Teacher Notes on: NMR Spectroscopy What is it? Nuclear Magnetic Resonance (NMR) spectroscopy is (arguably) the most powerful tool available for determining the structure of organic compounds. This technique relies on the ability of atomic nuclei to behave like a small magnet and align themselves with an external magnetic field. When irradiated with a radio frequency signal the nuclei in a molecule can change from being aligned with the magnetic field to being opposed to it. Therefore, it is called nuclear for the instrument works on stimulating the nuclei of the atoms to absorb radio waves. The energy frequency at which this occurs can be measured and is displayed as an NMR spectrum. The most common nuclei observed using this technique are 1 and 13 C, but also 31 P, 19 F, 29 Si and 77 Se NMR are available. What is it used for? Difference between NMR and MS To identify and/or elucidate detailed structural MS is destructive, whereas NMR is not. owever, a much information about chemical compounds. For smaller amount of material is needed for MS techniques. NMR example: and Mass Spectrometry (MS) are complementary techniques: Determining the purity of medicines before they leave the while MS can tell the weight (and thus the molecular formula) factory Identifying contaminants in food, cosmetics, or medications of a molecule, NMR can differentiate between structural elping research chemists discover whether a chemical isomers, and provide information about connectivities reaction has occurred at the correct site on a molecule between atoms within a molecule. Identifying drugs seized by police and customs agents Checking the structure of plastics, to ensure they will have the desired properties (Pictures sourced from: http://tinyurl.com/yfoxr5 ; http://tinyurl.com/yje6t3 ow does it work? To get the nuclei in a molecule to all align in the same direction, a very strong magnetic field is generated using a superconducting electromagnet, which requires very low temperatures to function. The coils of the magnet are surrounded by liquid helium (4K, or -269 C), which is prevented from boiling off too quickly by a surrounding layer of liquid nitrogen (-77 C). These coolants are all contained in double-layer steel with a vacuum between the layers, to provide insulation just like a thermos. There is a narrow hole through the middle of the magnet, and the sample tube and radio frequency coils ("probe ) are located there. Further Information on NMR Spectroscopy: http://tinyurl.com/yje6t3 http://tinyurl.com/yms9yb
ow to read the spectrum An NMR spectrum appears as a series of vertical peaks/signals distributed along the x-axis of the spectrum (Figures 1-4). Each of these signals corresponds to an atom within the molecule being observed. The position of each signal in the spectrum gives information about the local structural environment of the atom producing the signal. For example, the 13 C NMR spectrum of ethanol Fig. 1: 13 C NMR spectrum of ethanol (C3C2O) is shown in figure 1. The two carbons in ethanol are in different structural environments and hence each produces a signal in the NMR spectrum. The carbon attached to oxygen is deshielded due to the electronegative nature of oxygen and this shifts its signal towards the left in the spectrum. Whereas the carbon bonded only to hydrogens and carbon appears at the right of the spectrum. A similar effect is seen in the 1 NMR spectrum of ethanol. The two protons of the C2 group neighbouring the oxygen are further to the left in the spectrum, whilst the hydrogens of the C3 group that is most remote from the oxygen produce a signal towards the right of the spectrum. The signals in the 1 NMR spectrum do not necessarily appear as a single line, as can be seen in figure 2. The splitting pattern seen in these signals gives information as to how many hydrogens are present on the neighbouring carbon. Also, integration of the 1 NMR signals allows the number of hydrogens in each environment to be determined. 13 C NMR ppm C3 18.1 C2 57.8 1 NMR C 3 1.23 C 2 3.69 O 2.61 Table1: Peaks for ethanol 3 C C 2 O Fig. 2: 1 NMR spectrum of ethanol O O O O C 3 Aspirin The 13 C and 1 NMR spectra for aspirin are shown in figures 3 and 4, respectively. Aspirin, with nine different carbons produces a 13 C NMR spectrum with nine individual signals. Again, the positions of the signals indicate the individual structural environments of each carbon. Six signals are clustered around the 120-150ppm region, typical for carbons in an aromatic benzene ring. The two carbonyl carbons (C=O) appear characteristically towards the left of the spectrum (170 ppm) whilst the carbon of the C3 group, not being attached to an electronegative element or part of an aromatic ring, appears at the right of the spectrum. The 1 NMR spectrum of aspirin (figure 4) shows 6 signals, due to six different hydrogen environments. The signals in the 7-8 ppm range are typical for hydrogens attached to an aromatic (benzene) ring. The hydrogen of the carboxylic acid (COO) produces a broad signal at 11.2 ppm and the C3 group is at 2.2 ppm.
