en>Trappist the monk: /* References */replace mr template with mr parameter in CS1 templates; using AWB

2014-09-25T21:41:41Z

References: replace mr template with mr parameter in CS1 templates; using AWB

en>Hans Adler: refine stub

2010-04-26T13:55:35Z

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[[File:Menthol Proton Spectrum.jpg|thumb|431px|'''Example 1H NMR spectrum''' (1-dimensional) of a mixture of [[menthol]] [[enantiomer]]s plotted as signal intensity (vertical axis) vs. [[chemical shift]] (in ppm on the horizontal axis). Signals from spectrum have been assigned [[hydrogen]] atom groups (a through j) from the [[Chemical structure|structure]] shown at upper left.]]
'''Proton NMR''' (also '''Hydrogen-1 NMR''', or '''1H NMR''') is the application of [[nuclear magnetic resonance]] in [[NMR spectroscopy]] with respect to [[hydrogen-1]] [[Atomic nucleus|nuclei]] within the [[molecules]] of a substance, in order to determine the structure of its molecules.<ref>R. M. Silverstein, G. C. Bassler and T. C. Morrill, ''Spectrometric Identification of Organic Compounds'', 5th Ed., Wiley, '''1991'''.</ref> In samples where natural [[hydrogen]] (H) is used, practically all the hydrogen consists of the [[isotope]] 1H (hydrogen-1; i.e. having a [[proton]] for a nucleus). A full 1H atom is called [[Hydrogen-1|protium]].

Simple NMR spectra are recorded in [[solution]], and [[solvent]] protons must not be allowed to interfere. [[deuterium|Deuterated]] (deuterium = 2H, often symbolized as D) solvents especially for use in NMR are preferred, e.g. [[heavy water|deuterated water]], D2O, deuterated [[acetone]], (CD3)2CO, deuterated [[methanol]], CD3OD, [[deuterated dimethyl sulfoxide]], (CD3)2SO, and [[deuterated chloroform]], CDCl3. However, a solvent without hydrogen, such as [[carbon tetrachloride]], CCl4 or [[carbon disulphide]], CS2, may also be used.

Historically, deuterated solvents were supplied with a small amount (typically 0.1%) of [[tetramethylsilane]] (TMS) as an [[internal standard]] for calibrating the [[chemical shift]]s of each analyte proton. TMS is a [[tetrahedral molecular geometry|tetrahedral]] molecule, with all protons being chemically equivalent, giving one single signal, used to define a chemical shift = 0 ppm.
<ref>[http://orgchem.colorado.edu/Spectroscopy/nmrtheory/chemshift.html The Theory of NMR - Chemical Shift]</ref>
It is [[volatility (chemistry)|volatile]], making sample recovery easy as well. Modern spectrometers are able to reference spectra based on the residual proton in the solvent (e.g. the CHCl3, 0.01% in 99.99% CDCl3). Deuterated solvents are now commonly supplied without TMS.

Deuterated solvents permit the use of deuterium frequency-field lock (also known as deuterium lock or field lock) to offset the effect of the natural drift of the NMR's magnetic field <math>B_0</math>. In order to provide deuterium lock, the NMR constantly monitors the deuterium signal resonance frequency from the solvent and makes changes to the <math>B_0</math> to keep the resonance frequency constant.<ref>{{US patent reference| number = 4110681| y = 1978| m = 08| d = 29| inventor = Donald C. Hofer; Vincent N. Kahwaty; Carl R. Kahwaty| title = NMR field frequency lock system}}</ref> Additionally, the deuterium signal may be used to accurately define 0 ppm as the resonant frequency of the lock solvent and the difference between the lock solvent and 0 ppm (TMS) are well known.

Proton NMR spectra of most organic compounds are characterized by [[chemical shift]]s in the range +14 to -4 ppm and by [[spin-spin coupling]] between protons. The [[Integral|integration curve]] for each proton reflects the abundance of the individual protons.

Simple molecules have simple spectra. The spectrum of [[ethyl chloride]] consists of a triplet at 1.5 ppm and a quartet at 3.5 ppm in a 3:2 ratio. The spectrum of [[benzene]] consists of a single peak at 7.2 ppm due to the [[diamagnetic ring current]].

Together with [[Carbon-13 NMR]], proton NMR is a powerful tool for molecular structure characterization.

