Abstract
Energy profiles of seven halogen-bonded complexes were analysed with the topological energy partitioning called Interacting Quantum Atoms (IQA) at MP4(SDQ)/6–31 + G(2d,2p) level of theory. Explicit interatomic electron correlation energies are included in the analysis. Four complexes combine X2 (X = Cl or F) with HCN or NH3, while the remaining three combine ClF with HCN, NH3 or N2. Each complex was systematically deformed by translating the constituent molecules along its central axis linking X and N, and reoptimising its remaining geometry. The Relative Energy Gradient (REG) method (Theor. Chem. Acc. 2017, 136, 86) then computes which IQA energies most correlate with the total energy during the process of complex formation and further compression beyond the respective equilibrium geometries. It turns out that the covalent energy (i.e., exchange) of the halogen bond, X…N, itself drives the complex formation. When the complexes are compressed from their equilibrium to shorter X…N distance then the intra-atomic energy of N is in charge. When the REG analysis is restricted to electron correlation then the interatomic correlation energy between X and N again drives the complex formation, and the complex compression is best described by the destabilisation of the through-space correlation energy between N and the “outer” halogen.
Highlights
IntroductionThe halogen bond [1,2,3,4] is the second oldest non-covalent interaction after the hydrogen bond.According to the IUPAC definition [5] a halogen bond occurs when there is evidence of a net attractive interaction between an electrophilic region (sometimes referred to as the σ-hole [6]) associated with a halogen atom in a molecular entity and a nucleophilic region in another, or the same, molecular entity.It is known [7] that σ-holes are electron-deficient regions arising from the anisotropic distribution of electron density on the atoms of Group 14, 15, 16 and 17 elements when covalently bonded to electron-withdrawing groups yielding non-covalent bonds named as tetrels, pnictogens, chalcogens and halogens, respectively.The nature of the halogen bond interaction has been disputed for a long time
Seven halogen-bonded complexes for which the experimental geometry was determined using rotational spectroscopy have been selected for the current study
The small number of complexes investigated is determined by the prohibitive computational cost of the MP4(SDQ)-Interacting Quantum Atoms (IQA) method
Summary
The halogen bond [1,2,3,4] is the second oldest non-covalent interaction after the hydrogen bond.According to the IUPAC definition [5] a halogen bond occurs when there is evidence of a net attractive interaction between an electrophilic region (sometimes referred to as the σ-hole [6]) associated with a halogen atom in a molecular entity and a nucleophilic region in another, or the same, molecular entity.It is known [7] that σ-holes are electron-deficient regions arising from the anisotropic distribution of electron density on the atoms of Group 14, 15, 16 and 17 elements when covalently bonded to electron-withdrawing groups yielding non-covalent bonds named as tetrels, pnictogens, chalcogens and halogens, respectively.The nature of the halogen bond interaction has been disputed for a long time. According to the IUPAC definition [5] a halogen bond occurs when there is evidence of a net attractive interaction between an electrophilic region (sometimes referred to as the σ-hole [6]) associated with a halogen atom in a molecular entity and a nucleophilic region in another, or the same, molecular entity. It is known [7] that σ-holes are electron-deficient regions arising from the anisotropic distribution of electron density on the atoms of Group 14, 15, 16 and 17 elements when covalently bonded to electron-withdrawing groups yielding non-covalent bonds named as tetrels, pnictogens, chalcogens and halogens, respectively. The conclusion reached depends on the energy partitioning method and the Molecules 2020, 25, 2674; doi:10.3390/molecules25112674 www.mdpi.com/journal/molecules
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