CNMR vs HNMR and Intramolecular H-Bonding¶
When the protons are put into a magnetic field (\(B_\circ\)) and align themselves with or against it, these is a splitting in energy that occurs.
This excitation follows the EM relationship:
Where:
- \(B_\circ=\) the strength of the magnetic field
- \(\hbar=\frac{h}{2\pi}\)
- \(\Delta E=\) the difference in energy between the two spin states
- \(\gamma=\) a constant that is specific to the atom type. This gives us atomic specificity.
We can use a bigger magnetic field to increase the sensitivity of the NMR spectrometer, as \(\nu\propto B_\circ\). This results in higher resolution spectra:
When Shielding or deshielding occurs, what’s happening is that we get a change in the effective \(B_\circ\), that results in a change in \(\nu\)
\(\cnmr\) vs \(\hnmr\)¶
Both provide information about the number of chemically nonequivalent nuclei in the sample and both give us information about the electronic environment that those nuclei exist, however:
- A \(\cnmr\) spectra is \(\e{-4}\) times weaker than a \(\hnmr\) spectra, as a \(\ce{^13C}\) nucleus is only about \(1\%\) as intense as the \(\ce{^1H}\) nucleus and is significantly less abundant (\(\sim1.1\%\)) of a sample of carbon (most are \(\ce{^12C}\)).
- The \(\cnmr\) spectra is more spread out over a larger range than the \(\hnmr\) spectra, making it easier to identify, count and categorise the nuclei.
- Since the abundance of \(\ce{^13C}\) is so low, the likelihood of having two \(\ce{^13C}\) atoms next to each other in a molecules is icredibly low, and accounts ofr why we don’t see splitting in \(\cnmr\)
When to use \(\cnmr\) Spectroscopy¶
When the molecule is heavily unsaturated, the proton information may not give you much clarification to the structure. It may also be used when the \(\hnmr\) spectra are just too complex to be analysed alone
| Chloradane | Croconic Acid | Polyacetylene |
|---|---|---|
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Intramolecular H-Bonding¶
Since any shift of electron density will cause an NMR shift, an H-bonding oxygen will donate electron density from the hydrogen, deshielding it even further than it might already have been.
