A new microstructural model of solvatochromism is suggested and used to describe the solvent induced changes of both the electronic spectrum and structure of a simple pentamethinemerocyanine dye molecule. The electrostatic potential on the surface of a molecule is calculated from the net charges at the atomic centres, which are accessible from semi-empirical all-valence electron MO calculations. The "real" molecular potential thus obtained generates, in a solvent continuum, a reaction field. The re-action of this field on the molecule is included in the Hamiltonian of a quantum-chemical calculation reduced to the π-electron orbitals. The strength of the solvent interaction is proportional to a perturbation parameter λ and includes dispersion, polarisation and specific interaction forces such as hydrogen bonds. The results of the model are in good agreement with the solvent dependent experimental data such as transition energies, oscillator strengths, π-electron densities, and π-bond orders. Another consequence of the model is that the dipole moments of solvatochromic dyes must be strongly solvent dependent already in the ground state. © 1977.
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