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| | Hello! Let me start by stating my title - Ron Stephenson. Some time in the past I chose to reside in Arizona but I need to move for my family. What she enjoys doing is bottle tops gathering and she is trying to make it a profession. Bookkeeping is what she does.<br><br>my blog post - [http://Www.danceresults.com/UserProfile/tabid/148/userId/5790/Default.aspx Www.danceresults.com] |
| In [[nuclear magnetic resonance]] (NMR) spectroscopy, the '''chemical shift''' is the resonant frequency of a nucleus relative to a standard. Often the position and number of chemical shifts are diagnostic of the structure of a [[molecule]].<ref>''Spectrometric Identification of organic Compounds'' Silverstein, Bassler, Morrill 4th Ed. ISBN 047109700</ref><ref>''Organic Spectroscopy'' William Kemp 3rd Ed. ISBN 0-333-41767-4</ref><ref>''Basic <sup>1</sup>H - <sup>13</sup>C-NMR spectroscopy'' Metin Balei ISBN 0-444-51811-8</ref> Chemical shifts are also used to describe signals in other forms of spectroscopy such as [[photoemission spectroscopy]].
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| Some atomic nuclei possess 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. <sup>1</sup>H, <sup>13</sup>C, <sup>15</sup>N) 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. | |
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| ==Operating frequency==
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| The operating (or Larmor) frequency [[Omega|<math>\omega_0</math>]] of a magnet is calculated from the [[Larmor equation]]<ref>[http://nmrcentral.com/2011/08/chemical-shift/ Chemical Shift | NMRCentral]</ref>
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| : <math>\omega_{0} = \gamma B_0\,</math>
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| where <math>B_0</math> is the actual strength of the magnet in units like [[tesla (unit)|teslas]] or [[Gauss (unit)|gauss]], and [[Gamma|<math>\gamma</math>]] is the [[gyromagnetic ratio]] of the nucleus being tested which is in turn calculated from its [[magnetic moment]] [[Magnetic dipole moment|<math>\mu</math>]] and [[spin number]] <math>I</math> with the [[nuclear magneton]] <math>\mu_N</math> and the [[Planck constant]] h:
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| : <math>\gamma = \frac{\mu\,\mu_N}{hI}\,</math>
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| Thus, the proton operating frequency for a 1 [[Tesla (unit)|T]] magnet is calculated as:
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| : <math>
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| \omega _0 = \gamma B_0 = \frac{{2.79 \times 5.05 \times 10^{ - 27}\,{\rm{J/T}} }}{{6.62 \times 10^{ - 34}\,{\rm{Js}} \times \left( {1/2} \right)}} \times 1\,{\rm{T}} = 42.5\,{\rm{MHz}}
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| \,</math>
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| ==Chemical shift referencing==
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| Chemical shift [[Delta (letter)|δ]] is usually expressed in [[parts per million]] (ppm) by [[frequency]], because it is calculated from:
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| :<math>\delta = \frac{\mbox{difference between a resonance frequency and that of a reference substance}}{\mbox{operating frequency of the spectrometer}}</math>
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| Since the numerator is usually in [[hertz]], and the denominator in [[megahertz]], delta is expressed in ppm.
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| The detected frequencies (in Hz) for <sup>1</sup>H, <sup>13</sup>C, and <sup>29</sup>Si nuclei are usually referenced against TMS ([[tetramethylsilane]]) or [[DSS (NMR Standard)|DSS]], which is assigned the chemical shift of zero. Other standard materials are used for setting the chemical shift for other nuclei.
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| Thus, an NMR signal that absorbs at 300 Hz higher than does TMS at an applied frequency of 300 MHz has a chemical shift of:
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| :<math>\frac{300\,\rm Hz}{300\times10^6\,\rm Hz}=1\times10^{-6}= 1\,\rm ppm \,</math>
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| 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.
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| ==The induced magnetic field==
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| The electrons around a nucleus will circulate in a magnetic field and create a secondary [[Inductive effect|induced magnetic field]]. This field opposes the applied field as stipulated by [[Lenz's law]] and atoms with higher induced fields (i.e., higher electron density) are therefore called ''shielded'', relative to those with lower electron density. The chemical milieu of an atom can influence its electron density through the [[polar effect]]. Electron-donating [[alkyl]] groups, for example, lead to increased shielding while electron-withdrawing substituents such as [[Nitro compound|nitro groups]] lead to ''deshielding'' of the nucleus. Not only substituents cause local induced fields. Bonding electrons can also lead to shielding and deshielding effects. A striking example of this are the [[pi bonds]] in [[benzene]]. Circular current through the [[hyperconjugated]] system causes a shielding effect at the molecule's center and a deshielding effect at its edges. Trends in chemical shift are explained based on the degree of shielding or deshielding.
