A crude treatment of the p electrons of a conjugated molecule regards these electrons as moving in the particle-in-a-box potential of Fig. 2.1, where the box length is somewhat more than the length of the conjugated chain. The Pauli Exclusion Principle (Chapter 10) allows no more than two electrons to occupy each box level. (These two have opposite spins.) For 1,3-butadiene, CH2==CHCH==CH2, take the box length as 7.0 Ã… and use this model to estimate the wavelength of light absorbed when a p electron is excited from the highest-occupied to the lowest-vacant box level of the molecular electronic ground state. The experimental value is 217 nm.

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