Molecular structure of trifluorophosphine tetraborane(8), B4H8PF3, as determined in the gas phase by electron diffraction and ab initio computations

The molecular structure of trifluorophosphine tetraborane(8), B4H8PF3, has been studied in the gas phase by electron diffraction. The experimental data can be fitted using a model which represents the gas as consisting solely of the endo conformer with C-s symmetry, the PF3 group staggered with respect to the B(I)-H(1) bond. Important experimental structural parameters (r(alpha)degrees) are r[B(1)-B(2)] (hinge-wing) = 184.7(9) pm, r[B(1)-B(3)] (hinge-hinge)= 172.2(12) pm, r[B(2)-B(3)] = 179.9(10) pm, r[B(1)-P] = 179.8(9) pm, and r(P-F) (mean) = 152.8(1) pm; B(3)B(1)P = 131.6(11)degrees, and the dihedral (''butterfly'') angle between the planes B(1)B(2)B(3) and B(1)B(4)B(3) is 133.9(23)degrees. These values agree well with the ab initio (MP2/TZP level) optimized molecular geometry for the endo conformer; at the MP2/TZP//MP2/TZP + ZPE(HF/6-31G*) level, the exo conformer is predicted to represent ca. 2% of the compound vapor, consistent with the experimental B-11 NMR solution spectrum. The experimental and theoretical geometries are supported by comparison of the calculated (IGLO) B-11 NMR chemical shifts with the experimental NMR data.