The effect of replacing the hydrogen atoms in thioformaldehyde by halogen atoms (F, Cl) on the ionisation potential of the non-bonding electron is analysed by using the Hellman-Feynman theorem, regarding the nuclear charge of the substituent as a parameter in the many-electron Hamiltonian. The trends predicted by our theory nicely agree with the relevant ionisation potentials computed either by applying Koopmans’ theorem or by the ΔE_SCF method. For the carbonyls, avaible experimental data indicate the reliability of our prediction.

The dipole moment function of a diatomic molecule may be viewed as a continuous collective coordinate which encodes a lot of information about sharing of electrons between the atoms concerned. By its very nature it takes cognizance of certain many-body effects and should shape the constrained wavefunction or the one electron density in much the same way as would explicit inclusion of the same many-body effects. A case study of the problem with lithium hydride as the model system has been presented and the long range behaviour of the constrained density analysed. The spectrum of the constrained Fock operator is compared with that of the unconstrained one.

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.

Quantum chemical valence parameters of several carbonyl molecules in the ground and excited states are calculated by invoking the INDO-orthogonal gradient method in an MC-SCF framework. These parameters are then used to construct state-specific structural descriptions of these molecules in terms of superposition of several canonical structures. Photochemical reactivities of some of these molecules are sought to be explained on the basis of the picture that emerges.