Figure 3: 13 C NMR spectrum of aspirin Figure 4: 1 NMR spectrum of aspirin 13 C NMR ppm CO2 170.2 COC3 169.8 C3 21.0 6 aromatic C 122-151 1 NMR CO2 11.2 C 3 2.35 4 aromatic C 7.14-8.13 Table 2: Peaks for aspirin For some profiles of chemists see www.freeradical.org.au then follow the links to the community pages, then to teachers page Spotlight on Science- Maree and the Free Radicals Maree is undertaking a PhD in Free Radical Chemistry at The University of Melbourne. She is working on developing radical methodology to synthesis selenium containing anti-oxidants. Selenium is useful in the body as an anti-oxidant to mop up free radical damage and Maree s project is looking to more effectively synthesis such compounds. The main reaction she is researching is illustrated below. From her starting compound C 17 18 OS 2 Se [1] she adds an alkene [2], then irradiates it with a mercury lamp. This cleaves the Carbon-Sulfur bond to generate a benzyl radical that adds to the alkene to give the intermediate [3]. The intermediate radical then undergoes homolytic substitution at selenium to afford the product [4] and a by-product [5]. As the methodology which she is working on has never been tested before, she uses NMR to calculate the yield and identify the products of this free radical cyclisation reaction. As she knows what the spectra of all key compounds in the reaction [1-5], she is able to calculate the yield of the product [4] under different conditions allowing for the optimisation of the reaction. She does this through comparing the integration of certain peaks in the product with an internal standard. Each NMR analysis takes around 3 minutes, and Maree will probably complete hundreds for her PhD.
More complex NMR The spectra shown above are described as being onedimensional (1D), because we are looking at the individual resonance frequencies for the different nuclei in a molecule. As we move towards bigger molecules with more and more atoms, the 1D spectra become very complex, and two-dimensional (2D) spectroscopy becomes important in understanding the relationships and interactions between different atoms in the molecule. Figure 5: 2D 1, 13 C-correlation spectrum of a neuraminic acid derivative. (Source: Basic One- and Two-Dimensional NMR Spectroscopy, 3 rd Ed., Wiley- VC) There are many different types of 2D NMR experiments, which allow scientists to determine the chains of connectivity between atoms, bond angles, and sometimes even through-space distances for atoms not closely connected. In this way, the structures of large molecules, such as proteins, can be solved. Proteins form the molecular machinery in our bodies, and so understanding their structure and function is of great use to medical science, and for the chemistry of drug design. Figure 6: The structure of the protein SSB-2, which was elucidated by NMR at Bio21 in 2005. (Source: http://tinyurl.com/yf5aw9 ) NMR resonance technology is used by doctors, too, when they do an MRI scan of a patient. MRI stands for Magnetic Resonanace Imaging they leave the nuclear off the name so as not to scare the patients! (Pictures\ sourced from http://tinyurl.com/ynfc3q ; http://tinyurl.com/wro2j )
W ith the above examples as reference, try to solve the following exercises related to NMR spectroscopy. Exercise 1: An unknown compound has been shown to possess the molecular formula C38O. There are 3 possible structures (isomers) for this compound. Based on the 13 C NMR spectrum below, which is the likely structure. Exercise 2: Assign the carbons and protons that belong to each signal in the 1 and 13 C NMR spectra of ethyl acetate. O 3 C O C 2 C3 Ethyl acetate 13 C NMR spectrum of ethyl acetate 1 NMR spectrum of ethyl acetate
Exercise 3: An absent-minded chemistry professor has accidentally mixed up his unlabelled medication bottles. Fortunately he is able to analyse each of his bottles using 13 C NMR spectroscopy. Which one of the bottles (A or B) contains his adrenaline medication that is urgently needed to enable him to get through his next lecture? O O N C 3 O Adrenaline Bottle A Bottle B An example of an NMR spectra of Maree s selenium containing heterocycle www.freeradical.org.au