13.4 UV/VIS Spectroscopy The spectroscopy which utilizes the ultraviolet (UV) and visible (VIS) range of electromagnetic radiation, is frequently referred to as Electronic Spectroscopy . The term implies that these relatively high energy photons disturb the electron distribution within the molecule. Consequently, the MO description of the molecular electron distribution and its change during excitations is very useful. 13.4.1 The orbital basis of electronic spectroscopy In general, we may distinguish two (2) bonding type MO ( σ and π ), one (1) non-bonding type (n) lone pair MO, and two (2) anti-bonding type MO ( σ * and π *). This scheme is illustrated by Figure 13.34. Figure 13.34 A schematic illustration of six (6) transitions between three (3) types of occupied and two (2) types of unoccupied MO. Depending on the extent of conjugative interactions, these transitions may fall anywhere within the three well recognized segments of the spectrum. Table 13.5 Ranges of the electromagnetic radiation used in electronic spectroscopy. Radiation λ ν * nm Å cm -1 VIS 400-750 4000-7500 25000-13000 near UV 200-400 2000-4000 50000-25000 far UV a) <200 <2000 >50000 a) Also called vacuum UV The MO level diagrams of Figure 13.34 are practically the same as the one presented previously for formaldehyde (c.f. Figure 6.18). In the case of simple carbonyl compounds, irrespective of whether they are involved in conjugative stabilization or not, two types of transitions may be
observed: the low energy (long wavelength) n →π* and the higher energy (shorter wavelength) π→π* as illustrated by Figure 13.35. Figure 13.35 A typical electronic spectrum of a carbonyl compound. The intensity of the absorption is measured by the molar extinction coefficient ( ε ) and it is defined in terms of the incidental (I o ) and transmitted (I) light intensity, as well as the concentration of the solution (c) and the path length (l). ° ± ² ³ ´ µ = I I cl 0 10 log 1 ε [13.34a] The value of ε may be within a large range of values 0 ≤ ε ≤ 10 6 [13.34b] when c = 1 mol/L and l = 1cm. The greater the ε value, the more probable the absorption. In general, the n →π* absorption is practically “symmetry forbidden” or “overlap forbidden” because the electron is promoted from the plane of the molecule to a plane which is perpendicular to the molecular plane. This is not the case for the π→π* excitation and therefore, it is an allowed transition, and consequently, more intense.
Figure 13.36 A schematic illustration of the spatial arrangement of the two lobes of the lone pair n (which is like a 2p y of oxygen) and the four lobes (two + and two -) of the π* MO or a carbonyl functional group. The wavelength (or wavenumber) depends on the extent of conjugation. This is illustrated by the data summarized in Table 13.6 and the underlying principle is shown in Figure 13.37. Table 13.6 Carbonyl transitions as the function of conjugative interaction.
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