==Chemical shifts==
[[Chemical shift]] values, symbolized by '''δ''', are not precise, but typical - they are to be therefore regarded mainly as a reference. Deviations are in ±0.2 [[Parts per million|ppm]] range, sometimes more. The exact value of chemical shift depends on molecular structure and the [[solvent]], [[temperature]], [[magnetic field]] in which the spectrum is being recorded and other neighboring [[functional groups]]. Hydrogen nuclei are sensitive to the [[Orbital hybridisation|hybridization]] of the atom to which the hydrogen atom is attached and to [[electronic effect]]s. Nuclei tend to be deshielded by groups which withdraw electron density. Deshielded nuclei resonate at higher δ values, whereas shielded nuclei resonate at lower δ values.

Examples of electron withdrawing substituents are [[hydroxyl|-OH]], [[carboxylate|-OCOR]], [[alkoxy|-OR]], [[nitro compound|-NO2]] and [[halogen]]s. These cause a downfield shift of approximately 2-4 ppm for [[Hydrogen|H]] atoms on Cα and of less than 1-2 ppm for H atoms on Cβ. Cα is an [[aliphatic]] [[Carbon|C]] atom directly bonded to the substituent in question, and Cβ is an aliphatic C atom bonded to Cα. [[Carbonyl group]]s, [[olefin]]ic fragments and [[aromatic ring]]s contribute ''sp2'' hybridized carbon atoms to an aliphatic chain. This causes a downfield shift of 1-2 ppm at Cα.

Note that labile protons (-OH, [[amino|-NH2]], [[sulfhydryl|-SH]]) have no characteristic chemical shift. However such resonances can be identified by the disappearance of a peak when reacted with [[Heavy water|D2O]], as [[deuterium]] will replace a [[Hydrogen-1|protium]] atom. This method is called a '''D2O shake'''. [[Acidic]] protons may also be suppressed when a solvent containing acidic deuterium ions (e.g. [[methanol]]-''d''4) is used.

{| border="1" cellpadding="2" align="left" class="wikitable sortable"

!Width="200"|Functional group
!Width="100"|CH3
!Width="100"|CH2
!Width="100"|CH
|-
|CH2R
|0.8
|1.3
|1.6
|-
|C=C
|1.6
|2.0
|2.6
|-
|C≡C
|1.7
|2.2
|2.8
|-
|C6H5
|2.3
|2.6
|2.9
|-
|F
|4.3
|4.4
|4.8
|-
|Cl
|3.0
|3.4
|4.0
|-
|Br
|2.7
|3.4
|4.1
|-
|I
|2.2
|3.2
|4.2
|-
|OH
|3.3
|3.5
|3.8
|-
|OR
|3.3
|3.4
|3.7
|-
|OC6H5
|3.8
|4.0
|4.3
|-
|OCOR
|3.6
|4.1
|5.0
|-
|OCOC6H5
|3.9
|4.2
|5.1
|-
|OCOCF3
|4.0
|4.4
|/
|-
|CHO
|2.2
|2.4
|2.5
|-
|COR
|2.1
|2.2
|2.6
|-
|COOH
|2.1
|2.3
|2.6
|-
|COOR
|2.0
|2.3
|2.5
|-
|CONR2
|2.0
|2.1
|2.4
|-
|CN
|2.1
|2.5
|3.0
|-
|NH2
|2.5
|2.7
|3.0
|-
|NR2
|2.2
|2.4
|2.8
|-
|NRC6H5
|2.6
|3.0
|3.6
|-
|NR3+
|3.0
|3.1
|3.6
|-
|NHCOR
|2.9
|3.3
|3.7
|-
|NO2
|4.1
|4.2
|4.4
|-
|SR
|2.1
|2.5
|3.1
|-
|SOR
|2.6
|3.1
|/
|-
|=O (aliphatic aldehyde)
|/
|/
|9.5
|-
|=O (aromatic aldehyde)
|/
|/
|10
|-
|M-H (metal hydride)
|/
|/
|-5 to -15
|}
 

