Document Type


Journal/Book Title

Physical Chemistry Chemical Physics

Publication Date



Royal Society of Chemistry





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In addition to the standard halogen bond formed when NH3 approaches XCN (X=F,Cl,Br,I) along its molecular axis, a perpendicular approach is also possible, toward a π-hole that is present above the X-C bond. MP2/aug-cc-pVDZ calculations indicate the latter geometry is favored for X=F, and the σ-hole structure is preferred for the heavier halogens. The π-hole structure is stabilized by charge transfer from the NH3 lone pair into the π*(CN) antibonding orbital, and is characterized by a bond path from the N of NH3 to the C atom of XCN, a form of tetrel bond. The most stable 2:1 NH3/XCN heterotrimer for X=F and Cl is cyclic and contains a tetrel bond augmented by a pair of NH∙∙N H-bonds. For X=Br and I, the favored trimer is noncyclic, stabilized by a tetrel and a halogen bond.

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