Question
Molecules with a lower energy gap between ____ are more easily excited.
Answer
Molecules with a lower energy gap between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are more easily excited.
Note ↑: Organic σ bonds have a large gap between bonding and antibonding orbitals. To excite electrons in such bonds requires wavelengths much below the practical range (< 200 nm). As a result, the technique has found its major uses in the study of π systems, in which filled and unfilled orbitals are much closer in energy. Excitation of such electrons gives rise to π → π* transitions. Nonbonding (n) electrons are even more readily promoted through n → π* transitions (Figure 14-14). Because the number of molecular orbitals is equal to that of the component p orbitals, the simple picture presented in Figure 14-14 is rapidly complicated by extending conjugation: The number of possible transitions skyrockets, and with it the complexity of the spectra.
In Summary: UV and visible spectroscopy can be used to detect electronic excitations in
conjugated molecules. With an increasing number of molecular orbitals, there is an increasing
variety of possible transitions and hence number of absorption bands. The band of longest
wavelength is typically associated with the movement of an electron from the highest occupied
to the lowest unoccupied molecular orbital. Its energy decreases with increasing conjugation.
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Infrared (IR) spectroscopy measures ____.
Molecular vibrations can be seen as ____.
To record an IR spectrum, i____e is measured.
The infrared light range runs from λ = ____.
The useful absorptions for infrared spectroscopy occur at wavelengths of ____ nm.
On an IR spectrum, we use an analog of ____ called wavenumber.