Calculated properties and ring-chain rearrangements of triphosphirane (P3H3)

Ab initio quantum chemical calculations have been used to explore the P3H3 potential energy surface focussing on the ring-chain rearrangements of the three-membered ring in (PH)(3) (1), the parent triphosphirane. Relative ene rgies between stationary points were estimated using the QCISD(T)/6-311G(d, p) method based on MP2/631G(d,p) geometries and corrected for zero-point co ntributions. Ring strain, proton affinities, ionization and excitation ener gies and heats of formation have been evaluated using larger basis sets, e. g. 6-311++G(3df,2p). The cyclic trans-triphosphirane (1a) is the most stabl e P3H3 isomer and lies about 40 kJ/mol below the open-chain phosphanyldipho sphene (H2P-P=PH). The decrease of ring strain in three-membered rings when CH2 is replaced by PH is confirmed. Triphosphirane 1a is a virtually strai n-free ring and even gains some stabilization relative to three separate P- P single bonds. The reduced ring strain also helps diminish the phosphorus inversion barrier to 224 kJ/mol compared to the monocyclic isomers of (CH2) (PH)(2) and (CH2)(2)(PH) Compound la follows a pure ring-opening or a 1,2-h ydrogen shift rather than a combined motion pathway, in fundamental contras t with corresponding processes of diphosphirane and phosphirane. This is du e to the existence of an open-chain P3H3 phosphorane intermediate stabilize d by allylic conjugation. The pericyclic ring-opening of 1a is the most fav ored process but the energy barrier in the gas phase is about 180 kJ/mol hi gh. Electron density is largely delocalized within the three-membered P-3 r ing not only in the C-3v-symmetric 1b (all-cis) but also in 1a (C-s). The p roton affinity of 1a is similar to that of PH3. The proton affinities decre ase with n in cyclo-(CH3)(3-n)(PH)(n) and their values were obtained: PA(1a ) = 777 +/-10, PA(diphosphirane) = 799 +/-10 and PA(phosphirane) = 802 +/-1 0 kJ/mol. Heats of formation are evaluated as follows (Delta H degrees(fo) at 0 K in kJ/mol): 1a, 70 +/-10; cyclo-(PH)(2)(PH2)(+) (protonated 1a), 821 +/-10; diphosphirane, 85 +/-10; cyclo-(CH2)(PH) (PH2)(+) (protonated dipho sphirane), 814 +/-10; phosphirane, 86 +/-10; and protonated phosphirane, 81 2 +/-10 kJ/mol. All P rings remain cyclic following ionization to the radic al cations. Adiabatic ionization energies (IEa) are estimated as: la and di phosphirane, 9.3 +/-0.3 eV and phosphirane 9.5 +/-0.3 eV. The first UV abso rption band shifts toward the longer wavelength region on going from phosph irane to 1a. The GIAO/B3LYP computed magnetic shieldings for la and related molecules reveal a clear relationship between the narrow bond angles in th e rings and their unusually strong magnetic shielding. The similarity of th e predicted P-31-NMR signals in la and its heteroanalog diphosphirane, (CH2 )(PH)(2), can be rationalized in terms of a compensation of the carbon-subs tituent effect [downfield shift) and the bond-bending effect imposed by the ring (upfield shift).