1.3- Butadiene ultraviolet absorption

Ultraviolet absorption is particularly useful for the determination of compounds possessing conjugated unsaturation and as far as hydrocarbon gases are concerned, this includes diolefins such as 1,3-butadiene and its five carbon equivalents. 

Fahr, A. Nayak, A.K. Temperature dependent ultraviolet absorption cross sections of 1,3 butadiene and butadiyne. Chem. Phys. 1994, 189, 725-731. 

A series of random, block and graft copolymers of VC with styrene (S), butadiene (B), MMA, VAc, and VDC was characterized with the help of three consecutive detectors a differential refractometer (RI), an ultraviolet absorption detector (UV) and an automatic viscometer [33]. Figure 5.2 summarizes the relation between solution viscosity and molecular mass for these copolymers. 

Fig. 339. Ultraviolet absorption spectra of poly(styrene-butadiene) films at different UV irradiation times. (Reprinted with permission from [293], Pergamon Press Ltd, Oxford, England.)... 

Use the energy level expressions for benzene and for 1,3-butadiene to obtain two different values for the parameter p in the Hiickel theory, using the fact that the strongest ultraviolet absorption in benzene is at a wavelength 180 nm, while the strongest ultraviolet absorption in 1,3-butadiene is at 217 nm. Conpare these values with an accepted value of = —2.71 eV and explain why the values do not agree. 

Linear polyenes (butadiene, hexatriene, etc.) absorb ultraviolet radiation. They have absorption maxima at the approximate wavelengths given in Table 6-1. 

The description of the photochemical ring closure of butadiene that derives from this picture is as follows. Absorption of an ultraviolet (UV) photon by the butadiene generates the A state. The A state evolves along the correlation line to the A state of cyclobutene until it encounters an allowed crossing with the excited state. It hops over to that state and rolls down to the local minimum on the upper surface. From there it drops down to the maximum on the ground-state surface, allowing it either to return to the reactant or proceed on to ground-state cyclobutene. 

Ultraviolet (UV) spectroscopy is a method cd structure determination applicable specifically to conjugated systems. When a coi jugated molecule is irradiated with ultraviolet light, ener gy absorption occurs and a it electron is promoted from the highest occupied molecular orbital tHOMOl to the lowest unoccupied molecular orbital (L.UMO1. For I,3< butadiene, radiation of 217 nm ia required. As a general rule, the... 

The experimental results on the photochemical cyclobutene-to-butadiene ring opening are not as straightforward as for the thermal reaction. For simple alkyl-cyclobutenes, the photolysis must be done in the vacuum ultraviolet (<200 nm) because of the high energy of the absorption maximum. Cyclobutene ring opening... 

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. 

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... 

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