← Spectroscopic data
1 Reading the notation
Spectroscopy is written in shorthand. None of it is hard, but it is all assumed, so if a symbol has never been explained to you, it is not obvious. Everything you will meet on this page is defined here. All the functional groups, in one go Recommended: walks through every functional group you need — the fastest way to get the whole set straight before naming or spectra make sense.Symbols and units used in spectroscopy
8 termsCover the right column and check you can say what each one means.
- ¹³C (the small number, top-left)
- The raised number in front of an element is its mass number, protons + neutrons, so it names a specific isotope. ¹³C is carbon-13 (6 protons + 7 neutrons). It is written top-LEFT so it is not confused with a charge (top-right, e.g. Na⁺) or a subscript count (bottom-right, e.g. H₂O). ¹³C makes up just 1.1% of carbon nuclei, which is both why ¹³C NMR gives a small signal, and why every carbon-containing mass-spec peak has a tiny companion one mass unit higher.
- δ (delta: the "squiggly" symbol)
- The Greek lower-case letter delta, used here to mean chemical shift: a measure of how shielded or deshielded an atom is. A larger δ means the nucleus is more deshielded, it experiences a greater magnetic field, and it appears further LEFT on the spectrum.
- ppm (parts per million)
- The unit of chemical shift. The spectrum is measured in Hz relative to TMS, then divided by the machine’s resonance frequency. The shift relative to TMS is thousands of Hz while the machine runs at hundreds of millions of Hz, so the numbers come out as 10–100 millionths. Dividing this way also makes shifts consistent regardless of which machine was used. A 300 MHz and a 600 MHz instrument give the same ppm value.
- cm⁻¹ (wavenumber)
- The unit on the x-axis of an IR spectrum, the frequency of the IR radiation, expressed as a wavenumber. Higher wavenumber = higher frequency. IR spectra are conventionally plotted with wavenumber decreasing left to right.
- m/z
- Mass divided by charge, the x-axis of a mass spectrum. Generally most peaks result from a single electron being lost, so z = 1 and you can read m/z directly as the mass of the molecule or fragment.
- [M⁺·] (the molecular ion)
- The peak produced by the whole original molecule after losing one electron. Its m/z gives you the molar mass of the compound. It sits at the right-hand end of the spectrum and is the highest m/z peak, apart from isotope peaks.
- Transmittance (the y-axis of an IR spectrum)
- How much IR radiation passed through the sample. Absorptions therefore point downwards as dips, not upwards as peaks. This catches people out: a "strong absorption" is a deep trough.
- TMS
- Tetramethylsilane, (CH₃)₄Si. The traditional NMR reference, whose carbon signal is defined as 0 ppm. Modern machines usually calibrate off the solvent peak instead, which has itself been calibrated against TMS.