Ab initio supermolecular SCF calculations at the STO-3G level are reported for the diacetylene-hydrogen fluoride complexes. The reverse σ-complex is predicted to have somewhat higher stability and H-bond strength than the π-complexes.

An MCSCF model including the effects of solvent polarization is developed. The model is applied within the limitations of INDO approximations to look into the dominant effects of solvent polarization on the electronic structure in the excited states of a model system (e.g.nπ^* states of H₂CO). Important features of macroscopic solvation-induced reorganization of electron density and some consequence thereof are noted.

Gas phase proton affinities of formaldehyde and fluoroformaldehyde in the ground,^1,3nπ^* (lowest) and³ππ^* (lowest) states have been theoretically studied within the framework of the INDO-MC-SCF orthogonal gradient method developed earlier. Complete geometry optimization has been carried out for both the protonated and unprotonated bases in the ground and relevant excited states. Computed proton affinities (PA) of H₂CO in different electronic states are in the following order:$$PA({}^1S_0 ) > PA({}^3\pi \pi ^ * ) \sim PA({}^1n\pi ^ * ) > PA({}^3n\pi ^ * )$$ In F₂CO, however, the ordering turns out to be different viz PA(¹S₀)>PA(¹nπ^*)>PA(³nπ^*)>PA(³ππ^*) for protonation at the carbonyl oxygen. For protonation at the F atom the ordering is PA(¹S₀)>PA(³nπ^*)>PA(¹nπ^*)>PA(³ππ^*). Protonation at the oxygen atom is predicted to be energetically more favourable than protonation at one of the F atoms by approximately 30 kcal/mole in all the states studied. The role of Fermi correlation in shaping the difference in proton affinities of the singlet and tripletnπ^* states of H₂CO is discussed.