonance in physics. ... Current (I) flowing through a wire produces a magnetic field () around the wire. ...

The chemical shift is of great importance for NMR spectroscopy, a technique to explore molecular properties by looking at nuclear magnetic resonance phenomena. It has been suggested that NMR Data Processing be merged into this article or section. ... Pacific Northwest National Laboratorys high magnetic field (800 MHz, 18. ...

1 Operating frequency
2 Chemical shift referencing
3 The induced magnetic field
4 Factors causing chemical shifts
5 Magnetic properties of most common nuclei
6 See also
7 External links
- 7.1 Online tutorials[6]
8 References

Operating frequency

The operating frequency ω of a magnet is calculated from the Larmor equation Look up Î©, Ï‰ in Wiktionary, the free dictionary. ... The Larmor equation in physics, named for Irish physicist Joseph Larmor, is ω = γ B The equation describes the relationship between the angular frequency ω of a precessing proton, and the strength of the magnetic field B. Here γ is the gyromagnetic ratio, a constant unique to the nucleus of...

$omega_{0} = gamma * B_0,$

where $B 0$ 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 μ_N and the Planck constant h: SI unit. ... Carl Friedrich Gauss was a German mathematician and physicist. ... Gamma (uppercase Î“, lowercase Î³) is the third letter of the Greek alphabet. ... In physics, the gyromagnetic ratio or LandÃ© g-factor is a dimensionless unit which expresses the ratio of the magnetic dipole moment to the angular momentum of an elementary particle or atomic nucleus. ... A bar magnet. ... Look up MU, Mu, mu in Wiktionary, the free dictionary. ... It has been suggested that this article or section be merged into Spin (physics). ... The nuclear magneton (symbol ), is a physical constant of magnetic moment, defined by: where: is the elementary charge, is the reduced Plancks constant, is the proton rest mass In the SI system of units its value is approximately: = 5. ... Plancks constant, denoted h, is a physical constant that is used to describe the sizes of quanta. ...

$gamma = frac{mu,mu_N}{hI},$

Thus, the proton operating frequency for a 7 T magnet is calculated as: Tesla may refer to: // Nikola Tesla, a Serbian-American physicist, inventor, and electrical engineer. ...

$omega_{0} = frac{2.79*5.050,10^{-27},J/T}{6.62,10^{-34},J.s*1/2}*7,T*10^{-6}, = 297 Mhz,$

Chemical shift referencing

Chemical shift δ is usually expressed in parts per million (ppm) by frequency, because it is calculated from: Look up Î”, Î´ in Wiktionary, the free dictionary. ... Parts per million (ppm) is a measure of concentration that is used where low levels of concentration are significant. ... FreQuency is a music video game developed by Harmonix and published by SCEI. It was released in November 2001. ...

$delta = frac{mbox{difference in precession frequency between two nuclei}}{mbox{operating frequency of the magnet}}$

Since the numerator is usually in hertz, and the denominator in megahertz, delta is expressed in ppm. The hertz (symbol: Hz) is the SI unit of frequency. ... MegaHertz (MHz) is the name given to one million (106) Hertz, a measure of frequency. ...

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. Tetramethylsilane, or TMS, is a chemical compound with the formula Si(CH3)4 or SiMe4 (Me = CH3). ...

Thus, an NMR signal at 300 Hz from TMS at an applied frequency of 300MHz has a chemical shift of:

$frac{300hz}{300*10^6}*10^6 = 1,ppm ,$

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. Lenzs law (pronounced (IPA) ) was formulated by German physicist Heinrich Lenz in 1833 and gives the direction of the induced electromotive force (emf) resulting from electromagnetic induction. ...

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
the frequency is lower
the nucleus is more deshielded and the induced secondary field higher
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.

Factors causing chemical shifts

Important factors influencing chemical shift are electron density, electronegativity of neighboring groups and anisotropic induced magnetic field effects. Electronegativity is a measure of the ability of an atom or molecule to attract electrons in the context of a chemical bond. ...

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 cyclooctatetraenyl anion move upfield to 6.75 ppm and its dianion even at 5.56 ppm 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). ... 1,3,5,7-Cyclooctatetrene (COT) is an unsaturated derivative of cyclooctane, with the formula C8H8. ...

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. It has been suggested that this article or section be merged with electronegativity. ... Pacific Northwest National Laboratorys high magnetic field (800 MHz, 18. ... In chemistry, an alkyl halide is an organic molecule of the form R_X, where X is a halide and R contains a carbon atom bonded to other functional groups or hydrogens. ... Typical chemical shifts of 13C in 13C-NMR Carbon-13 NMR is the application of nuclear magnetic resonance in spectroscopy with respect to carbon. ...

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. This article is being considered for deletion in accordance with Wikipedias deletion policy. ... The chemical structure of ethylene, the simplest alkene. ...

The protons in aromatic compounds a shifted downfield even further with a signal for benzene at 7.73 ppm as a consequence of a diamagnetic ring current. Image File history File links No higher resolution available. ... In chemistry, an aromatic molecule is one in which electrons are free to cycle around circular arrangements of atoms, which are alternately singly and doubly bonded to one another. ... Benzene, also known as benzol, is an organic chemical compound with the formula C6H6. ... A diagram of an aromatic ring current. ...

Alkyne protons by contrast are 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. Alkynes are hydrocarbons that have at least one triple bond between two carbon atoms. ...

Image File history File links No higher resolution available. ...

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: Isotopes are any of the several different forms of an element each having different atomic mass (mass number). ... In physics, spin refers to the angular momentum intrinsic to a body, as opposed to orbital angular momentum, which is the motion of its center of mass about an external point. ... A bar magnet. ... The elementary charge (symbol e or sometimes q) is the electric charge carried by a single proton, or equivalently, the negative of the electric charge carried by a single electron. ... FreQuency is a music video game developed by Harmonix and published by SCEI. It was released in November 2001. ... SI unit. ...

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

A representation of the 3D structure of myoglobin, showing coloured alpha helices. ... The structure of part of a DNA double helix Deoxyribonucleic acid (DNA) is a nucleic acid that contains the genetic instructions for the development and function of living organisms. ...

External links

Online tutorials^[6]

Problem set 1, advanced^[7]
Problem set 2, moderate
Problem set 3, for beginners^[8]
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

^ Spectrometric Identification of organic Compounds Silverstein, Bassler, Morrill 4th Ed. ISBN 047109700
^ Organic Spectroscopy William Kemp 3th Ed. ISBN 0333417674
^ Basic ¹H - ¹³C-NMR spectroscopy Metin Balei ISBN 0444518118
^ In units of the Nuclear magneton
^ CRC Handbook of Chemistry and Physics 65Th Ed
^ 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.
^ 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.
^ For Problem set 3, you will need Adobe Reader

Categories: Nuclear chemistry | Spectroscopy | Nuclear physics | Nuclear magnetic resonance The nuclear magneton (symbol ), is a physical constant of magnetic moment, defined by: where: is the elementary charge, is the reduced Plancks constant, is the proton rest mass In the SI system of units its value is approximately: = 5. ... The CRC Press, LLC is a publishing group which specializes in producing technical books in a wide range of subjects. ... IR spectrum of a thin film of liquid ethanol. ... Mass spectrometry is a technique for separating ions by their mass-to-charge (m/z) ratios. ...

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