Electrostatics explains the reverse Lewis Acidity of BH3 and Boron Trihalides: Infrared Intensities and a Relative

Leonardo Duarte, Wagner E. Richter, Roy E. Bruns, Paul Popelier

Research output: Contribution to journalArticlepeer-review


The reaction path for the formation of the BX3-NH3 (X = H, F, Cl, Br) complexes was divided into two
process: (i) rehybridization of the acid while adopting a pyramidal geometry, and (ii) complex formation from
the pyramidal geometries of the acid and base. The Interacting Quantum Atoms (IQA) method investigated
the Lewis acidity trend of these compounds. This topological analysis suggests that the boron-halogen bond
exhibits a considerable degree of ionicity. A Relative Gradient Analysis (REG) analysis on IQA energies
indicates that the acid-base complex formation is highly dependent on the electrostatic energy. With
increasing halogen electronegativity, a higher degree of ionicity of the B-X is observed, causing an increase
in the absolute value of X and B charges. This increases not only the attractive electrostatic energy between
acid and base but also enhances the repulsive energy. The latter is the main factor behind the acidity trend
exhibited by the trihalides. Changes in geometry are relevant only for complexes where BH3 acts as acid,
where lower steric hindrance facilitates the adoption of the pyramidal geometry observed in the complex.
The CCTDP analysis shows that infrared intensities of BX3-NH3 are determined mostly by the atomic charges,
and not by the charge transfer or polarization. The opposite is observed in covalent analogues.
Original languageEnglish
JournalThe Journal of Physical Chemistry A
Publication statusPublished - 2021


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