Molecular and Electronic Structure of Phosphonium Cyclopropylide (H₃P=C(CH₂)₂): A Theoretical Study

Theoretical studies have been carried out for a phosphonium cyclopropylide model, H₃P = C(CH₂)₂. The molecule was shown to have a ground-state equilibrium geometry (structure I) with a pyramidal carbanion center, the details being in satisfactory agreement with experimental findings for the triphenylphosphine cyclopropylide homologue. One of the PH₃ hydrogen atoms is affected by the carbanion (partial) negative charge, and its position shows substantial distortion away from the idealized tetrahedral phosphorus geometry. In the energy profile for the carbanion inversion the stationary point of maximum energy (two imaginary vibrational frequencies) occurs not for the planar carbanion configuration but well beyond in a situation with a dihedral angle H0PC1C2 of 123° instead of 90°. The rotated configuration (a genuine transition state) is virtually the eclipsed analogue of structure I. The barriers to inversion and rotation are predicted to be 6.3 and 5.8 kcal, respectively. Calculated bond distances, bond angles, dihedral angles, total energy values, and total electron populations on atoms are given for all three conformations. Atomic orbital components and energies of frontier orbitals of the ground state are also discussed.