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2.3: Effect of Non-bonding Electrons

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  • Compare the UV absorption spectrum of benzene and pyridine.

    Benzene has a set of conjugated \(\pi\)-bonds and the lowest energy transition would be a \(\pi\)-\(\pi\)* transition as shown in Figure \(\PageIndex{17}\).

    Figure \(\PageIndex{17}\). Representation of the lowest energy transition in benzene.

    The UV/VIS absorption spectrum for benzene is shown in Figure \(\PageIndex{18}\).

    Figure \(\PageIndex{18}\). Ultraviolet absorption spectrum of benzene.

    Benzene absorbs radiation in the vacuum ultraviolet over the range from 160-208 nm with a \(\lambda\)max value of about 178 nm. Pyridine has a similar conjugation of double bonds comparable to what occurs in benzene.

    Figure \(\PageIndex{19}\) shows filled bonding molecular orbitals (BMOs), empty anti-bonding molecular orbitals (ABMOs) and the location of non-bonding electrons.
    Figure \(\PageIndex{19}\). Representation of the relative energies of bonding molecular orbitals (BMOs), anti-bonding molecular orbitals (ABMOs) and non-bonding electrons.

    For pyridine, the lowest energy transition involves the n-\(\pi\)* orbitals and this will be much lower in energy than the \(\pi\)-\(\pi\)* transition in pyridine or benzene. The UV/VIS absorption spectrum of pyridine is shown in Figure \(\PageIndex{20}\).

    Figure \(\PageIndex{20}\). UV/VIS absorption spectrum of pyridine.

    The shift toward higher wavelengths when compared to benzene is quite noticeable in the spectrum of pyridine, where the peaks from 320-380 nm represent the n-\(\pi\)* transition and the peak at about 240 nm is a \(\pi\)-\(\pi\)* transition. Note that intensity and therefore the molar absorptivity of the n-\(\pi\)* transition is lower than that of the \(\pi\)-\(\pi\)* transition. This is usually the case with organic compounds.

    Dye molecules absorb in the visible portion of the spectrum. They absorb wavelengths complementary to the color of the dye. Most \(\pi\)-\(\pi\)* transitions in organic molecules are in the ultraviolet portion of the spectrum unless the system is highly conjugated. Visible absorption is achieved in dye molecules by having a combination of conjugation and non-bonding electrons. Azo dyes with the N=N group are quite common, one example of which is shown in Figure \(\PageIndex{21}\).

    Figure \(\PageIndex{21}\). Structure of yellow azo dye.
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