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

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 N, CO, HCN, HO, HS, HCCH, CH, PH, NH or (CH)N and XY is one of the di-halogens Cl, BrCl, Br, ICl, IBr or I) 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 of an electron transferred from the Lewis base B to atom X and the fraction δ of an electron transferred simultaneously from atom X to atom Y. Systematic relationships between the δ values for the six series are established. It is shown that, in reasonable approximation, δ decays exponentially as the first ionisation energy I of the Lewis base B increases, that is δ = A exp(-bI). It is concluded from the results for the series BBrCl, BBr, BICl, BIBr and BI that the coefficients A and b in regression fits to the corresponding logarithmic version ln(δ) = ln(A) -b(I) 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 PH as a Lewis base appears to be anomalous. Values of δ and δ 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.