8.1
Cambio químico: Referencias internas y efectos de los disolventes
En una muestra de RMN, es difícil medir con precisión las frecuencias de absorción absoluta de los núcleos. Se agrega un compuesto de referencia inter…
La medición precisa de las frecuencias absolutas de absorción de los núcleos en una muestra es difícil. Para superar esto, se agrega un compuesto de referencia interno estándar y se mide la diferencia entre sus frecuencias de absorción.
Los compuestos de referencia internos, como el tetrametilsilano o TMS, son químicamente inertes, solubles en disolventes de RMN y fácilmente removibles.
Con protones de metilo altamente blindados que producen una señal intensa a una frecuencia más baja que la mayoría de las moléculas orgánicas, TMS es una referencia principal en espectroscopia de RMN de protones, carbono y silicio.
Si el compuesto de referencia no es inerte, se mantiene en un tubo capilar dentro del tubo de RMN y se denomina referencia externa.
Además, los disolventes de RMN deuterados contienen protones residuales cuya señal también puede utilizarse como referencia secundaria.
La señal del propio deuterio se puede utilizar para controlar el campo magnético del instrumento mediante una técnica llamada bloqueo.
La señal de deuterio se compara constantemente con una frecuencia de referencia y se ajusta si hay alguna variación.
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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.