Diferenças entre edições de "Espectroscopia NMR"

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===Chemical shift===
{{Main|Chemical shift}}
Depending on the local chemical environment, different protons in a molecule resonate at slightly different frequencies. Since both this frequency shift and the fundamental resonant frequency are directly proportional to the strength of the magnetic field, the shift is converted into a ''field-independent'' dimensionless value known as the [[chemical shift]]. The chemical shift is reported as a relative measure from some reference resonance frequency. (For the nuclei <sup>1</sup>H, <sup>13</sup>C, and <sup>29</sup>Si, TMS ([[tetramethylsilane]]) is commonly used as a reference.) This difference between the frequency of the signal and the frequency of the reference is divided by frequency of the reference signal to give the chemical shift. The frequency shifts are extremely small in comparison to the fundamental NMR frequency. A typical frequency shift might be 100&nbsp;Hz, compared to a fundamental NMR frequency of 100&nbsp;MHz, so the chemical shift is generally expressed in parts per million ([[Parts per million|ppm]]).<ref name = keeler-2>{{citecitar web | publisher publicado= [[University of California, Irvine]] | work obra= Understanding NMR Spectroscopy | authorautor = James Keeler | format formato= reprinted at [[University of Cambridge]] | url = http://www-keeler.ch.cam.ac.uk/lectures/Irvine/chapter2.pdf | title título= Chapter 2: NMR and energy levels | accessdate acessodata= 2007-05-11}}</ref> To detect such small frequency differences the applied magnetic field must be constant throughout the sample volume. High resolution NMR spectrometers use [[Shim (magnetism)|shims]] to adjust the homogeneity of the magnetic field to parts per billion ([[Parts per billion|ppb]]) in a volume of a few cubic centimeters.
 
[[ImageImagem:Lipscomb-NMR-hexaborene-B6H10.png|thumb|right|Example of the chemical shift: NMR spectrum of hexaborane B<sub>6</sub>H<sub>10</sub> showing peaks shifted in frequency, which give clues as to the molecular structure. (click to read interpretation details)]]
 
By understanding different chemical environments, the chemical shift can be used to obtain some structural information about the molecule in a sample. The conversion of the raw data to this information is called ''assigning'' the spectrum. For example, for the <sup>1</sup>H-NMR spectrum for ethanol (CH<sub>3</sub>CH<sub>2</sub>OH), one would expect three specific signals at three specific chemical shifts: one for the C''H''<sub>3</sub> group, one for the C''H''<sub>2</sub> group and one for the O''H'' group. A typical CH<sub>3</sub> group has a shift around 1 ppm, a CH<sub>2</sub> attached to an OH has a shift of around 4 ppm and an OH has a shift around 2&ndash;3 ppm depending on the solvent used.
Modern analysis software allows analysis of the size of peaks to understand how many protons give rise to the peak. This is known as [[integral|integration]]&mdash;a mathematical process which calculates the area under a curve. The analyst must integrate the peak and not measure its height because the peaks also have ''width''&mdash;and thus its size is dependent on its area not its height. However, it should be mentioned that the number of protons, or any other observed nucleus, is only proportional to the intensity, or the integral, of the NMR signal, in the very simplest one-dimensional NMR experiments. In more elaborate experiments, for instance, experiments typically used to obtain [[carbon-13]] NMR spectra, the integral of the signals depends on the relaxation rate of the nucleus, and its scalar and dipolar coupling constants. Very often these factors are poorly known - therefore, the integral of the NMR signal is very difficult to interpret in more complicated NMR experiments.
 
===J-coupling=== --><!-- This section is linked from [[Hyperfine coupling]]
{{Main|J-coupling}}
{| class=wikitable style="text-align:center" align=right
Coupling to ''n'' equivalent (spin ½) nuclei splits the signal into a ''n''+1 '''multiplet''' with intensity ratios following [[Pascal's triangle]] as described on the right. Coupling to additional spins will lead to further splittings of each component of the multiplet e.g. coupling to two different spin ½ nuclei with significantly different coupling constants will lead to a ''doublet of doublets'' (abbreviation: dd). Note that coupling between nuclei that are chemically equivalent (that is, have the same chemical shift) has no effect of the NMR spectra and couplings between nuclei that are distant (usually more than 3 bonds apart for protons in flexible molecules) are usually too small to cause observable splittings. ''Long-range'' couplings over more than three bonds can often be observed in [[Cyclic compound|cyclic]] and [[aromatic]] compounds, leading to more complex splitting patterns.
 
