spectra (200 - 700 nm) n ® s * Transitions Transitions are not seen in typical UV-Vis. ® s * transitions) shows an absorbance maximum at 125 Methane (which has only C-H bonds, and can only undergo s Possible electronic transitions of p, s, and n electrons are s ® s * TransitionsĪn electron in a bonding s orbital is excited to This appears as a continuous absorption band. This is because the superposition of rotational and vibrational transitions on the electronic transitions gives a combination of overlapping lines. The spectrum of a molecule containing these chromophores is complex. These vibrations and rotations also have discrete energy levels, which can be considered as being packed on top of each electronic level.Ībsorbing species containing p, s, and n electronsĪbsorption of ultraviolet and visible radiation in organic molecules is restricted to certain functional groups ( chromophores) that contain valence electrons of low excitation energy. In a molecule, the atoms can rotate and vibrate with respect to each other. When an atom or molecule absorbs energy, electrons are promoted from their ground state to an excited state. Transitions involving d and f electrons (not covered in this.Transitions involving charge-transfer electrons.There are three types of electronic transition which can be considered The absorption of UV or visible radiation corresponds to the excitation of outerĮlectrons. The absorption from the carbonyl group in diethyl ketone.įor a comprehensive discussion of Beer's Law, click here The UV region for the carbonyl group in acetone is of the same wavelength as For example, the absorption that is observed in Spectrum will show a number of absorption bands corresponding to structural Is a constant of proportionality, called the absorbtivity.ĭifferent molecules absorb radiation of different wavelengths. To the path length, b, and the concentration, c, of the absorbing Increases as attenuation of the beam increases. Many molecules absorb ultraviolet or visible light.
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