==Spin-spin couplings==
[[File:1H NMR Ethyl Acetate Coupling shown.png|thumb|450px|'''Example 1H NMR spectrum''' (1-dimensional) of [[ethyl acetate]] plotted as signal intensity vs. [[chemical shift]]. There are three different types of [[hydrogen|H]] atoms in ethyl acetate regarding NMR. The hydrogens (H) on the CH3COO- ([[acetate]]) group are not coupling with the other H atoms and appear as a singlet, but the -CH2- and -CH3 hydrogens of the [[ethyl group]] (-CH2CH3) are coupling with each other, resulting in a quartet and triplet respectively.]]
The [[chemical shift]] is not the only indicator used to assign a molecule. Because nuclei themselves possess a small magnetic field, they influence each other, changing the energy and hence frequency of nearby nuclei as they resonate—this is known as [[spin-spin coupling]]. The most important type in basic NMR is ''scalar coupling''. This interaction between two nuclei occurs through [[chemical bond]]s, and can typically be seen up to three bonds away.

The effect of scalar coupling can be understood by examination of a proton which has a signal at 1ppm. This proton is in a hypothetical molecule where three bonds away exists another proton (in a CH-CH group for instance), the neighbouring group (a [[magnetic field]]) causes the signal at 1 ppm to split into two, with one peak being a few [[hertz]] higher than 1 ppm and the other peak being the same number of hertz lower than 1 ppm. These peaks each have half the area of the former '''singlet''' peak. The magnitude of this splitting (difference in frequency between peaks) is known as the [[J-coupling|coupling constant]]. A typical coupling constant value would be 7 Hz.

The coupling constant is independent of magnetic field strength because it is caused by the magnetic field of another nucleus, not the spectrometer magnet. Therefore it is quoted in [[hertz]] (frequency) and not ppm ([[chemical shift]]).

In another molecule a proton resonates at 2.5 ppm and that proton would also be split into two by the proton at 1 ppm. Because the magnitude of interaction is the same the splitting would have the same coupling constant 7 Hz apart. The spectrum would have two signals, each being a '''doublet'''. Each doublet will have the same area because both doublets are produced by one proton each.

The two doublets at 1 ppm and 2.5 ppm from the fictional molecule CH-CH are now changed into CH2-CH:
*The total area of the 1 ppm CH2 peak will be twice that of the 2.5 ppm CH peak.
*The CH2 peak will be split into a doublet by the CH peak—with one peak at 1 ppm + 3.5 Hz and one at 1 ppm - 3.5 Hz (total splitting or coupling constant is 7 Hz).

In consequence the CH peak at 2.5 ppm will be split ''twice'' by each proton from the CH2. The first proton will split the peak into two equal intensities and will go from one peak at 2.5 ppm to two peaks, one at 2.5 ppm + 3.5 Hz and the other at 2.5 ppm - 3.5 Hz—each having equal intensities. However these will be split again by the second proton. The frequencies will change accordingly:
*The 2.5 ppm''' + '''3.5 Hz signal will be split into 2.5 ppm + 7 Hz and 2.5 ppm
*The 2.5 ppm''' - '''3.5 Hz signal will be split into 2.5 ppm and 2.5 ppm - 7 Hz

The net result is not a signal consisting of 4 peaks but three: one signal at 7 Hz above 2.5 ppm, two signals occur at 2.5 ppm, and a final one at 7 Hz below 2.5 ppm. The ratio of height between them is 1:2:1. This is known as a '''triplet''' and is an indicator that the proton is three-bonds from a CH2 group.

This can be extended to any CHn group. When the CH2-CH group is changed to CH3-CH2, keeping the chemical shift and coupling constants identical, the following changes are observed:
*The relative areas between the CH3 and CH2 subunits will be 3:2.
*The CH3 is coupled to two protons into a 1:2:1 '''triplet''' around 1 ppm.
*The CH2 is coupled to ''three'' protons.
Something split by three identical protons takes a shape known as a '''quartet''', each peak having relative intensities of 1:3:3:1.

A peak is split by ''n'' identical protons into components whose sizes are in the ratio of the ''n''th row of [[Pascal's triangle]]:
n
0 singlet 1
1 doublet 1 1
2 triplet 1 2 1
3 quartet 1 3 3 1
4 quintet 1 4 6 4 1
5 sextet 1 5 10 10 5 1
6 septet 1 6 15 20 15 6 1
7 octet 1 7 21 35 35 21 7 1
8 nonet 1 8 28 56 70 56 28 8 1

Because the ''n''th row has ''n''+1 components, this type of splitting is said to follow the "''n''+1 rule": a proton with ''n'' neighbors appears as a cluster of ''n''+1 peaks.

