title: Source of Cooperativity in Halogen-Bonded Haloamine Tetramers.
abstract: Inspired by the isostructural motif in α-bromoacetophenone oxime crystals, we investigated halogen-halogen bonding in haloamine quartets. Our Kohn-Sham molecular orbital and energy decomposition analysis reveal a synergy that can be traced to a charge-transfer interaction in the halogen-bonded tetramers. The halogen lone-pair orbital on one monomer donates electrons into the unoccupied σ*N-X orbital on the perpendicular N-X bond of the neighboring monomer. This interaction has local σ symmetry. Interestingly, we discovered a second, somewhat weaker donor-acceptor interaction of local π symmetry, which partially counteracts the aforementioned regular σ-symmetric halogen-bonding orbital interaction. The halogen-halogen interaction in haloamines is the first known example of a halogen bond in which back donation takes place. We also find that this cooperativity in halogen bonds results from the reduction of the donor-acceptor orbital-energy gap that occurs every time a monomer is added to the aggregate.
authors.affiliation: Department of Theoretical and Structural Chemistry, Faculty of Chemistry, University of Łódź, Pomorska 163/165, 90-236, Łódź, Poland. justyna@uni.lodz.pl.
authors.forename: Justyna
authors.identifier: http://orcid.org/0000-0003-4150-0506
authors.initials: J
authors.lastname: Dominikowska
authors.affiliation: Department of Theoretical Chemistry and Amsterdam Center for Multiscale Modeling (ACMM), VU University Amsterdam, De Boelelaan 1083, 1081 HV, Amsterdam, The Netherlands.|Institute for Molecules and Materials (IMM), Radboud University Nijmegen, Heyendaalseweg 135, 6525 AJ, Nijmegen, The Netherlands.
authors.forename: F Matthias
authors.identifier: http://orcid.org/0000-0003-4655-7747
authors.initials: FM
authors.lastname: Bickelhaupt
authors.affiliation: Department of Theoretical and Structural Chemistry, Faculty of Chemistry, University of Łódź, Pomorska 163/165, 90-236, Łódź, Poland.
authors.forename: Marcin
authors.identifier: http://orcid.org/0000-0002-0032-0878
authors.initials: M
authors.lastname: Palusiak
authors.affiliation: Department of Theoretical Chemistry and Amsterdam Center for Multiscale Modeling (ACMM), VU University Amsterdam, De Boelelaan 1083, 1081 HV, Amsterdam, The Netherlands. c.fonsecaguerra@vu.nl.
authors.forename: Célia
authors.identifier: http://orcid.org/0000-0002-2973-5321
authors.initials: C
authors.lastname: Fonseca Guerra
keywords: computational chemistry; cooperative effects; density functional calculations; halogen bonds; molecular orbital theory