The Allotropy of Phosphorus

Elemental phosphorus exists in a wide variety of allotropic forms, most of them polymeric [9, Chap. 2]. Its monomeric forms, in addition to atomic P, are diatomic P2 and the more stable tetrahedral P4, whereas Pe and Pg species are conspicuously absent. Bock and Muller [10] investigated the P4 2P2 equilibrium by photoelectron spectroscopy, and detected no other species up to 1470 K. The experimental enthalpy of reaction (Ai/g p = 55 kcal/mol) was reproduced reasonably well by the value computed with MNDO = 43 kcal/mol). 

Of the two computations that had already been published, both using pseudopotential methods, one [13] found = -47 kcal/mol for the dissociation Pg 2 P4, whereas the other [14] obtained = +10 kcal/mol, in qual- 

Although the most reliable calculations agree that the dimerization of P4 to Pg is endothermic, the computed energy difference is not large enough to explain the complete non-observability of Pg. It seems that three necessary conditions must be fulfilled  

There is a symmetry-imposed barrier to the dimerization that is not easily circumvented. 

No such barrier inhibits polymerization to amorphous red phosporus, a process that takes place readily in the liquid phase [6, p. 86]. 

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