Department of Chemistry, Virginia Commonwealth University, Richmond, Virginia 23284-2006, United States.
J Phys Chem A. 2022 Aug 11;126(31):5160-5166. doi: 10.1021/acs.jpca.2c03715. Epub 2022 Aug 2.
The gas-phase sequential association of methanol onto protonated benzonitrile (CHCNH) and the proton-bound dimer (CHCN)H have been examined experimentally by equilibrium thermochemical measurements and computationally by density functional theory (DFT). The bonding enthalpy (Δ°) for the association of methanol with protonated benzonitrile (25.2 kcal mol) reflects the strong electrostatic interaction provided by the formation of an ionic hydrogen bond in the CHCNHOHCH cluster in excellent agreement with a DFT-calculated binding energy of 24.9 kcal mol. The sequential bonding enthalpy within the (CHCN)H(OHCH) clusters decreases from 25.2 to 10.6 kcal mol for the eighth solvation step ( = 8), which remains more than 25% above the enthalpy of vaporization of liquid methanol (8.4 kcal mol). The nonbulk convergence of Δ° with eight solvent molecules is attributed to the external solvation of a benzonitrile molecule by an extended hydrogen bonding network of protonated methanol clusters H(CHOH). In the external solvation of protonated benzonitrile by methanol, the proton resides on the methanol subcluster and the neutral benzonitrile molecule remains outside and bonded to the surface of the protonated methanol cluster. The bonding enthalpy of methanol to the proton-bound benzonitrile dimer (CHCN)H(NCCH) is measured to be 18.0 kcal mol, in good agreement with a DFT-calculated value of 17.1 kcal mol, which reflects the association of the proton with the lower proton affinity methanol molecule, thus forming a highly stable structure of protonated methanol terminated by two ionic hydrogen bonds to the two benzonitrile molecules. The external solvation of benzonitrile by methanol ices in space allows benzonitrile to remain on the ice grain surface rather than being isolated inside the ice. This could provide accessibility for reactions with incoming ions and molecules or for photochemical processes by UV irradiation, leading to the formation of complex organics on the surface of ice grains.
甲醇在气相中与质子化苯甲腈（CHCNH）和质子束缚二聚体（CHCN）H 的连续缔合已通过平衡热化学测量实验和密度泛函理论（DFT）计算进行了研究。甲醇与质子化苯甲腈（25.2 kcal/mol）缔合的键合焓（Δ°）反映了形成 CHCNHOHCH 簇中离子氢键时提供的强静电相互作用，与 DFT 计算的结合能 24.9 kcal/mol 非常吻合。在（CHCN）H（OHCH）簇中，随着第八个溶剂化步骤（=8），连续键合焓从 25.2 降低到 10.6 kcal/mol，仍然超过液体甲醇蒸发焓（8.4 kcal/mol）的 25%以上。与八个溶剂分子的非块状收敛性Δ°归因于甲醇质子化簇 H（CHOH）的扩展氢键网络对外层苯甲腈分子的质子化苯甲腈的外部溶剂化。在甲醇对外层质子化苯甲腈的溶剂化中，质子位于甲醇亚簇上，中性苯甲腈分子仍位于外部并与质子化甲醇簇的表面键合。甲醇与质子束缚的苯甲腈二聚体（CHCN）H（NCCH）的键合焓测量值为 18.0 kcal/mol，与 DFT 计算值 17.1 kcal/mol 非常吻合，这反映了质子与较低质子亲和力甲醇分子的缔合，从而形成由两个离子氢键稳定的质子化甲醇结构，两个苯甲腈分子。甲醇冰中对苯甲腈的外部溶剂化使苯甲腈能够留在冰粒表面而不是被隔离在冰内部。这可以为与进入的离子和分子的反应提供通道，或者为 UV 照射的光化学过程提供通道，从而导致冰粒表面形成复杂有机物。
