title: Tetraphosphacubane: An Unexpectedly Strong Base in the Gas Phase.
abstract: The experimental determination of physical properties of tetra-tert-butylphosphocubane (TtBuPC) is of paramount importance for understanding the reactivity of this fascinating molecule.The gas-phase basicity of TtBuPC measured by FT-ICR spectroscopy (GB = 221.8 kcal mol(-)(1), PA = 230.5 kcal mol(-)(1)) is surprisingly high although protonation in solution is only achieved under drastic conditions. A molecular orbital treatment, including electron correlation effects, predicts the unsubstituted parent compound phosphacubane (PC) to be a carbon base. Carbon protonation implies a P-C bond fission which alleviates the strain of the system. A similar behavior is also predicted for the tetramethyl derivative. However, TtBuPC is found to be a phosphorus base, because the strong repulsive interactions which appear between the tert-butyl substituents destabilize significantly the C-protonated form. These effects decrease dramatically when just one tert-butyl group is removed and both P- and C-protonated species become almost degenerate. As for other strained systems, the PAs of PC and TtBuPC are only adequately reproduced when G2-type [6-311+G(3df,2p)] basis sets are used.
authors.affiliation: Instituto de Química Médica (CSIC), Juan de la Cierva, 3, E-28006 Madrid, Spain.
authors.forename: José-Luis M.
authors.initials: JL
authors.lastname: Abboud
authors.forename: Marta
authors.initials: M
authors.lastname: Herreros
authors.forename: Rafael
authors.initials: R
authors.lastname: Notario
authors.forename: Otilia
authors.initials: O
authors.lastname: Mó
authors.forename: Manuel
authors.initials: M
authors.lastname: Yáñez
authors.forename: Manfred
authors.initials: M
authors.lastname: Regitz
authors.forename: José
authors.initials: J
authors.lastname: Elguero