8.1
化学位移：内参和溶剂效应
在核磁共振信号中，能够精确测量原子核的绝对吸收频率是很困难的。此时则需要添加标准的内参化合物，并测量参比信号和样品信号之间的频率差来解决这类问题。
核磁共振波谱中常用的内参化合物是四甲基硅烷（TMS）。四甲基硅烷是优选的，因为它具有化学惰性，从而使其溶于核磁共振的溶剂中，并且能够易于去除。此外，在四…
精确测量样品中核的绝对吸收频率较为困难。为克服这一问题，需加入一种标准内参化合物，并测量其与样品核吸收频率之间的差值。
内标化合物（如四甲基硅烷，即TMS）化学性质惰性，可溶于核磁共振溶剂，且易于去除。
由于三甲基硅烷（TMS）具有高度屏蔽的甲基质子，其信号强度高且共振频率低于大多数有机分子，因此是质子、碳和硅核磁共振波谱分析中的主要参考标准。
如果参比化合物具有反应活性，则将其置于核磁共振管内的毛细管中，称为外部参比。
此外，氘代核磁共振溶剂中含有残留质子，其信号也可用作次级参考。
氘本身的信号可通过一种称为锁场的技术来监测仪器的磁场。
氘信号会持续与参考频率进行比较，若存在任何偏差，将进行相应调整。
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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.