This lesson describes how to read and interpret proton NMR spectra of organic compounds, including peak splitting, the meaning of chemical shift due to deshielding, as well as peak integration.
Identifying Organic Compounds
Forensic scientists often try to piece together shreds of chemical evidence from a crime scene. Many of the chemical compounds in our world are organic (contain carbon), so it is often possible to identify lots of different chemicals. In this lesson, we discuss just such a tool for uniquely identifying compounds: NMR spectroscopy.
What Is NMR Spectroscopy?
Nuclear Magnetic Resonance (NMR) Spectroscopy uses the magnetic properties of nuclei to discover the properties of the nuclei's parent atom. Let's break that down a bit.
Nuclei of atoms have certain magnetic moments, just like magnets have + and - poles. These magnetic moments are due to something called spin.
Each element has a different amount of nuclear spin. Nuclei with an odd number of protons and/or neutrons will have a net spin, whereas those with an even number of both protons and neutrons will not. Only nuclei with net spin can be detected with NMR.
Think of a group of people looking for apartments. They will choose different apartments in a complex, but if they all want to pay the same price, they have to get equally valuable apartments. Nuclei can occupy different 'apartments'or spin states that are equally 'valuable'--that is, at the same energy level.
What happens if some more expensive, but nicer, apartments open up? The folks who align their more expensive desires with the increased rent will move in, but some people will keep their cheaper apartments.
When nuclei are subjected to an external magnetic field (literally a giant magnet), the protons or neutrons in them that happen to be aligned with the magnetic field will be at a different energy than the ones not aligned. A small majority of nuclei will stay at the lower energy level.
The sample is now zapped with radiofrequency radiation, making the nuclei jump to the higher-energy state. It's like getting a year-end bonus; now you can afford that nicer apartment (at least, temporarily)!
Nuclei temporarily living in the higher-energy state eventually fall back to their lower-energy state, and this emits the same energy as the radiofrequency energy they received earlier. When this process occurs, the NMR instrument records the resulting signal as a peak in the NMR spectrum.
Not all families have the same financial situation because they have different numbers of kids at different ages. Similarly, not all nuclei will give off the exact same energy because of their electrons.
The electrons surrounding the nuclei also have spin, which affect peaks in the NMR spectrum. In a given molecule, electrons are not only located on individual atoms, but are somewhat smeared out across the molecule, too.
A particular H atom on an organic molecule will not necessarily have an identical chemical environment to one several atoms away. Consider the propane molecule CH3 CH2 CH3
The H atoms on both ends have identical environments: they are all bonded to a C atom, and each has two H 'neighbors'~. The H atoms on the central carbon are different though: each has only one H neighbor, and its attached C atom is bonded to two other C atoms.
We therefore observe two peaks in the 1 H NMR spectrum for propane: one due to the CH2 hydrogens, and one from the CH3 hydrogens. The H atoms in propane interact with the external magnetic field, but the nuclei themselves are little magnets, and they can interact with each other too.
The CH2 hydrogens can 'see' the CH3 hydrogens and vice versa. This causes the NMR peak from CH2 hydrogens to split into seven separate peaks, whereas the peak from the CH3 hydrogens splits into three separate peaks.
If a chemically distinct H atom sees N identical H atoms bonded to a neighboring C atom(s), the original peak will split into a set of 'N + 1' peaks in the spectrum. We call this the N + 1 rule, and we can use it to identify a molecule from its NMR spectrum.
The X-Axis of an NMR Spectrum: Chemical Shift
Many molecules have other functional groups. Nuclei surrounded by electron-withdrawing functional groups are said to be deshielded. The external magnetic field has a stronger effect on deshielded nuclei because they are somewhat lacking in electron density. For example, consider the ethanal molecule CH3 CHO.
Just as before, we predict two sets of peaks (there are two sets of chemically identical H atoms): one from the CH3 hydrogens, and one from the aldehyde H. The N + 1 rule tells us that the CH3 hydrogens would be split into two peaks (a doublet), and the aldehyde H into four peaks (a quartet).
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But why is the aldehyde H at a lower frequency (lower energy)? The electronegative O atom is better at partially 'stealing'electron density, making it easier for the magnetic field to affect this H to a greater extent than the C on the CH3 does for its H atoms. It's like saying it's easier for a childless couple to make the transition to a more expensive apartment because they lack kids (electrons).
As a result, the aldehyde quartet is found shifted to a lower energy. We call this a chemical shift. Functional groups have fairly predictable chemical shifts. The shifts are recorded in parts-per-million (ppm), which simply tells us the amount of deshielding relative to a different functional group.
One of the most shielded (as opposed to deshielded) types of H atoms are found on the molecule Si(CH3) 4, so we often add this molecule to a sample of an organic molecule and assign the peak at 0 ppm to Si(CH3) 4. You can find a table of chemical shifts for functional groups online.
Ethanal's peaks are not the same size. If we calculate the area under the doublet peaks and compare to that under the quartet peaks, we find the ratio to be 1:3:
Ethanal NMR spectrum with integration
This is exactly the ratio of aldehyde H atoms (1) to CH3 H atoms (3)! Therefore, the area under a set of peaks tells us the relative number of H's responsible for that set. These days, calculating the areas of the sets of peaks, called peak integration, is done by computers.
Nuclear Magnetic Resonance (NMR) Spectroscopy uses the magnetic properties of nuclei to discover the properties of the nuclei's parent atom.
Nuclei surrounded by electron-withdrawing functional groups are deshielded. This lack of electron density allows a stronger affect from external magnetic field.
If a chemically distinct H atom sees N identical H atoms bonded to a neighboring C atom(s), the original peak will split into a set of N + 1 peaks in the spectrum, known as the N + 1 rule.
Calculating areas of sets of peaks is called peak integration.
If we know the number and types of peaks (doublet, triplet, etc.), their chemical shifts, and the peak integrations, it's usually possible to determine the identity of an organic molecule.
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