Knowledge Bank

The absorbance (extinction) of the secondary band of a number of chlorinated and fluorinated benzene derivatives in non-polar solvents is discussed. The molar absorptivities (extinction coefficients) vary from $0.2\times 103$ to $3.0\times 103$. In opposition to the generally accepted view, it is shown that the molar absorptivity coefficient is not proportional to the square of the spectroscopic moment throughout the entire range. Indeed, for coefficients above $0.4\times 103$, the relationship becomes linear with respect to the spectroscopic moment. Evidence for the presence of important ''ortho'' effects is presented.