Molecular interaction with electromagnetic radiation

The various spectroscopies are the primary method for determining the structure of
compounds. If the molecule is not too large or complex, the determination should be very
accurate. These are simply done and rapid. They can be combined to give overlapping
This is not chemistry in the sense of reactions but it is very interesting puzzle solving. Once
you understand the rules, you will like it (except of course in exams).
The chapter begins with background information on how these techniques work which is
interesting but not essential to using them to determine structures. In the NMR, the
information sequence is the logical progression from the simple to the more complex. But do
not think that the first item, chemical shift, or the second, integration, is the important
component. The splitting or multiplicity is the key element in H-NMR. IR is much more
straightforward; memorize a few absorption numbers to identify functional groups. MS is also
straightforward. We will not do UV-Vis becauase it is not very useful for structure
identification. It is an extremely important tool for quantitating substances and is used widely.
1. Molecular interaction with electromagnetic radiation.
Molecules have electromagnetic fields derived from their electrons and nuclei. We
saw earlier that plane-polarized light interacts by being rotated by an enantiomer. As seen
below, energy varies across the spectrum and matches that required for various interactions.

The Electromagnetic Spectrum energy increases going to the left. The electromagnetic radiation interacts with the
electromagnetic fields of the electrons to raise their energy levels from one state to the next.
The nature of that interaction depends on the energy available. Ultraviolet and visible have
sufficient energy to effect electronic transitions. Infrared has sufficient energy only to effect
transitions between vibrational energy states. Microwave has only enough energy to effect
transitions between rotationaly energy states. Thus the radiation absorbed tells us different
information. Radio waves have insufficient energy to effect molecules but affect nuclear spin
energy states found in magnetic fields. This latter interaction is most important because it is
used in Nuclear Magnetic Resonance spectroscopy.

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I’m a life long learner who loves reading, painting, sci-fi and travelling. As the publisher of The Sciencelock, I edits the website Features and writes articles across the publication.

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