Isomerism in OBe3F3 + cation: anab initio study

Abstract

Ab initio MP2/6-31G*//HF/6-31G*+ZPE(HF/6-31G*) calculations of the potential energy surface in the vicinity of stationary points and the pathways of intramolecular rearrangements between low-lying structures of the OBe3F3+ cation detected in the mass spectra of μ4-Be4O(CF3COO)6 were carried out. Ten stable isomers with di- and tricoordinate oxygen atoms were localized. The relative energies of six structures lie in the range 0–8 kcal mol−1 and those of the remaining four structures lie in the range 20–40 kcal mol−1. Two most favorable isomers, aC2v isomer with a dicoordinate oxygen atom, planar six-membered cycle, and one terminal fluorine atom and a pyramidalC3v isomer with a tricoordinate oxygen atom and three bridging fluorine atoms, are almost degenerate in energy. The barriers to rearrangements with the breaking of one fluorine bridge are no higher than 4 kcal mol−1, except for the pyramidalC3v isomer (∼16 kcal mol−1). On the contrary, rearrangements with the breaking of the O−Be bond occur with overcoming of a high energy barrier (∼24 kcal mol−1). A planarD3h isomer with a tricoordinate oxygen atom and linear O−Be−H fragments was found to be the most favorable for the OBe3H3+ cation, a hydride analog of the OBe3F3+ ion; the energies of the remaining five isomers are more than 25 kcal mol−1 higher.