8.1
Déplacement chimique : références internes et effets des solvants
Dans un échantillon RMN, la mesure précise des fréquences d’absorption absolues des noyaux est difficile. Un composé de référence interne standard est…
Il est difficile de mesurer avec précision les fréquences d’absorption absolues des noyaux dans un échantillon. Pour surmonter ce problème, un composé de référence interne standard est ajouté et la différence entre leurs fréquences d’absorption est mesurée.
Les composés de référence internes, comme le tétraméthylsilane, ou TMS, sont chimiquement inertes, solubles dans les solvants RMN et facilement amovibles.
Avec des protons méthyles hautement blindés qui produisent un signal intense à une fréquence plus basse que la plupart des molécules organiques, la TMS est une référence majeure en spectroscopie RMN des protons, du carbone et du silicium.
Si le composé de référence n’est pas inerte, il est conservé dans un tube capillaire à l’intérieur du tube RMN et appelé référence externe.
De plus, les solvants RMN deutérés contiennent des protons résiduels dont le signal peut également être utilisé comme référence secondaire.
Le signal du deutérium lui-même peut être utilisé pour surveiller le champ magnétique de l'instrument par une technique appelée verrouillage.
Le signal de deutérium est constamment comparé à une fréquence de référence et ajusté en cas de variation.
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Q1: Why is tetramethylsilane used as an internal reference in NMR spectroscopy?
Tetramethylsilane (TMS) is the primary internal reference compound in proton, carbon, and silicon NMR spectroscopy because it is chemically inert, soluble in NMR solvents, and easily removable. Its highly shielded methyl protons produce an intense signal at a lower frequency than most organic molecules, making it ideal for standardizing measurements.
Q2: What is the purpose of measuring frequency differences between a reference compound and sample signals?
Precise measurement of absolute absorption frequencies of nuclei is difficult in NMR. By measuring the frequency difference between an internal reference compound and sample signals, chemists overcome this limitation and obtain reliable, standardized chemical shift values that are independent of the instrument's magnetic field strength.
Q3: When is an external reference used instead of an internal reference in NMR?
An external reference is used when a suitable inert internal reference compound is unavailable. The external reference is kept in a capillary tube within the NMR tube, allowing measurement of frequency differences without direct contact between the reference and sample, preventing unwanted chemical interactions.
Q4: How can deuterated NMR solvents serve as secondary references?
Deuterated NMR solvents such as CDCl3, D2O, and (CD3)2SO contain residual protons whose signals can be used as secondary references. These residual proton signals provide an alternative reference point when the primary internal reference is unavailable or unsuitable for a particular analysis.
Q5: What is deuterium locking and why is it important in NMR spectroscopy?
Deuterium locking is a technique that uses the deuterium signal to monitor and stabilize the instrument's magnetic field. The deuterium signal is constantly compared to a reference frequency and adjusted if any variation occurs, ensuring consistent field strength and reliable spectroscopic measurements throughout data acquisition.
Q6: How do solvent effects influence NMR reference selection?
Solvent choice affects reference selection because the reference compound must be soluble in the chosen NMR solvent. Deuterated solvents like CDCl3 and D2O are commonly used because they dissolve both samples and references effectively while their residual protons or deuterium signals provide reliable reference points for chemical shift measurements.
Q7: What properties make tetramethylsilane ideal for standardizing chemical shift measurements?
TMS possesses several ideal properties: it is chemically inert and does not react with samples, it is soluble in standard NMR solvents, it is easily removable after analysis, and its highly shielded methyl protons yield an intense signal at lower frequency than most organic molecules, providing a clear, reliable reference point.