For example, in the proton spectrum for ethanol described above, the CH<sub>3</sub> group is split into a ''triplet'' with an intensity ratio of 1:2:1 by the two neighboring CH<sub>2</sub> protons. Similarly, the CH<sub>2</sub> is split into a ''quartet'' with an intensity ratio of 1:3:3:1 by the three neighboring CH<sub>3</sub> protons. In principle, the two CH<sub>2</sub> protons would also be split again into a ''doublet'' to form a ''doublet of quartets'' by the hydroxyl proton, but intermolecular exchange of the acidic hydroxyl proton often results in a loss of coupling information.
 
Coupling to any spin ½ nuclei such as phosphorus-31 or fluorine-19 works in this fashion (although the magnitudes of the coupling constants may be very different). But the splitting patterns differ from those described above for nuclei with spin greater than ½ because the [[spin quantum number]] has more than two possible values. For instance, coupling to deuterium (a spin 1 nucleus) splits the signal into a ''1:1:1 triplet'' because the spin 1 has three spin states. Similarly, a spin 3/2 nucleus splits a signal into a ''1:1:1:1 quartet'' and so on.
The above description assumes that the coupling constant is small in comparison with the difference in NMR frequencies between the inequivalent spins. If the shift separation decreases (or the coupling strength increases), the multiplet intensity patterns are first distorted, and then become more complex and less easily analyzed (especially if more than two spins are involved). Intensification of some peaks in a multiplet is achieved at the expense of the remainder, which sometimes almost disappear in the background noise,
although the integrated area under the peaks remains constant.
In most high-field NMR, however, the distortions are usually modest and the characteristic distortions (''roofing'') can in fact help to identify related peaks.
 
Second-order effects decrease as the frequency difference between multiplets increases, so that high-field (i.e. high-frequency) NMR spectra display less distortion than lower frequency spectra. Early spectra at 60&nbsp;MHz were more prone to distortion than spectra from later machines typically operating at frequencies at 200&nbsp;MHz or above.
 
====Magnetic inequivalence====
More subtle effects can occur if chemically equivalent spins (i.e. nuclei related by symmetry and so having the same NMR frequency) have different coupling relationships to external spins. Spins that are chemically equivalent but are not indistinguishable (based on their coupling relationships) are termed magnetically inequivalent.
For example, the 4 H sites of 1,2-dichlorobenzene divide into two chemically equivalent pairs by symmetry, but an individual member of one of the pairs has different couplings to the spins making up the other pair.
Magnetic inequivalence can lead to highly complex spectra which can only be analyzed by computational modeling. Such effects are more common in NMR spectra of aromatic and other non-flexible systems, while conformational averaging about C-C bonds in flexible molecules tends to equalize the couplings between protons on adjacent carbons, reducing problems with magnetic inequivalence.
{{Details|2D-NMR}}
 
'''[[Correlation spectroscopy]]''' is one of several types of two-dimensional nuclear magnetic resonance (NMR) spectroscopy or [[2D-NMR]]. This type of NMR experiment is best known by its [[acronym]], [[Correlation spectroscopy|COSY]]. Other types of two-dimensional NMR include J-spectroscopy, exchange spectroscopy (EXSY), [[Nuclear Overhauser effect]] spectroscopy (NOESY), total correlation spectroscopy (TOCSY) and heteronuclear correlation experiments, such as [[HSQC]], [[HMQC]], and [[HMBC]]. Two-dimensional NMR spectra provide more information about a molecule than one-dimensional NMR spectra and are especially useful in determining the structure of a [[molecule]], particularly for molecules that are too complicated to work with using one-dimensional NMR. The first two-dimensional experiment, COSY, was proposed by Jean Jeener, a professor at Université Libre de Bruxelles, in 1971{{Citationcarece neededde fontes|datedata=Julyjulho de 2010}}. This experiment was later implemented by Walter P. Aue, Enrico Bartholdi and [[Richard R. Ernst]], who published their work in 1976.<ref>Martin, G.E; Zekter, A.S., ''Two-Dimensional NMR Methods for Establishing Molecular Connectivity''; VCH Publishers, Inc: New York, 1988 (p.59)</ref>
 
==Solid-state nuclear magnetic resonance==
{{Main|Nuclear magnetic resonance spectroscopy of nucleic acids}}
 
Nucleic acid NMR is the use of NMR spectroscopy to obtain information about the structure and dynamics of [[nucleic acid]] molecules, such as [[DNA]] or [[RNA]]. {{As of|2003}}, nearly half of all known RNA structures had been determined by NMR spectroscopy.<ref name="furtig">{{cite journalcitar periódico|doi=10.1002/cbic.200300700 | pmid=14523911}}</ref>
 
