1.3- Butadiene ultraviolet spectrum

Analogous results were reported by Hay and co-workers (33, 54). In the polymerization of butadiene initiated by BuLi/TMED, the rate was found to increase with increasing TMED to lLi 2TMED, but additional TMED had no further effect. The ultraviolet spectrum of the polybutadienyl anion suggested chelating agent separated ion pairs in... 

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... 

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.]...

RetinalS. The structure and photophysics of rhodopsins are intimately related to the spectroscopic properties of their retiny1-polyene chromophore in its protein-free forms, such as the aldehyde (retinal), the alcohol (retinol or vitamin A), and the corresponding Schiff bases. Since most of the available spectroscopic information refers to retinal isomers (48-55), we shall first center the discussion on the aldehyde derivatives. Three bands, a main one (I) around 380 nm and two weaker transitions at 280 nm and 250 nm (II and III), dominate the spectrum of retinals in the visible and near ultraviolet (Fig. 2). Assignments of these transitions are commonly made in terms of the lowest tt, tt excited states of linear polyenes, the spectroscopic theories of which have been extensively discussed in the past decade (56-60). In terms of the idealized C2h point group of, for example, all-trans butadiene, transitions are expected from the Ta ground state to B , A, and A" excited states... 

The principal feature in the optical spectrum of linear polyenes is the strongly allowed absorption that lies in the near ultraviolet for the shortest polyene butadiene and shifts to... 

---