Ultraviolet Spectrum of 1,3-Butadiene

Ultraviolet excitation of 1,3-butadiene results in the promotion of an electron from 021 the highest occupied molecular orbital (HOMO), to the lowest unoccupied molecular orbital (LUMO). 

On irradiation with ultraviolet light ihp), 1,3-butadiene absorbs energy and a tt electron is promoted from the highest occupied molecular orbital, or HOMO, to the lowest unoccupied molecular orbital, or LUMO. Since the electron is promoted from a bonding tt molecular orbital to an antibonding it molecular orbital, we call this a tt — tt excitation (read as pi to pi star ). The energy gap between the HOMO and the LUMO 

The exact amount of UV light absorbed is expressed as the sample s molar absorptivity (e), defined by the equation 

Molar absorptivity is a physical constant, characteristic of the particular substance being observed and thus characteristic of the particular ir electron 

Unlike IR and NMR spectra, which show many absorptions for a give molecule, UV spectra are usually quite simple—often only a single peak The peak is usually broad, however, and we identify its position by notin the wavelength at the very top of the peak (A gx. read as lambda max ] 

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Figure 7.5 Ultraviolet-visible spectrum of dehydrohalogenated copolymers of styrene-l-chloro-1,3-butadiene. [Redrawn with permission from A. Winston and P. Wichacheewa, Macromolecules 6 200 (1973), copyright 1973 by the American Chemical Society.]...

In the free-electron molecular orbital model, the electrons actually move in three dimensions. For 1,3-butadiene represent the electrons as particles in a three-dimensional box with a length in the x direction equal to 694 pm, width in the y direction equal to 268 pm, and height in the z direction equal to the width. Find the wavelength of the photons absorbed in the longest-wavelength absorption due to changes in the quantum numbers Uy and n. Explain why the representation as a one-dimensional box can successfully be used to understand the near-ultraviolet spectrum. 

Figure 14-15 Ultraviolet spectrum of 2-methyl-1,3-butadiene in methanol, Amax = 222.5 nm (e = 10,800). The indentations at the sides of the main peak are called shoulders.

Let us consider the ultraviolet spectrum of butadiene (Table 11.1). Assuming two electrons loosely spin-paired in each orbital and neglecting the interaction between the electrons, we can fill two energy levels with four electrons. Then the lowest energy excitation will be firom the highest occupied molecular orbital (HOMO) n = 2 to the lowest unoccupied molecular orbital (LUMO) n = 3. We can estimate the wavelength of this transition as a one-electron jump from n = 2... 

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