Symmetric C···H···C Hydrogen Bonds Predicted by Quantum Chemical Calculations

Intrinsic dynamic and static natures of APn--X+--BPn σ(3c-4e) type interactions (APn = BPn = 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 APn--X+--BPn (APn = BPn: 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 APn and BPn of APn(CH2CH2CH2)3BPn (1-4) and APn(CH2CH2CH2CH2)3BPn (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 APn-X/BPn-X interactions in APn--X+--BPn 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(APn)→ σ*(X-BPn) but also σ(APn-C)→σ*(X-BPn), σ(APn-C/BPn-C)→np(X+) and n(X)→ns(Pn+). The direction and magnitude of the p-character of n(APn) 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 APn--X+--BPn were also examined by analysing the charge distributions on APn--X+--BPn. These results would provide fundamentally important insight into designing molecules with high functionality containing X+ in symmetric and nonsymmetric structures.