Knowledge Bank

The Doub and Vandenbelt ultraviolet spectral $data1$ on 191 substituted benzenes in aqueous solution have been reexamined. The frequency shifts of the $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}A-1Lb$ transition (Doub and Vandenbelt's Secondary Band''), relative to benzene, have been found to obey the $F\""{o}$rster $formula:^{2, 3} \begin{equation} \Delta \nu = \Sigma_{m}l_{m} + [\Sigma_{m}v_{m} \exp (2\pi m/3)]^{2}\end{equation}$ Parameters, l and v, derived from some of the data (30 compounds) have been used with formula (1) to predict the remainder of the data on $^{1}L_{b}$ bands (80 compounds) to within $\pm 1kK$. The frequency shifts of the $^{1}A-^{1}L_{a}$ transition (First Primary Band'') obey the following formula: $\Delta \nu =K[\Sigma m\Delta \nu m+(X0-X3)2]$ The first term is the sum of the ``monoshifts,'' or shifts observed in the monosubstituted benzenes. The second term involves the difference of donor and acceptor strengths for substituents para to each other and is attributed to the mixing of the $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}La$ state with a low lying substituent-to-substituent charge-transfer state. The parameter K is dependent on the number of substituents present, and is in some cases less than unity. The observed monoshifts and values of X derived from combinations of 13 substituents can be used to predict the spectral positions of the remaining 150 compounds to within $\pm 1.5kK$, even for frequency shifts up to 25 kK.