Theory of hypochromism

Abstract

The explanation of hypochromism recently proposed by Bolton and Weiss is examined critically, and a more detailed theory is derived from the semi-classical radiation formalism. The theory is applied to compute the relative hypochromism for several positions of two ethylene molecules, and results are compared with a previous derivation from the static exciton theory. The present results are approximately three orders of magnitude larger, and are comparable to the large hypochromism observed in polynucleotides for similar intermolecular distances. The formula used by Bolton and Weiss is found to be erroneous, in that a resonance line width is misinterpreted to be equivalent to a molecular band width parameter, but the correct formula changes the results of Bolton and Weiss only by a factor π/2. The present theory is shown to be equivalent to classical dispersion theory, but it does not require the usual dipole approximations. There is a clear conflict with static exciton theory, which also is derived from semiclassical radiation theory and is equivalent to the usual theory of molecular spectra. Thus the status of the latter theory must also be questioned.