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Ab initio molecular orbital calculations (at the MP4/6-3lG(d,p) level using MP2/6-31G(d,p) geometries and ZPE corrections) on the cycloaddition of the model HC=P + HC=P system suggest that the head-to-head dimerization of phosphaalkyne giving 1,4-diphosphacyclobutadiene is inherently favored over the head-to-tail approach yielding 1,3-diphosphacyclobutadiene (by 37 kJ/mol); thus the experimental observation of the latter can be understood by a steric effect of the bulky substituents employed or a catalytic effect of metal fragments. Both dimers undergo, however, further reactions giving the most stable structure, diphosphabicyclo [1.1.0] butane. The latter molecule, also trapped by metal fragments, is likely to be an intermediate of the thermal tetramerization of HCP, leading to tetraphosphacubane.

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Ab initio quantum chemical calculations have been used to about 180 kJ/mol high. Electron density is largely delocalized within the three-membered P 3 ring not only in explore the P 3 H 3 potential energy surface focussing on the ring-chain rearrangements of the three-membered ring in the C 3v -symmetric 1b (all-cis) but also in 1a (C s ). The proton affinity of 1a is similar to that of PH 3 . The proton affinities (PH) 3 (1), the parent triphosphirane. Relative energies between stationary points were estimated using the decrease with n in cyclo-(CH 3 ) 3 -n (PH) n and their values were obtained: PA(1a) = 777 ±10, PA(diphosphirane) = 799 QCISD(T)/6-311G(d,p) method based on MP2/6-31G(d,p) geometries and corrected for zero-point contributions. Ring ±10 and PA(phosphirane) = 802 ±10 kJ/mol. Heats of formation are evaluated as follows (∆H°f 0 at 0 K in kJ/mol): strain, proton affinities, ionization and excitation energies and heats of formation have been evaluated using larger 1a, 70 ±10; cyclo-(PH) 2 (PH 2 ) + (protonated 1a), 821 ±10; diphosphirane, 85 ±10; cyclo-(CH 2 )(PH)(PH 2 ) + (protonated basis sets, e.g. 6-311++G(3df,2p). The cyclic transtriphosphirane (1a) is the most stable P 3 H 3 isomer and lies diphosphirane), 814 ±10; phosphirane, 86 ±10; and protonated phosphirane, 812 ±10 kJ/mol. All P rings remain about 40 kJ/mol below the open-chain phosphanyldiphosphene (H 2 P-P=PH). The decrease of ring strain in threecyclic following ionization to the radical cations. Adiabatic ionization energies (IE a ) are estimated as: 1a and membered rings when CH 2 is replaced by PH is confirmed. Triphosphirane 1a is a virtually strain-free ring and even diphosphirane, 9.3 ±0.3 eV and phosphirane 9.5 ±0.3 eV. The first UV absorption band shifts toward the longer wavelength gains some stabilization relative to three separate P-P single bonds. The reduced ring strain also helps diminish the region on going from phosphirane to 1a. The GIAO/B3LYP computed magnetic shieldings for 1a and related molecules phosphorus inversion barrier to 224 kJ/mol compared to the monocyclic isomers of (CH 2 )(PH) 2 and (CH 2 ) 2 (PH). reveal a clear relationship between the narrow bond angles in the rings and their unusually strong magnetic shielding. Compound 1a follows a pure ring-opening or a 1,2-hydrogen shift rather than a combined motion pathway, in fundamentalThe similarity of the predicted 31 P-NMR signals in 1a and its heteroanalog diphosphirane, (CH 2 )(PH) 2 , can be rationalized contrast with corresponding processes of diphosphirane and phosphirane. This is due to the existence of an open-chain in terms of a compensation of the carbon-substituent effect (downfield shift) and the bond-bending effect imposed by the P 3 H 3 phosphorane intermediate stabilized by allylic conjugation. The pericyclic ring-opening of 1a is the most ring (upfield shift). favored process but the energy barrier in the gas phase is eral, the existence of polyphosphorus compounds as rings

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