NMR spectroscopy
Techniques and analysis
Nuclear magnetic resonance is the single most informative tool available for working out the structure of an organic compound. A proton spectrum tells you how many kinds of hydrogen a molecule contains, what each one is attached to, and how many hydrogens sit on the neighbouring carbons; a carbon spectrum counts the distinct carbons. Learning to read a spectrum is a matter of asking those questions in order, every time, rather than trying to recognize the whole picture at once.
Shielding and chemical shift
Where a signal appears, and why
A hydrogen nucleus placed in a magnetic field can align with the field or against it, and the energy gap between those two states is what the spectrometer measures. The nucleus does not feel the applied field in full, however. The electrons around it circulate and generate a small opposing field, so the nucleus is shielded and comes into resonance at slightly lower frequency. Anything that pulls electron density away from a hydrogen atom, e.g. an electronegative atom, a positive charge, or a π system, reduces that shielding and moves the signal downfield, to the left of the spectrum.
Positions are reported on the δ scale in parts per million (ppm), measured from tetramethylsilane at 0 ppm. Using a ratio rather than an absolute frequency makes the number independent of the instrument, so a signal at 2.1 ppm is at 2.1 ppm on a 300 MHz instrument and on a 600 MHz instrument. Two effects account for most of what you see: induction, which explains why the hydrogens next to oxygen or halogens sit at 3–4 ppm, and anisotropy, the circulation of electrons in a π system, which is why hydrogens on arenes appear near 7 ppm and aldehyde hydrogens near 10 ppm rather than merely somewhat downfield.
Equivalence and integration
How many signals, and how many hydrogens in each?
The number of signals tells you how many chemically distinct kinds of hydrogen the molecule has. Two hydrogens are equivalent if a symmetry operation of the molecule interchanges them, or if free rotation averages their environments, as it does for the three hydrogens of a methyl group. Working through the structure and grouping the hydrogens by symmetry before looking at the spectrum is the most reliable way to start; if you predict four signals and the spectrum shows three, your structure is probably wrong.
The area under each signal is proportional to the number of hydrogens producing it, and the instrument reports that area as an integral. Integration gives a ratio, not an absolute count, so a 3:2 ratio might mean three and two hydrogens or six and four; the molecular formula from the mass spectrum is needed. Note that this is a genuinely quantitative measurement in 1H NMR, unlike the carbon spectrum, where signal heights are not reliable.
Spin–spin splitting
Reading the neighbours off the multiplet
A hydrogen also senses the tiny magnetic fields of the hydrogens on the adjacent carbons, transmitted through the bonds. Each neighbour can be aligned with or against the applied field, and the resulting combinations split the signal into a multiplet. For a set of n equivalent neighbours the signal appears as n + 1 lines with intensities predicted by Pascal's triangle: e.g. three neighbours give a quartet in a 1:3:3:1 pattern. Splitting is mutual, so a triplet somewhere in the spectrum implies a partner elsewhere, and the two multiplets share the same coupling constant J, measured in hertz.
Because J is a real physical coupling and not a chemical shift, it does not change with field strength, which is how you confirm that two multiplets belong together. The size of J is also structurally informative: a typical open-chain coupling is 6–8 Hz, a trans alkene coupling is much larger at 12–18 Hz, and a cis alkene coupling is smaller at 6–12 Hz. Hydroxyl and amine hydrogens usually appear as broad singlets, since they are weakly acidic and exchange between molecules faster than the coupling can be resolved.
Carbon-13 NMR
Counting the carbon skeleton
Only the 13C isotope is magnetically active, and it makes up only about 1.1% of natural carbon. The signals are therefore weak and need many scans, but the low abundance has a convenience attached: two 13C nuclei are almost never adjacent, so carbons do not split one another. Routine spectra are also run with the protons decoupled, which removes C–H coupling as well. The result is a spectrum of singlets; one for each chemically distinct carbon, and that is read essentially as a count.
The shift range is roughly 0–220 ppm, about twenty times wider than the proton range, so signals rarely overlap and small structural differences are easier to see. The same shielding arguments apply: sp3 carbons fall below 100 ppm, alkene and aromatic carbons around 110–160 ppm, and carbonyl carbons sit at 160–220 ppm, with ketones and aldehydes furthest downfield. One caution: because decoupling enhances different carbons to different extents, peak intensity is not a reliable count, and quaternary carbons are often noticeably small.
Reference
Chemical shift ranges
Approximate values. Substituents shift these by a few tenths of a ppm, and two effects on the same hydrogen add.
Self-check
Six questions before you move on
Try to work out an answer on paper, then reveal to check. If your reasoning is right but the answer is wrong, you are closer than you think.
Why is a chemical shift quoted in ppm rather than in hertz?
The frequency separation from TMS scales with the strength of the magnet, so a value in Hertz only means something for one instrument. Dividing by the operating frequency gives a ratio that is the same on every spectrometer.
Aromatic hydrogens appear near 7 ppm, well downfield of a vinyl hydrogen. Induction alone does not explain it. What does?
Anisotropy. The applied field drives a ring current in the delocalized π system, and the induced field reinforces the applied field in the region outside the ring, where the hydrogens sit. They are deshielded as a result.
C3H7Cl gives a spectrum with a triplet, a sextet and a triplet. Which isomer is it, and what does each multiplet correspond to?
1-Chloropropane. Three signals with the middle CH2 split by five neighbours means an unbranched chain: CH3 at about 1.0 ppm (triplet), the central CH2 near 1.8 ppm (sextet), and the CH2Cl near 3.5 ppm (triplet). 2-Chloropropane would show only two signals, a doublet and a septet.
Two alkene isomers show couplings of 15 Hz and 9 Hz. Which is which, and would those numbers change on a 600 MHz instrument?
15 Hz is the trans isomer and 9 Hz the cis; the dihedral relationship makes trans coupling larger. J is a through-bond interaction, not a shift, so the values are unchanged at any field strength.
Why do carbons not split one another in a routine 13C spectrum?
Only 13C is magnetic and it is just 1.1% abundant, so the chance of two of them being adjacent in the same molecule is miniscule. Coupling to the attached hydrogens is removed separately by broadband decoupling, leaving singlets.
A C8H10 aromatic compound shows only three 13C signals in the aromatic region and one methyl signal. What is it?
p-Xylene. One methyl signal means both methyls are equivalent, and the para arrangement leaves only three distinct ring carbons. The ortho and meta isomers each give four aromatic signals, and ethylbenzene would give two aliphatic signals.