8.1
化学シフト: 内部参照と溶媒の効果
NMR サンプルでは、原子核の絶対吸収周波数を正確に測定することが困難です。標準内部参照化合物を追加し、参照信号とサンプル信号の周波数差を測定します。
NMR 分光法で一般に使用される内部標準化合物はテトラメチルシラン (TMS) です。 TMS は化学的に不活性であり、NMR 溶媒に可溶で、容…
試料中の原子核の絶対吸収周波数を正確に測定することは困難です。これを克服するために、標準的な内部参照化合物を添加し、それらの吸収周波数の差を測定します。
テトラメチルシラン(TMS)などの内部参照化合物は、化学的に不活性で、NMR溶媒に可溶で、容易に除去できます。
TMSは、ほとんどの有機分子よりも低い周波数で強いシグナルを生成する高度にシールドされたメチルプロトンを備えており、プロトン、カーボン、シリコンNMR分光法の主要な基準となっています。
参照化合物が不活性でない場合、NMRチューブ内のキャピラリーチューブに保持され、外部参照と呼ばれます。
さらに、重水素化NMR溶媒には残留陽子が含まれており、そのシグナルは二次参照としても使用できます。
重水素自体からの信号は、ロックと呼ばれる技術によって機器の磁場を監視するために使用できます。
重水素信号は常に基準周波数と比較され、変動がある場合は調整されます。
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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.