Basicity of some proto-typical carbonyls in the ground and some low lying excited states: Application of the orthogonal gradient method of orbital optimization in an INDO-MC-SCF framework-I

Abstract

Gas phase proton affinities of formaldehyde and fluoroformaldehyde in the ground,1,3 nπ * (lowest) and3 ππ * (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(1 S 0)>PA(1 nπ *)>PA(3 nπ *)>PA(3 ππ *) for protonation at the carbonyl oxygen. For protonation at the F atom the ordering is PA(1 S 0)>PA(3 nπ *)>PA(1 nπ *)>PA(3 ππ *). 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.