Ability of Peripheral H Bonds to Strengthen a Halogen Bond

Abstract

Quantum calculations study the manner in which the involvement of a halogen atom as a proton acceptor in one or more H bonds (HBs) affects the strength of the halogen bond (XB) it can form with a nucleophile aligned with the X σ-hole. A variety of Lewis acids wherein X = F, Cl, Br, and I are attached to a tetrel atom C or Ge engaged in a XB with nucleophile NH3. One, two, and three HF molecules were positioned perpendicular to the XB axis so that they could form a HB to the X atom. Each such HB strengthened the XB by an increment of 1 kcal/mol or more that does not attenuate as each new HB is added, potentially increasing the interaction energy manyfold. Additionally, the presence of one or more HBs facilitates the formation of a XB by molecules which are reluctant to engage in such a bond in the absence of these auxiliary interactions. Even the F atom, which avoids such a XB, can be coaxed to participate in a XB of moderate strength by one or more of these external HBs.

Boat and Chair Shaped Hexahalogen Synthons

“…The remaining Br atoms form apices of two triangles, each of which is fused with one of the parallel edges of the parallelogram. Although the occurrence of cyclic chair shaped synthons was earlier predicted by virtue of density functional theory by Scheiner in ICl molecules, [46] the crystalline evidence for the same is presented for the first time.…”

Non‐classical Non‐covalent σ‐Hole Interactions in Protein Structure and Function: Concepts for Potential Protein Engineering Applications

“…[44] There is some indication from QM studies that with multiple enhancing H-bonds, even fluorine can serve as a potent X-bond donor. [45] Thus, the concept of the synergistic HBeXB seems to be general across chemistry and biochemistry and greatly expands the range of energies and geometries available for molecular design applications compared to Xbonds alone.…”

“…Surveys of the Cambridge Structural Database (CSD) and the PDB indicates that the requirements for HB e XB interactions are prevalent in both small and large molecule systems [44] . There is some indication from QM studies that with multiple enhancing H‐bonds, even fluorine can serve as a potent X‐bond donor [45] . Thus, the concept of the synergistic HB e XB seems to be general across chemistry and biochemistry and greatly expands the range of energies and geometries available for molecular design applications compared to X‐bonds alone.…”

Enhancement of Halogen Bond Strength by Intramolecular H-Bonds

“…A recent set of quantum calculations examined this question in a systematic fashion. A set of Lewis acids were constructed of the general R 3 TX type where T represented either a tetrel C or Ge atom, R was H or F; three choices of halogen atom X included Cl, Br, and I.…”

“…This HB/XB cooperativity has also been shown to play an active role in protein structure and function 50,51 and catalysis. 52,53 A recent set of quantum calculations 54 examined this question in a systematic fashion. A set of Lewis acids were constructed of the general R 3 TX type where T represented either a tetrel C or Ge atom, R was H or F; three choices of halogen atom X included Cl, Br, and I.…”