Intrinsic dynamic and static natures of Pn--X--Pn σ(3c-4e) type interactions (Pn = Pn = N, P, As and Sb; X = H, F, Cl, Br and I) in bicyclo[3.3.3] and bicyclo[4.4.4] systems and their behaviour, elucidated with QTAIM dual functional analysis.

The intrinsic dynamic and static natures of Pn--X--Pn (Pn = Pn: N, P, As and Sb; X = H, F, Cl, Br and I) in 1a-8c were elucidated with the quantum theory of atoms-in-molecules dual functional analysis (QTAIM-DFA). Species 1a-8c were formed by incorporating X between Pn and Pn of Pn(CHCHCH)Pn (1-4) and Pn(CHCHCHCH)Pn (5-8). The relative stabilities between the symmetric and nonsymmetric structures along with their transition states were investigated. Various natures from typical hydrogen bonds (t-HB) to classical covalent bonds were predicted for the Pn-X/Pn-X interactions in Pn--X--Pn with QTAIM-DFA. The secondary interactions of H-H and X-C were also detected. The vdW to molecular complexes through charge transfer natures were predicted for them. Natural bond orbital analysis clarified that the CT terms were caused by not only n(Pn)→ σ*(X-Pn) but also σ(Pn-C)→σ*(X-Pn), σ(Pn-C/Pn-C)→n(X) and n(X)→n(Pn). The direction and magnitude of the p-character of n(Pn) were the factors that determined the types of donor-acceptor interactions. Estimating the order of the interaction strengths was attempted. The σ(3c-4e) characters of Pn--X--Pn were also examined by analysing the charge distributions on Pn--X--Pn. These results would provide fundamentally important insight into designing molecules with high functionality containing X in symmetric and nonsymmetric structures.