The thermodynamic stability of P8, A CBS‐Q study

The thermodynamic stabilities of P₂, P₄, and three P₈ cage structure were investigated through high‐precision CBS‐Q calculations. The CBS‐Q values for the bond energy of P₂ (ΔE_o: +115.7 kcal mol^?1) and the formation of P₄ from P₂ (Δ E_o:‐56.6 kcal mol^?1) were in excellent agreement with the experimental values (E_o: +117 and ‐56.4 kcal mol^?1 respectively). Among the P₈ cages, the cubane structure was the least stable (Δ E_o +37 kcal vs. 2×P₄). The most stable P₈ isomer adopts a cuneane structure resembling S₄N₄, and is more stable than white phosphorus at T = 0 K (Δ E_o ?3.3 kcal mol^?1), but still unstable under standard conditions for entropic reasons (Δ G^o of +8.1 kcal mol^?1 vs. 2×P₄). The CBS‐Q energies represent significant revisions (6–20 kcal mol^?1) of previous computational predictions obtained by high‐level single method calculations. © 2005 Wiley Periodicals, Inc. Heteroatom Chem 16:453–457, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20119