But What Is Its Structure? Probing the Molecule with NMR
By Lydia from SLN More Blogs by This AuthorFrom the Science Bits in a World of Bytes Blog Series
Nobody thinks of science without thinking of cool equipment. Anton von Leeuwenhoek’s microscope and Galileo Galilei’s telescope brought huge changes in understanding our world. Equipment has gradually become more sophisticated, allowing to probe deeper into our world around us. We can determine the identity and structure of a purified compound with a tiny amount. A technique often used for structural determination is nuclear magnetic resonance, or NMR spectroscopy.
The history of NMR goes back to 1926, when Wolfgang predicted the concept of nuclear spin. This was confirmed by Stern in 1932, when he detected the nuclear magnetic moment. The first NMR experiments were run in 1945 by Felix Bloch (liquid) and Edward Purcell (paraffin); for this, they shared the 1952 Nobel Prize. The first commercial NMR instrument came to market in 1961. The first pulsed Fourier Transforming NMR instrument was used in 1964. Varieties of two-dimensional NMR, COSY and NOESY, were first demonstrated in 1974 and 1979, respectively. NMR technology has grown steadily stronger, increasing speed and resolution.
The basic determination made by most use of NMR is the carbon-hydrogen framework of a molecule. The “nuclear” part of the name refers to the nucleus of an atom in a molecule. Nuclei with odd numbers of neutrons or protons, such as 1H, 14N, and 13C, have a property called spin that can be up or down. They behave like tiny magnets and thus interact with an external magnetic field. If given the proper amount of energy, they “spin-flip” to a higher state of energy. When this occurs, they are in resonance with the applied radiation, adding “magnetic resonance” to complete the description of the technique.
NMR often starts with dissolving or suspending the sample in deuterochloroform, CDCl3. The “deutero” part is a hydrogen atom with a proton and neutron. It will absorb differently, allowing the instrument to lock in on ordinary hydrogen or whatever nucleus it probes. The NMR technique used most often starts with a pulse of waves to get the nuclei to snap to attention. More pulses are sent through at intervals, knocking down the nuclei. The magnetic field is varied, so all different nuclei will resonate. The waves pass through and land on the receptor, where they are converted into electrical signals, which are in turn amplified and analyzed. The signal seen on the computer is often the average of many runs.
The readout looks like this:
The field strength increases left to right, so the left side is called the downfield side, and the right side the upfield side. Nuclei on the left side require less of a magnetic field to resonate, so they are called “deshielded.” The nuclei on the right side are more shielded by electrons and require a higher magnetic field to resonate. The numbers along the bottom are the chemical shift, where the nucleus absorbs. The absorption is usually measured with reference to tetramethylsilane (TMS), (CH3)4Si, a very shielded species. The peaks for the nuclear species might be split into doublets based on interaction with nearby nuclei.
Hydrogen-1 NMR is fast and easy because almost all hydrogen nuclei are hydrogen-1. Its spectrum runs from 0 to about 10 delta. On the low end, from 0 to 1.5 delta are the hydrogens on saturated compounds, like methyl (-CH3) hydrogens. They are very shielded by electrons. Next, from about 1.5 to 2.5 delta, are hydrogen atoms near but not part of a double bond. The double bond withdraws some electron density, but not as much as the atoms in the next category. When an atom of oxygen, nitrogen, fluorine, chlorine, or bromine is attached to a carbon atom, the hydrogen that is also attached to the carbon has electron density drawn away even more, so its peak will be found between 2.5 and 4.5 delta. A hydrogen on a double bond, called a vinyl hydrogen, will be shifted farther down, from 4.5 to 6.5 delta. Benzene rings’ hydrogens have their own section on the spectrum. My organic chemistry professor said that the electrons, when hit with a pulse, treat the ring like a racetrack and zoom around. This deshields the hydrogen so much that its shift is farthest downfield, between 6.5 and 8.5 delta.
The peaks also, as I said earlier, might not be whole. Depending on how many neighboring protons there are, it could be split into parts. The splitting in general follows the n+1 rule. If there are 2 neighboring protons, the signal will be split into a triplet. These multiplets can be split if there are different kinds of hydrogen on either side of the one in question. For example, while doing NMR on a mystery molecule for a spectroscopy assignment last year, I had a doublet of triplets, as does the cover image of this article. I looked at some other people’s spectra and saw even nastier conglomerations of peaks.
Another interesting feature of the 1H NMR spectrum is the area under the peaks. If you set the area under the shortest peak to one, then the ratio between the area of that peak and larger ones shows the relative amount of protons in each peak. For example, if a peak has three times the area of the smallest one, then there are three times as many protons represented by that particular peak.
13C NMR follows the same principle and deshielding trends but has a greater spread on the spectrum, from 0 delta to about 220 delta. The regions for different kinds of bonds shows much more overlap than hydrogen, so a straight 13C NMR is better used for confirmation of a structure or formula than determination. Another weakness of carbon NMR is that it takes a lot longer to do a run; when running my mystery molecule through the NMR, a run for 13C took about an hour to get a good run, while 1H took around a minute. This is necessary because 13C is a rare isotope, comprising about 1.1 percent of all carbon atoms. A 13C run will have a noisy (more squiggly) baseline than one of 1H and will not show splitting due to isotopic rarity.
In another way, 13C does some things 1H doesn’t. A special technique called DEPT (distortionless enhancement by polarization transfer) helps probe different types of carbon groups in a molecule. There are three varieties, DEPT-45, DEPT-90, and DEPT-135. The head NMR prof explained it this way: DEPT knocks atoms down 45, 90, or 135 degrees and measures what happens as the atom goes back to vertical. DEPT-45 isolates carbons with one, two, or three protons (hydrogen atoms) attached. DEPT-90 shows only CH carbons. In DEPT-135, -CH2- groups have peaks below the baseline, while -CH3 and CH are above the baseline.
During my sophomore year, we organic students struggling with learning NMR rejoiced when the instrument broke down, and we had to analyze our product some other way. Despite that, I have come to recognize the value of this method. It is non-destructive and fast (well, at least 1H NMR) and so is a good way to probe molecules.
Brust, Gregory. "Nuclear Magnetic Resonance Spectroscopy." The University of Southern Mississippi, 13 Jul 2005. Web. 25 Mar 2013.
"History of NMR." Nuclear Magnetic Resonance: The Basics. N.p., 29 Nov 2005. Web. 25 Mar 2013.
"Isotopes of Carbon." Lawrence Berkeley National Laboratory, 24 Jan 2003. Web. 25 Mar 2013.
McMurry, John. Organic Chemistry. 7th ed. Belmont, CA: Brooks/Cole, 2008. Print.
Reddy, Ravinder. "Introduction to NMR." . N.p., 24 Sep 2008. Web. 25 Mar 2013.