With 2-methylpropane, (CH3)3CH, as another example: the CH proton is attached to three identical methyl groups containing a total of 9 identical protons. The C-H signal in the spectrum would be split into '''ten''' peaks according to the (n + 1) rule of multiplicity. Below are NMR signals corresponding to several simple multiplets of this type. Note that the outer lines of the nonet (which are only 1/8 as high as those of the second peak) can barely be seen, giving a superficial resemblance to a septet.

[[Image:J-Coupling-simple-multiplets.gif]]

When a proton is coupled to two different protons, then the coupling constants are likely to be different, and instead of a triplet, a doublet of doublets will be seen. Similarly, if a proton is coupled to two other protons of one type, and a third of another type with a different, smaller coupling constant, then a triplet of doublets is seen. In the example below, the triplet coupling constant is larger than the doublet one. By convention the pattern created by the largest coupling constant is indicated first and the splitting patterns of smaller constants are named in turn. In the case below it would be erroneous to refer to the quartet of triplets as a triplet of quartets. The analysis of such multiplets (which can be much more complicated than the ones shown here) provides important clues to the structure of the molecule being studied.

[[Image:J-Coupling-complex-multiplets.gif]]

The simple rules for the spin-spin splitting of NMR signals described above apply only if the chemical shifts of the coupling partners are substantially larger than the coupling constant between them. Otherwise there may be more peaks, and the intensities of the individual peaks will be distorted (second-order effects).

==Carbon satellites and spinning sidebands==
Occasionally, small peaks can be seen shouldering the main 1H NMR peaks. These peaks are not the result of proton-proton coupling, but result from the coupling of 1H atoms to an adjoining [[carbon-13]] (13C) atom. These small peaks are known as [[Carbon-13 NMR satellite|carbon satellite]]s as they are small and appear around the main 1H peak i.e. [[satellite]] (around) to them. Carbon satellites are small because only very few of the molecules in the sample have that carbon as the rare NMR-active 13C isotope. As always for coupling due to a single spin-1/2 nucleus, the signal splitting for the H attached to the 13C is a doublet. The H attached to the more abundant 12C is not split, so it is a large singlet. The net result is a pair of evenly-spaced small signals around the main one. If the H signal would already be split due to H–H coupling or other effects, each of the satellites would also reflect this coupling as well (as usual for complex splitting patterns due to dissimilar coupling partners). Other NMR-active nuclei can also cause these satellites, but carbon is most common culprit in the proton NMR spectra of organic compounds.

Sometimes other peaks can be seen around 1H peaks, known as [[spinning sidebands]] and are related to the rate of spin of an [[NMR tube]]. These are experimental artifacts from the spectroscopic analysis itself, not an intrinsic feature of the spectrum of the chemical and not even specifically related to the chemical or its structure.

Carbon satellites and spinning sidebands should not be confused with impurity peaks.<ref>{{cite journal |author=Gottlieb HE, Kotlyar V, Nudelman A |title=NMR Chemical Shifts of Common Laboratory Solvents as Trace Impurities |journal=J. Org. Chem. |volume=62 |issue=21 |pages=7512–7515 |date=October 1997 |pmid=11671879 |doi = 10.1021/jo971176v}}</ref>

==See also==
*[[Mass spectrometry]]
*[[Pople Notation]] – letter designations for coupled spin-systems

==References==
{{reflist}}
{{refbegin}}
{{refend}}

==External links==
* [http://www.wfu.edu/~ylwong/chem/nmr/h1/ 1H-NMR Interpretation Tutorial]

* [http://riodb01.ibase.aist.go.jp/sdbs/cgi-bin/cre_index.cgi?lang=eng Spectral Database for Organic Compounds]
* [http://www.chem.wisc.edu/areas/reich/handouts/nmr-h/hdata.htm Proton Chemical Shifts]
* [http://nmr.chinanmr.cn/guide/eNMR/1dcont.html 1D Proton NMR] 1D NMR experiment

{{NMR by isotope}}

{{DEFAULTSORT:Proton Nmr}}
[[Category:Nuclear magnetic resonance]]
[[Category:Spectroscopy]]

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en>Trappist the monk: /* References */replace mr template with mr parameter in CS1 templates; using AWB

en>Hans Adler: refine stub