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| 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:
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| * the applied effective magnetic field is lower, if the resonance frequency is fixed, (as in old traditional CW spectrometers)
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| * the frequency is higher, when the applied magnetic field is static, (normal case in FT spectrometers)
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| * the nucleus is more deshielded
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| * the signal or shift is '''downfield''' or at '''low field''' or paramagnetic
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| Conversely a lower chemical shift is called a '''diamagnetic shift''', and is '''upfield''' and more shielded.
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| ==Diamagnetic shielding==
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| In real molecules protons are surrounded by a cloud of charge due to adjacent bonds and atoms. In an applied magnetic field ('''B'''<sub>0</sub>) electrons circulate and produce an induced field ('''B'''<sub>i</sub>) which opposes the applied field. The effective field at the nucleus will be '''B''' = '''B'''<sub>0</sub> − '''B'''<sub>i</sub>. The nucleus is said to be experiencing a diamagnetic shielding.
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| ==Factors causing chemical shifts==
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| Important factors influencing chemical shift are electron density, [[electronegativity]] of neighboring groups and anisotropic induced magnetic field effects.
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| 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.
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| A nucleus in the vicinity of an [[electronegative]] atom experiences reduced electron density and the nucleus is therefore deshielded. In [[proton NMR]] of [[alkyl halide|methyl halides]] (CH<sub>3</sub>X) 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.
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| [[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 [[alkene]]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.
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| :[[Image:NMR alkenes.png|400px|Induced magnetic field of alkenes in external magnetic fields, field lines in grey]]
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| 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]].
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| [[Alkyne]] 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.
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| :[[Image:NMR alkynes.png|400px|Induced magnetic field of alkynes in external magnetic fields, field lines in grey]]
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| ==Magnetic properties of most common nuclei==
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| <sup>1</sup>H and <sup>13</sup>C 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 <sup>1</sup>H) of the nuclei in question, the other factor for rare use being their slender representation in nature/organic compounds.
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| {| class="wikitable" align="center"
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| ![[Isotope]]
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| !Width="150"|Occurrence<br> in nature<br> (%)
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| ![[Spin (physics)|spin]] number l
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| !Width="150"|[[Magnetic moment]] μ<ref>In units of the [[nuclear magneton]]</ref><br>
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| !Width="150"|Electric quadrupole moment<br> ([[elementary charge|''e'']]×10<sup>−24</sup> cm<sup>2</sup>)
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| !Width="150"|[[Operating frequency]] at 7 [[Tesla (unit)|T]]<br> (MHz)
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| !Width="150"|Relative sensitivity
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| |-
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| |<sup>1</sup>H
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| |99.984
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| |1/2
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| |2.79628
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| |300.13
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| |1
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| |-
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| |<sup>2</sup>H
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| |0.016
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| |1
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| |0.85739
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| |2.8 x 10<sup>−3</sup>
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| |46.07
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| |0.0964
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| |-
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| |<sup>10</sup>B
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| |18.8
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| |3
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| |1.8005
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| |7.4 x 10<sup>−2</sup>
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| |32.25
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| |0.0199
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| |-
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| |<sup>11</sup>B
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| |81.2
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| |3/2
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| |2.6880
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| |2.6 x 10<sup>−2</sup>
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| |96.29
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| |0.165
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| |-
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| |<sup>12</sup>C
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| |98.9
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| |0
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| |-
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| |<sup>13</sup>C
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| |1.1
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| |1/2
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| |0.70220
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| |75.47
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| |0.0159
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| |-
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| |<sup>14</sup>N
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| |99.64
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| |1
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| |0.40358
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| |7.1 x 10<sup>−2</sup>
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| |21.68
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| |0.00101
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| |-
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| |<sup>15</sup>N
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| |0.37
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| |1/2
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| |−0.28304
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| |30.41
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| |0.00104
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| |-
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| |<sup>16</sup>O
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| |99.76
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| |0
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| |-
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| |<sup>17</sup>O
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| |0.0317
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| |5/2
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| |−1.8930
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| |−4.0 x 10<sup>−3</sup>
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| |40.69
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| |0.0291
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| |-
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| |<sup>19</sup>F
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| |100
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| |1/2
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| |2.6273
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| |282.40
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| |0.834
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| |-
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| |<sup>28</sup>Si
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| |92.28
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| |0
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| |-
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| |<sup>29</sup>Si
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| |4.70
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| |1/2
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| |−0.55548
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| |59.63
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| |0.0785
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| |-
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| |<sup>31</sup>P
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| |100
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| |1/2
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| |1.1205
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| |121.49
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| |0.0664
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| |-
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| |<sup>35</sup>Cl
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| |75.4
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| |3/2
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| |0.92091
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| |−7.9 x 10<sup>−2</sup>
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| |29.41
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| |0.0047
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| |-
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| |<sup>37</sup>Cl
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| |24.6
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| |3/2
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| |0.68330
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| |−6.2 x 10<sup>−2</sup>
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| |24.48
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| |0.0027
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| |-
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| |Colspan=7|Magnetic properties of common nuclei<ref>[[CRC Handbook of Chemistry and Physics]] 65Th Ed</ref>
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| |}
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| '''<sup>1</sup>H, <sup>13</sup>C, <sup>15</sup>N, <sup>19</sup>F''' and '''<sup>31</sup>P''' are the five nuclei that have the greatest importance in NMR experiments:
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| * <sup>1</sup>H because of high sensitivity and vast occurrence in organic compounds
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| * <sup>13</sup>C 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 <sup>12</sup>C, which has a spin of 0 and therefore is NMR inactive.
