Use of the DFT method with the B3LYP functional offers an efficient way for
determining geometries of phosphole structures. It also provides inversion
barriers in good agreement with experiment. The molecular orbital MP2 meth
od is better suited to calculate the relative stability of the considered i
somers. Combination of geometric (Julg index) and magnetic (NICS) criteria
leads to an interesting approach to analyze the pi-electron delocalization
and/or aromaticity in conjugated heterocyclic systems and can be used for i
nvestigation of much larger oligomers. It thus allows us to confirm that pl
anarizing the phosphorus atom in phosphole monomers leads to a greater conj
ugation over the butadienic pi-system and that substituents, characterized
by a strongly pronounced pi-system, exhibit a large extent of conjugation w
ith the pi-diene moiety of the heterocyclic system. In push-pull systems su
ch as 2-BH2-5-NH2-1H-phosphole, the electron delocalization along the pi-di
ene system is even more pronounced than in phospholes with strongly pronoun
ced pi-systems. The employed approach is also used to analyze the relations
hip between electron conjugation and the phosphorus inversion barrier. More
relevant for chemical applications is the result that the computational st
udy provides a method of fine-tuning of phosphole building blocks, which ma
y enable us to obtain higher pi-conjugation along the polymer backbone.