NMR Spectroscopy

NMR spectroscopy Definition

Nuclear Magnetic Resonance Spectroscopy, or “NMR,” is a fine-tuned chemistry tool that we use to find out information about an unknown compound’s magnetic properties. It does so by recording the spectral patterns given off by the nuclei within a sample’s atoms. It is wild to think that each singular atom gives off enough of a resonance shift to be traced by NMR machinery, but this is greatly dependent and affected by the effective magnetic field at the nucleus. We can draw upon these resonance patterns to begin to understand the details of a molecule’s three-dimensional structure and the functional groups that form it.

Years upon years of research has helped scientists create the immense spectral library that we now use as a reference tool for taking some of the mystery out of identifying unknown substances. We now know which shifts to expect from which functional groups and can infer structural, chemical, and magnetic information from an NMR reading alone. It is thus no overstatement to say that NMR has helped us develop a deeper understanding of the world around us.

Nuclear magnetic resonance (NMR)

Nuclear Magnetic Resonance (NMR) equipment

NMR and Unknown Compounds

The resonance readings we obtain from NMR spectroscopy lends us the ability to decipher an unknown compound’s molecular structure and its “purity.” Purity is defined by whether or not the compound contains remnants of a solvent (such as methanol or chloroform solvent). If we were testing a mystery substance, then, we would compare the spikes in our NMR readings to the vast spectral libraries in existence and then make inferences about the compounds basic structure.

Our readings let us infer a couple of things. First, the length of a spike in our NMR readings will reflect the relative proportion of said atom in our sample’s skeleton. Therefore, we can estimate how many hydrogen atoms or methane groups are in our unknown compound based on the relative length of the spike compared to those given off by the other atoms. Second, NMR spectroscopy also gives us information about the relative position of our atoms. A hydrogen atom can give off several different resonance signals depending on its neighboring atoms or groups. For example, a hydrogen located next to a polar group, such as an oxygen-containing carboxyl group, will give off a higher NMR reading than a hydrogen neighbored by non-polar methane groups. A hydrogen bonded to a polar atom, on the other hand, will have an even higher NMR reading due to its increased resonance shift. In general, the pattern we find is that polar relationships will give off high NMR readings while non-polar accomplish the opposite. NMR works spectacularly well for functional groups. Hydroxyl groups, amine groups, carboxyl groups, and more have characteristic NMR resonance shifts that are an automatic tell. When we find them, we can really begin to visualize our molecule.

Once our basic structure is understood, our NMR results can help us infer several things about the compound’s molecular conformation and its physical properties (i.e. boiling, melting points, phase changes) based on the molecular components present and their polarity.

The Principles of NMR

The guiding principle behind NMR lies in the fact that nuclei have two special properties: an ability to spin, and they are charged. These properties cause nuclei to react like a magnet. If we were to run an electric current or apply an external magnetic field, this would allow an upward energy transfer to a higher state. This energy transfer is reflected at a certain wavelength and radio frequency. Once the spin returns to its baseline, the emitted energy from the drop will be read by the NMR machinery. Likewise, the application of an external magnetic field can cause a nucleus to either spin in the direction or against the direction of the magnetic field. The nucleus with the opposite spin will have a higher energy while the one with the same direction will have a lower energy. Furthermore, the resonant frequency as it applies to NMR spectroscopy will be affected by the electron shielding of the atoms in our molecule of interest. This, of course, will rely heavily on the inherent chemical environment of the nucleus. But the general rule is that the more electronegative the nucleus is, the higher resonant frequency we will expect. Further, the more electronegative, or “electron withdrawing” the group is (for instance, like CH3 or methyl group) will give off the lowest chemical shift, while the most “electron donating” groups will have the highest chemical shifts. The rise in chemical shift can be due to many factors, including the delocalization of current that occurs in aromatic groups that can distribute current across the groups.

NMR Patterns and Signal Splitting

While we’ve briefly discussed a few things to look for when interpreting NMR readings, it’s important to discuss chemical splitting in more detail and summarize a few points.

Interpreting NMR Signals:

  1. The number of signals will reflect the number of equivalent (“like”) protons
  2. The intensity or size of the signal will infer a ratio of that specific type of proton
  3. The position of the signal will infer “chemical shift,” where the position of the peak in the NMR spectrum will indicate how de-shielded or shielded the proton was
  4. Signal Splitting will be represented by the number of peaks/lines that a proton signal will split into depending on the other proton neighbors.

Signal splitting is an important concept in NMR and gives us vital information about the protons in our molecule. This signal splitting phenomenon is one where we see a proton signal will “split” into several smaller peaks all dependent on their hydrogen neighbors. When there are no adjacent hydrogens, we will observe a single peak. When one hydrogen is adjacent to the particular hydrogen, the resonance will split in two, or a doublet. When two hydrogens are found on adjacent atoms, we will see three peaks called a triplet signal. On the other hand, when there are three hydrogens on the adjacent atoms we will see the resonance split to four peaks called a quartet.

The NMR resonance will be predictably split into N + 1 peaks, where N represents the number of hydrogens on the adjacent atoms.

Quiz

1. Which of the following patterns is correct about NMR spectroscopy?
A. More electronegative or electron-withdrawing atoms will give off higher chemical shifts
B. More electropositive atoms will give off higher chemical shifts
C. More electronegative or electron-withdrawing atoms will give off lower chemical shifts
D. None of the above are true

Answer to Question #1
C is correct. As discussed above, the more electronegative or electron withdrawing a group is, the lower the chemical shift it will give. This is an important pattern to remember when interpreting NMR results.

2. When observing protons in an NMR, having two adjacent hydrogens will likely give which of the following signals?
A. A singlet
B. 2 signals
C. 3 signals
D. A quartet

Answer to Question #2
C is correct. What is described here is signal splitting, which obeys the N+1 rule. Having two adjacent hydrogens will split the signal into three peaks, or a triplet.

References

  • NMR Lab (2017). “Uses of NMR Spectroscopy.” NMR lab. Date accessed 8 Jan 2017. <http://chem.ch.huji.ac.il/nmr/whatisnmr/whatisnmr.html>
  • King, Bruce (2017). “Splitting.” Wake Forest University. Date accessed 8 Jan 2017. <http://users.wfu.edu/ylwong/chem/nmr/h1/splitting.html>