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| * <sup>15</sup>N because of being a key component of important biomolecules such as [[protein]]s and [[DNA]]
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| * <sup>19</sup>F because of high relative sensitivity
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| * <sup>31</sup>P because of frequent occurrence in organic compounds and moderate relative sensitivity
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| ==Other chemical shifts==
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| 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]], 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.<ref>''A Short History of Three Chemical Shifts'' Shin-ichi Nagaoka Vol. 84 No. 5 May '''2007''' [[Journal of Chemical Education]] 801, {{doi|10.1021/ed084p801}}</ref>
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| ==See also==
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| * [[MRI]]
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| * [[Nuclear Magnetic Resonance]]
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| * [[Nuclear magnetic resonance spectroscopy of carbohydrates]]
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| * [[Nuclear magnetic resonance spectroscopy of nucleic acids]]
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| * [[Nuclear magnetic resonance spectroscopy of proteins]]
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| * [[Protein NMR]]
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| * [[Relaxation (NMR)]]
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| * [[Solid-state NMR]]
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| * [[Zeeman effect]]
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| * [[Random Coil Index]]
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| ==References==
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| {{Reflist}}
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| ==External links==
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| *[http://www.chem.wisc.edu/areas/reich/handouts/nmr-h/hdata.htm www.chem.wisc.edu]
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| *[http://www.bmrb.wisc.edu BioMagResBank]
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| *[http://wwwchem.csustan.edu/Tutorials/NMRTABLE.HTM wwwchem.csustan.edu]
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| *[http://www.chem.wisc.edu/areas/reich/handouts/nmr-h/hdata.htm Proton chemical shifts]
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| *[http://www.chem.wisc.edu/areas/reich/handouts/nmr-c13/cdata.htm Carbon chemical shifts]
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| * Online tutorials (these generally involve combined use of [[IR spectroscopy|IR]], [[Proton NMR|<sup>1</sup>H NMR]], [[Carbon NMR|<sup>13</sup>C NMR]] and [[mass spectrometry]])
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| **[http://www.chem.ucla.edu/~webspectra/ Problem set 1, advanced] (see also this [http://drx.ch.huji.ac.il/nmr/whatisnmr/whatisnmr.html link] for more background information on spin-spin coupling)
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| **[http://orgchem.colorado.edu/hndbksupport/spectprob/problems.html Problem set 2, moderate]
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| <!-- apparently a dead link **[http://www.chem.brown.edu/chem36/problems/SpectroscopyProbs/ Problem set 3, for beginners]-->
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| **[http://www.chem.uni-potsdam.de/tools/kombi1.htm Problem set 4, moderate, German language (don't let that scare you away!)]
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| **[http://www.nd.edu/~smithgrp/structure/workbook.html Problem set 5, the best!]
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| **Combined solutions to problem set 5 [http://www.nd.edu/~smithgrp/structure/answers1-32.GIF (Problems 1–32)] and [http://www.nd.edu/~smithgrp/structure/answers33-64.GIF (Problems 33–64)]
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| {{DEFAULTSORT:Chemical Shift}}
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| [[Category:Nuclear chemistry]]
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| [[Category:Spectroscopy]]
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| [[Category:Nuclear physics]]
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| [[Category:Nuclear magnetic resonance]]
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| [[pl:Spektroskopia NMR#Przesunięcie chemiczne]]
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