Nucleic acid NMR uses similar techniques as protein NMR, but has several differences. Nucleic acids have a smaller percentage of hydrogen atoms, which are the atoms usually observed in NMR, and because [[Nucleic acid double helix|nucleic acid double helices]] are stiff and roughly linear, they do not fod back on themselves to give "long-range" correlations.<ref name="addess">{{citecitar booklivro|lastúltimo =Addess|firstprimeiro =Kenneth J.|titletítulo=Bioorganic Chemistry: Nucleic Acids|yearano=1996|publisherpublicado=Oxford University Press|locationlocal=New York|isbn=0195084675|coauthorscoautor=Feigon, Juli|editor=Hecht, Sidney M.|chaptercapítulo=Introduction to <sup>1</sup>H NMR Spectroscopy of DNA}}</ref> The types of NMR usually done with nucleic acids are [[Proton NMR|<sup>1</sup>H or proton NMR]], [[Carbon-13 NMR|<sup>13</sup>C NMR]], [[Nitrogen-15 NMR|<sup>15</sup>N NMR]], and [[Phosphorus-31 NMR|<sup>31</sup>P NMR]]. [[Two-dimensional NMR]] methods are almost always used, such as correlation spectroscopy (COSY) and total coherence transfer spectroscopy (TOCSY) to detect through-bond nuclear couplings, and [[nuclear Overhauser effect]] spectroscopy (NOESY) to detect couplings between nuclei that are close to each other in space.<ref name="wemmer">{{citecitar booklivro|lastúltimo =Wemmer|firstprimeiro =David|titletítulo=Nucleic acids: Structures, Properties, and Functions|yearano=2000|publisherpublicado=University Science Books|locationlocal=Sausalito, California|isbn=0935702490|editor=Bloomfield, Victor A., Crothers, Donald M., and Tinoco, Ignacio|chaptercapítulo=Chapter 5: Structure and Dynamics by NMR}}</ref>
 
Parameters taken from the spectrum, mainly NOESY cross-peaks and [[J-coupling|coupling constants]], can be used to determine local structural features such as [[glycosidic bond]] angles, [[dihedral angle]]s (using the [[Karplus equation]]), and sugar pucker conformations. For large-scale structure, these local parameters must be supplemented with other structural assumptions or models, because errors add up as the double helix is traversed, and unlike with proteins, the double helix does not have a compact interior and does not fold back upon itself. NMR is also useful for investigating nonstandard geometries such as [[Nucleic acid double helix#Bending|bent helices]], non-Watson–Crick basepairing, and [[coaxial stacking]]. It has been especially useful in probing the structure of natural RNA oligonucleotides, which tend to adopt complex conformations such as [[stem-loop]]s and [[pseudoknot]]s. NMR is also useful for probing the binding of nucleic acid molecules to other molecules, such as proteins or drugs, by seeing which resonances are shifted upon binding of the other molecule.<ref name="wemmer"/>
{{Main|Nuclear magnetic resonance spectroscopy of carbohydrates}}
 
Carbohydrate NMR is the application of NMR to structural and conformational analysis of [[carbohydrate]] molecules. The study of [[carbohydrate chemistry]] today relies heavily on NMR spectroscopy. It is a tool that allows the carbohydrate chemist to determine the structure of [[monosaccharides]] and [[oligosaccharides]] from synthetic and natural sources. It is also a useful tool for determining sugar conformations.
 
Modern high field strength NMR instruments used for carbohydrate samples, typically 500&nbsp;MHz or greater, are able to run a suite of 1D and 2D experiments to determine primary structure and conformation of carbohydrate compounds.
 
==Ligações externas==
*{{citecitar web | publisher publicado= [[University of California]] | title título= Understanding NMR Spectroscopy | authorautor = James Keeler | format formato= reprinted at [[University of Cambridge]] | url = http://www-keeler.ch.cam.ac.uk/lectures/Irvine/ | accessdate acessodata= 2007-05-11}}
*[http://www.cis.rit.edu/htbooks/nmr/ The Basics of NMR] - Uma visão não-técnica da teoria da RMN, equipamento e técnicas pelo Dr. Joseph Hornak, Professor de Química do RIT (em inglês)
*[http://scion.duhs.duke.edu/vespa/gamma/ GAMMA and PyGAMMA Libraries] - <!--GAMMA is an open source C++ library written for the simulation of Nuclear Magnetic Resonance Specroscopy experiments. PyGAMMA is a Python wrapper around GAMMA.--> (em inglês)
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