5.3: Factors That Influence NMR Chemical Shift

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So, what kinds of structural factors influence downfield or upfield shifting?

1. Inductive effects. Consider the molecules tetramethylsilane, ethane, and methylamine. The dipole moment of the C-Si, C-C, and C-N bonds are quite different and are based on electronegativity. We observe this effect in the NMR since TMS will have a shift at 0 ppm, ethane comes at 8.4 ppm, and methylamine is shifted downfield to 26.6 ppm. Thus, electronegative atoms will move the frequency downfield, but it is important to note that inductive effects only last for 1-2 bonds.

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2. Resonance. In anisole (or methoxybenzene), the lone pairs on oxygen are delocalized into the aromatic ring. We can draw a resonance structure that places negative charge on one of the ring carbon atoms, and then draw two more resonance structures that place negative charge on alternating carbon atoms. The net result is that each of those carbons atoms will have a partial negative charge. This means there is more electron density at those positions. This has a shielding effect on the chemical shift, so these carbons will resonate more upfield.

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3. Substitution. Methane, ethane, and propane have different chemical shifts for the carbon atoms. Increasing the substitution will shift the resonant frequency downfield. Thus, the trend is that quaternary carbons are most downfield, followed by tertiary, secondary, and primary, which is the most shielded and most upfield of the four.

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4. Magnetic anisotropies. Electron movement in bonds will create magnetic fields, which of course can change the local magnetic environment of a particular nucleus. Consider the molecule benzene. The electrons in the ring are fully delocalized and are moving in a circle around the ring. This creates a magnetic field itself, with two different zones, a shielding zone and a deshielding zone. Nuclei that are in the deshielding zone, such as the protons shown, will be shifted downfield. Any nucleus in the shielding zone will be shifted upfield. The same thing occurs for alkynes, and it is the reason why the methyl groups in this alkyne are shifted so far upfield. Another example is the ketone, whose carbon atoms lie in the deshielding zone, and are thus shifted more downfield.

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Chemical Shift for ¹³C:
Here are some common functional groups and their corresponding chemical shift ranges for the ¹³C nucleus:

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¹H NMR Spectroscopy

We’ve talked a lot about ¹³C NMR (mainly because it is simpler than other nuclei), but there are a few key differences in ¹H NMR spectroscopy that make it indispensable for structure elucidation.

The natural abundance of ¹H is 99.9%, while the natural abundance for ¹³C is 1.1%. This means you can run a ¹H spectrum in a matter of seconds, but it will take much longer to obtain a ¹³C spectrum.
The magnetogyric ratio is greater for ¹H, so there is no overlap in the spectra.
Integration of peaks (area under the curve) corresponds directly to the number of ¹H nuclei with that chemical shift.
Proton indirect coupling reveals \(σ\)-connectivity (we will cover this second semester)

Chemical Shift for ¹H:
Here are some common functional groups and their corresponding chemical shift ranges for the ¹H nucleus:

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