Nuclear Magnetic Resonance (NMR) spectroscopy is a powerful analytical technique used to determine molecular structure. It measures the energy absorption of atomic nuclei, primarily hydrogen protons, when subjected to a strong magnetic field. To ensure structural data is comparable across different instruments, a universal reference point is necessary. Without a common marker, the specific frequencies measured by one NMR machine would not relate directly to those measured by another.
Defining Tetramethylsilane and the Need for a Standard
The compound that fulfills this role is Tetramethylsilane, commonly abbreviated as TMS. Chemically, TMS has the formula \(Si(CH_3)_4\), consisting of a central silicon atom bonded to four equivalent methyl groups. This structure means the twelve hydrogen atoms share an identical chemical environment, causing them to resonate at the same frequency. A spectroscopic reference standard acts as a fixed landmark against which all other signals are measured, accounting for subtle instrumental differences caused by variations in magnetic field strength.
Unique Chemical Properties That Make TMS the Ideal Reference
TMS is the ideal reference due to its unique combination of chemical and physical attributes. The silicon atom is less electronegative than carbon, causing a substantial accumulation of electron density around the TMS protons. This high electron density strongly shields the protons from the external magnetic field, which is why the TMS signal appears far upfield from nearly all other organic proton signals. This isolation is advantageous because the reference signal rarely interferes with the signals of the sample being analyzed.
TMS is chemically inert, meaning it does not react with the vast majority of organic samples. Its low boiling point, approximately \(26.5^\circ\text{C}\), allows it to be easily evaporated and removed after the experiment.
How TMS Sets the Zero Point on the Chemical Shift Scale
The positioning of the TMS signal establishes the fundamental reference for the NMR spectrum’s horizontal axis, known as the chemical shift scale. The resonance frequency of the TMS protons is universally defined as \(0\) ppm, or parts per million, on the delta (\(\delta\)) scale. All other proton signals in the sample are measured relative to this defined zero point. Signals that resonate at higher frequencies than TMS are assigned positive ppm values, which typically range from \(0\) to \(12\) ppm for most organic compounds.
The use of the ppm scale allows NMR data to be universally compared regardless of the instrument’s operating frequency. Chemical shift is calculated by taking the difference between the sample signal’s frequency and the TMS frequency, and then dividing that difference by the spectrometer’s operating frequency. Because this calculation normalizes the data, a proton signal that appears at \(2.5\) ppm on a \(300\) MHz machine will also appear at \(2.5\) ppm on a \(600\) MHz machine. This normalization removes instrument-dependent variables.
Practical Usage and Alternatives to TMS
For most organic chemistry applications using non-aqueous solvents, a small amount of TMS is added directly to the sample solution, functioning as an internal standard. Internal referencing is the most accurate method because the reference and the sample nuclei are exposed to the exact same magnetic environment. However, TMS is not soluble in water, making it unsuitable for biological or aqueous samples.
Alternatives to TMS
For aqueous solutions, an alternative internal standard is required, such as the sodium salt of DSS (2,2-dimethyl-2-silapentane-5-sulfonate). A common modern practice is to reference the spectrum to the residual proton signal present in the deuterated solvent used. For instance, deuterated chloroform (\(CDCl_3\)) contains residual protonated solvent (\(CHCl_3\)), which produces a known signal at \(7.26\) ppm. Spectrometers can be automatically calibrated by setting this known solvent peak to its literature value, eliminating the need to physically add TMS. While TMS remains the primary standard, these secondary referencing methods are widely used for convenience and accuracy.