Why Do B-P and Al-P Polymers Differ? Structures, Stability, and Electronic Properties of Chain and Ring [H2PEH2](n) Oligomers (E=B, Al; n=1-15)

Chain and ring [H2PEH2](n) oligomers (E=B, Al; n=2-15) have been computationally studied at the B3LYP/def2-TZVP level of theory. A strong tendency to cyclization was found for the chain Al-P oligomers with n>7, whereas analogous long B-P oligomers can exist as chain structures. Subsequent oligomerization enthalpies approach each other very closely for the ring and chain oligomers with an increase of n. However, these oligomers have very different electronic properties with band gaps below 0.2 eV for the chain [H2PBH2](15) and more than 5 eV for the ring isomer [H2PEH2](15). The molecular orbitals in the ring oligomers are highly delocalized, whereas for the chain isomers, the HOMO and LUMO are localized at the different ends of the oligomeric chain.