Electronic spectroscopy Hiickel approximations

Values of a and )3 are measured spectroscopically, and the electronic spectroscopy of many tt electron systems shows that the Hiickel approximation works fairly well. Many transitions between tt electronic states occur in the visible or ultraviolet region of the spectrum. These transitions are the cause of color in conjugated tt electron systems. In the Hiickel approximation, all of the tt molecular orbitals end up with a value of energy having the form E = a + K)8, where the value for K depends on the system. Therefore, only the values of K and )8 determine the molecule s tt energy level pattern, which is what is probed in an experimental spectrum. However, because of how it is defined, )3 has a similar value for most tt systems about —75 kj/mol. The value for a can be determined from atomic spectra. Because a specific value for a is not necessary in understanding the pattern of the tt electronic states, its value is not usually a matter of concern. (For carbon atoms, a is about — 1120 kJ/mol, which is much larger than )8.)... 

E, of ethylene with two electrons is 2p according to the Hiickel approximation where the interelectron repulsion is neglected. This energy gap is approximately -250kJ/mol for ethylene and has been confirmed by UV-spectroscopy [2],... 

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