Halogen bonding in the gas phase: a comparison of the iodine bond in B⋯ICl and B⋯ICF3 for simple Lewis Bases B.

Abstract

Methods for observing the rotational spectra of the halogen-bonded complexes B⋯ICl and B⋯ICF3 (B = N2, CO, HC≡CH, H2C=CH2, H2O, H2S, PH3 or NH3) and deriving from them properties such as angular geometry, radial geometry, the strength of the intermolecular bond, and the extent of electron redistribution on complex formation are described. Comparison of various properties reveals several similarities between the two series. Thus, the B⋯ICF3 obey a set of rules which were originally proposed to rationalise the angular geometries of hydrogen-bonded complexes of the type B⋯HX, but which were subsequently found to apply to their halogen-bonded analogues B⋯XY, where XY is a dihalogen molecule, including ICl. Important for establishing the validity of these rules in both series B⋯ICl and B⋯ICF3 were the complexes with B = H2O or H2S. The configuration at O in H2O⋯ICF3 and H2O⋯ICl is effectively planar. On the other hand, the configuration at S in H2S⋯ICF3 and H2S⋯ICl is permanently pyramidal. Ab initio calculations of potential energy functions for inversion at O or S performed at the CCSD(T)(F12*)/cc-pVDZ-F12 level of theory confirmed these conclusions. Comparison of the intermolecular stretching force constants k σ show that the series B⋯ICF3 is systematically more weakly bound than B⋯ICl. Interpretation of k σ in terms of nucleophilicities N B of B and electrophilicities E IR of ICl and ICF3 reveals that [Formula: see text]. Experimental and ab initio values of distances r(Z⋯I), where Z is the acceptor atom/region of B, show that, for a given B, the intermolecular bond of B⋯ICF3 is longer than that of B⋯ICl. The electronic charge redistributed from B to ICF3 on formation of B⋯ICF3 is probably negligibly small.