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What a dot structure is actually showing is where electrons almost certainly are located, therefore resonance structures indicate a split in those same probabilities. The true situation is that no one can say for certain exactly where any individual electron is at any specific moment, but rather electron location can be expressed as a probability only. Rather, the electrons are portrayed as if they were moving instead. The nuclei of the atoms are not moving when they are represented by resonance structure drawings. The result of all that complexity is simply this: molecules with resonance structures are treated as mixtures of their multiple forms, with a greater percentage of probability given to the most stable configurations. Changes in molecular shape occur so rapidly, and on such a tiny scale, that the actual physical locations of individual electrons cannot be precisely known (due to Heisenberg's Uncertainty Principle). The actual situation on the molecular scale is that each configuration of the molecule contributes a percentage to the possible configurations, resulting in a "blend" of the possible structures. Just as entropic principles cannot be applied to individual molecules, it is impossible to say whether or not any given individual molecule with a resonance structure is literally in one configuration or another. It is sometimes best to use analogies to introduce a topic, but then explain the differences and inevitable complications as further details on a complicated subject. In science, analogies can provide an aid to understanding, but analogies should not be taken too literally. Resonance is easily misunderstood in part because of the way certain chemistry textbooks attempt to explain the concept. This spectrum is detailed enough to serve as a useful "fingerprint" for ethanol, and also is simple enough that we will be able to account for the origin of each line.Resonance refers to structures that are not easily represented by a single electron dot structure but that are intermediates between two or more drawn structures. The overall result is a spectrum such as the one shown in Figure 9-23. When a substance such as ethanol, \(CH_3-CH_2-OH\), the hydrogens of which have nuclei (protons) that are magnetic, is placed in the transmitter coil and the magnetic field is increased gradually, at certain field strengths radio-frequency energy is absorbed by the sample and the ammeter indicates an increase in the flow of current in the coil. The arrows through the nuclei represent the average component of their nuclear magnetic moment in the field direction.Ī schematic diagram of an NMR instrument is shown in Figure 9-22. Transitions between the two states constitute the phenomenon of nuclear magnetic resonance. Figure 9-21: Schematic representation of the possible alignments of a magnetic nucleus (here hydrogen) in an applied magnetic field. The two orientations are not equivalent, and energy is required to change the more stable alignment to the less stable alignment. The nuclear magnets can be aligned either with the field direction, or opposed to it. There are two possible alignments of this magnetic nucleus with respect to the direction of the applied field, as shown in Figure 9-21. To illustrate the procedure with a simple example, consider the behavior of a proton \(\left( ^1H \right)\) in a magnetic field. In NMR spectroscopy, we measure the energy required to change the alignment of magnetic nuclei in a magnetic field. The nuclei of many kinds of atoms act like tiny magnets and tend to become aligned in a magnetic field.

The principle on which this form of spectroscopy is based is simple.

Nuclear magnetic resonance (NMR) spectroscopy is extremely useful for identification and analysis of organic compounds.