Microwave Spectrum, Structure, and Dipole Moment of Difluorophosphine Borane

The structure of difluorophosphine borane (HF2P–BH3) has been determined from the microwave spectra of six isotopic species. The expected staggered conformation and Cs symmetry have been confirmed. The structural parameters are: d(PH) = 1.409 ± 0.004 Å; d(PF) = 1.552 ± 0.006 Å; d(PB) = 1.832 ± 0.009 Å; d(BHa) = 1.226 ± 0.005 Å; d(BHs) = 1.200 ± 0.007 Å; ∠HaBHa = 112.7 ± 0.5°; ∠HaBHs = 115.9 ± 0.4°; ∠FPF = 100.0 ± 0.5°; ∠FPH = 98.62 ± 0.25°; ∠BPF = 117.7 ± 0.3°. Two angles (PBHs = 109.9 ± 0.3°; PBHa = 99.89 ± 0.26°) indicate a tilt of the borane group away from the fluorine atoms. The value of the dipole moment obtained from Stark splittings is 2.504 ± 0.030 D. A range for the barrier to internal rotation is estimated as 3600–4500 cal/mole. The change in structural parameters of HF2PBH3 compared to similar compounds, the stability of HF2PBH3 towards dissociation, and the tilted borane group are discussed.