Hidden Bond Anomalies: The Peculiar Case of the Fluorinated Amine Chalcogenides

Bond anomalies have been investigated for a set of 53 molecules with either N-F, Ti-P, Cr-H, Pb-C, or Pb-F bonds for which reverse rather than inverse bond length-bond strength relationships have been previously claimed. The intrinsic strength of each bond investigated was determined utilizing the associated local stretching force constant obtained at the CCSD(T)/aug-cc-pVTZ level of theory. For the metal containing molecules, LC-omega PBE calculations with the aug-cc-pVTZ (Cr, Pb) and the 6-31++G(d,p) basis set (Ti) were carried out. For bonds containing a metal atom, any bond anomaly could not be confirmed. Previously reported results were due to ill-defined bond strength descriptors or lacking accuracy. In the case of the fluoro amines, methyl fluoro amines, and the fluor amine oxides, direct or hidden bond anomalies were detected, which result from two or more opposing electronic effects: a dominant bond shortening effect due to electron withdrawal and a bond weakening due to lone pair repulsion or hybridization defects. Bond anomalies can be disguised by a complex interplay of electronic effects. These hidden bond anomalies could be identified in this work for the fluoro amine chalcogenides.