Pearcy Adam C, Mason Kyle A, El-Shall M Samy
Department of Chemistry , Virginia Commonwealth University , Richmond , Virginia 23284-2006 , United States.
J Phys Chem A. 2019 Feb 21;123(7):1363-1371. doi: 10.1021/acs.jpca.8b09094. Epub 2018 Dec 3.
We report on the gas phase association of the small polar and aprotic solvent molecules acetonitrile (CHCN) and acetone (CHCOCH) with the halogenated benzene radical cations (CHX, X = F, Cl, Br, and I) using the mass-selected ion mobility technique and density functional theory calculations. The association energies (-Δ H°) of CHCN (CHCOCH) with CHF and CHI are similar [13.0 (13.3) and 13.2 (14.1) kcal/mol, respectively] but higher than those of CHCN (CHCOCH) with CHCl and CHBr [10.5 (11.5) and 10.9 (10.6) kcal/mol, respectively]. However, the electrostatic potentials of the lowest energy structures of CHBr(CHCN) and CHBr(CHCOCH) or CHI(CHCN) and CHI(CHCOCH) complexes clearly show the formation of the ionic halogen bonds (IXBs) C-Br- -NCCH and C-Br- -OC(CH) or C-I- -NCCH and C-I- -OC(CH) driven by positively charged σ-holes on the external sides of the C-Br and C-I bond axes of the bromobenzene and iodobenzene radical cations, respectively. For the CHF(CHCN) complex, the dominant interaction involves a T-shaped structure between the N atom of CHCN and the C atom of the C-F bond of CHF. The structure of the CHCl(CHCN) complex shows the formation of unconventional ionic hydrogen bonds (uIHBs) between the N atom of CHCN and the C-H bonds of the CHCl cation. Similar results are obtained for the association of acetone with the halogenated benzene radical cations. The formation of IXBs of the iodobenzene cation with acetonitrile or acetone involves a significant entropy loss (-Δ S° = 25-27 cal /(mol K)) resulting from the formation of more ordered and highly directional structures between the nitrogen or oxygen lone pair of electrons of acetonitrile or acetone, respectively, and the electropositive region around the iodine atom of the iodobenzene cation. In comparison, for the association of acetonitrile or acetone with the fluorobenzene, chlorobenzene, and bromobenzene cations, -Δ S° = 16-23 cal/(mol K), consistent with the formation of less ordered structures and loose interactions. The lowest energy structures of the CHBr(CHCOCH) and CHI(CHCOCH) clusters show a novel combination of ionic halogen bonding and hydrogen bonding where the oxygen atom of one acetone molecule forms the halogen bond while the oxygen atom of the second acetone molecule becomes the hydrogen acceptor from the methyl group of the first acetone molecule.
我们使用质量选择离子迁移技术和密度泛函理论计算，报道了小极性非质子溶剂分子乙腈（CH₃CN）和丙酮（CH₃COCH₃）与卤代苯自由基阳离子（C₆H₅X，X = F、Cl、Br和I）的气相缔合情况。CH₃CN（CH₃COCH₃）与C₆H₅F和C₆H₅I的缔合能（-ΔH°）相似[分别为13.0（13.3）和13.2（14.1）kcal/mol]，但高于CH₃CN（CH₃COCH₃）与C₆H₅Cl和C₆H₅Br的缔合能[分别为10.5（11.5）和10.9（10.6）kcal/mol]。然而，CH₃Br(CH₃CN)和CH₃Br(CH₃COCH₃)或CH₃I(CH₃CN)和CH₃I(CH₃COCH₃)配合物最低能量结构的静电势清楚地表明，分别由溴苯和碘苯自由基阳离子C-Br和C-I键轴外侧带正电的σ-空穴驱动，形成了离子型卤素键（IXBs）C-Br-…-NCCH₃和C-Br-…-OC(CH₃)₂或C-I-…-NCCH₃和C-I-…-OC(CH₃)₂。对于CH₃F(CH₃CN)配合物，主要相互作用涉及CH₃CN的N原子与CH₃F的C-F键的C原子之间的T形结构。CH₃Cl(CH₃CN)配合物的结构表明，在CH₃CN的N原子与CH₃Cl阳离子的C-H键之间形成了非常规离子型氢键（uIHBs）。丙酮与卤代苯自由基阳离子的缔合也得到了类似结果。碘苯阳离子与乙腈或丙酮形成IXBs涉及显著的熵损失（-ΔS° = 25 - 27 cal /(mol·K)），这是由于分别在乙腈或丙酮的氮或氧孤对电子与碘苯阳离子碘原子周围的正电区域之间形成了更有序和高度定向的结构。相比之下，对于乙腈或丙酮与氟苯、氯苯和溴苯阳离子的缔合，-ΔS° = 16 - 23 cal/(mol·K)，这与形成较无序的结构和松散的相互作用一致。CH₃Br(CH₃COCH₃)和CH₃I(CH₃COCH₃)簇的最低能量结构显示了离子型卤素键和氢键的新颖组合，其中一个丙酮分子的氧原子形成卤素键，而第二个丙酮分子的氧原子成为第一个丙酮分子甲基的氢受体。
