Systematic behaviour of electron redistribution on formation of halogen-bonded complexes BXY, as determined via XY...

Equilibrium nuclear quadrupole coupling constants associated with the di-halogen molecule XY in each of 60 complexes BXY (where B is one of the Lewis bases N₂, CO, HCN, H₂O, H₂S, HCCH, C₂H₄, PH₃, NH₃ or (CH₃)₃N and XY is one of the di-halogens Cl₂, BrCl, Br₂, ICl, IBr or I₂) have been calculated ab initio. The Townes-Dailey model for interpreting the changes in the coupling constants when XY enters the complex was used to describe the electron redistribution in the di-halogen molecule in terms of the fraction δ_i of an electron transferred from the Lewis base B to atom X and the fraction δ_p of an electron transferred simultaneously from atom X to atom Y. Systematic relationships between the δ_i values for the six series are established. It is shown that, in reasonable approximation, δ_i decays exponentially as the first ionisation energy I_B of the Lewis base B increases, that is δ_i = A exp(-bI_B). It is concluded from the results for the series BBrCl, BBr₂, BICl, BIBr and BI₂ that the coefficients A and b in regression fits to the corresponding logarithmic version ln(δ_i) = ln(A) -b(I_B) of the equation are not strongly dependent on either the halogen atom X directly involved in the halogen bond in BXY or, for a given X, on the nature of Y. The behaviour of PH₃ as a Lewis base appears to be anomalous. Values of δ_i and δ_p calculated by the quantum theory of atoms-in-molecules and natural bond orbital methodologies are very close to those from application of the Townes-Dailey approach described.