In the Classroom
edited by
applications & analogies
Ron DeLorenzo Middle Georgia College Cochran, GA 31014
An Analogy To Assist Understanding of Splitting Patterns in NMR Spectra Dianne A. Thoben and Thomas H. Lowry* Department of Chemistry, Smith College, Northampton, MA 01063 When students are first learning about proton NMR spectroscopy, they are frequently confused about the origins of the three principal features of the signal from a particular proton or group of equivalent protons. The confusion arises because, although the chemical shift and integration are properties solely of the proton under observation, the splitting pattern arises from interaction with that proton’s neighbors in the molecule. Correct interpretation of the spectrum requires that the student have a clear understanding of this difference; otherwise a triplet, for example, is likely to be interpreted as arising from a group of either three or two protons, Figure 1. The 60 MHz proton NMR spectrum of diethyl ether. rather than from a proton or group of protons coupled to two equivalent Norman Bates is doing to her while we are watching her neighbors. and so is an attribute of her interaction with him, just An analogy from the film industry, known as the as the splitting pattern is an attribute of the interacpoint of view shot, can give students a way of thinking tion of the X protons with the neighboring A protons. about this problem that will help them arrive at the corHaving recorded the signal from the X protons, the rect interpretation. In a famous scene in the Alfred pen continues to the right until it comes to the resonance Hitchcock movie Psycho, Norman Bates stabs a woman position of the A protons. Now the camera is focused on in the shower at the Bates Motel. During this scene, the Norman Bates and we are taking the point of view of camera is looking most of the time from Norman’s point the woman. He is standing at a different location (outof view at the woman’s face; it is from her expression of side the shower); in the spectrum his location corresonds horror that the viewer deduces her reaction to what to chemical shift δ = 1.1. He is also bigger than the Norman is doing to her. We apply this idea to an NMR woman, and the spectrum shows an integration correspectrum in the following way. Figure 1 shows the specsponding to three protons. These two pieces of informatrum of diethyl ether, which reveals two types of protion are attributes solely of Norman Bates. Although in tons: A (more shielded, chemical shift δ = 1.1) and X (less the movie his face is in shadow and not clearly visible, shielded, chemical shift δ = 3.3). In a field sweep experiwe can imagine his expression, which corresonds to the ment, the pen moves from left to right across the chart. triplet splitting pattern; it is determined by the woman’s When it comes to the resonance position of the X proreaction to what is happening to her, and so is again an tons, the camera is focused on the woman in the shower. attribute of the mutual interaction between the two inShe is standing at a certain location (inside the shower); dividuals. in the spectrum her location corresponds to chemical The point of view device is used often in horror movshift δ = 3.3. She is of a medium size; in the spectrum ies. A more contemporary example can be found in Jaws, her size corresonds to the integration showing two prowhere protons A could be the shark from the point of tons. These two pieces of information are attributes view of Richard Dreyfus, and the protons X could be Risolely of the woman in the shower, just as the resonance chard Dreyfus from the point of view of the shark. position and area of integration are attributes solely of We believe that this analogy, based as it is on an the X protons. The expression on her face is the quartet aspect of contemporary culture that all students will splitting pattern. It is determined by her reaction to what have experienced, namely how the movies contrive to *Corresponding author.
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convey emotions in dramatic scenes, will prove useful in helping them interpret NMR spectra.
Journal of Chemical Education • Vol. 74 No. 1 January 1997
