Aromaticity in P 8 allotropes and (CH) 8 analogues: significance of their 40 valence electrons?

The currently unknown phosphorus allotrope P 8 is of interest since its 40 total valence electrons is a "magic number" corresponding to a filled 1S 2 1P 6 1D 10 1S 2 1F 14 2P 6 shell such as found in the relatively stable main group element clusters Al 13 - and Ge 9 4- . However, P 8 still remains as an elusive structure not realized experimentally. The lowest energy P 8 structure by a margin of ∼9 kcal mol -1 is shown by density functional theory to be a cuneane analogue with no PP double bonds and two each of P 5 , P 4 , and P 3 rings. Higher energy P 8 structures are polycyclic systems having at most a single PP double bond. These P 8 systems are not "carbon copies" of the corresponding (CH) 8 hydrocarbons with exactly one hydrogen atom bonded to each carbon atom. Thus the lowest energy (CH) 8 structure is cyclooctatetraene with four CC bonds followed by benzocyclobutene with three CC bonds. The cuneane (CH) 8 structure is a relatively high energy isomer lying ∼36 kcal mol -1 above cyclooctatetraene. The cubane P 8 and (CH) 8 structures are even higher energy structures, lying ∼37 and ∼74 kcal mol -1 in energy above the corresponding global minima. Our results demonstrate differences in medium sized aggregates of elemental phosphorus and isolobal hydrocarbon species.