2444-120711

2444-120711 - 1 Ultraviolet Spectroscopy The sample is...

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Ultraviolet Spectroscopy 1 • The sample is dissolved in a solvent that does not absorb UV light in the region where the molecule of interest absorbs UV light : water, ethanol, methanol, etc., with no conjugated ! -bonds. The wavelength of UV light can be varied in the instrument during an analysis so the detection system displays a "spectrum" that plots wavelength versus absorption that shows how much energy was absorbed for a given wavelength of UV light. • This is the ultraviolet spectrum for that particular molecule. • When light is absorbed, it registers as a "peak" at that wavelength, and the intensity of that peak is a measure of how much light is absorbed. The intensity of the absorption is expressed as the extinction coefFcient ( ! ) and the larger the extinction coefFcient, the stronger the signal. • The wavelength with the largest extinction coefFcient (the largest peak) is referred to as " max . A typical ultraviolet spectrum shows the difference between a conjugated and a nonconjugated ketone. ! max absorption peaks
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Ultraviolet Spectroscopy 2 • The UV spectrum is a measure of the presence or absence of certain functional groups in a molecule. • The normal range of ultraviolet radiation is 400 nm to 200 nm. This corresponds to 71.5 kcal mol –1 for a frequency of 400 nm and 143 kcal mol –1 for a frequency of 200 nm. A higher frequency in nm
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2444-120711 - 1 Ultraviolet Spectroscopy The sample is...

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