Department of Chemistry, Texas A&M University, College Station, Texas 77843, United States.
J Phys Chem B. 2020 Mar 19;124(11):2081-2087. doi: 10.1021/acs.jpcb.9b10576. Epub 2020 Mar 6.
Here, cryogenic ion mobility-mass spectrometry (cryo-IM-MS) is used to investigate intracluster proton transfer reactions of 4-aminobenzoic acid during the transition from solution to the gas phase. Previous studies have shown that protonation of the amine group of 4-aminobenzoic acid (4-ABAH) is favored in solution (N-protomer), whereas protonation of the carboxylic acid group is favored in the gas phase (O-protomer). Results from cryo-IM-MS (80 K) studies of hydrated 4-ABAH ions, 4-ABAH(HO), are interpreted as evidence that the proton transfer reaction occurs through a water bridge at = 6 connecting the -NH and -COOH groups, that is a Grotthuss mechanism. The weak binding energy of water molecules imposes limits for obtaining first-principles collisional cross sections (CCSs) of hydrated ions; consequently, candidate structures for 4-ABAH(HO) ions are derived by correlating experimental arrival-time distributions to theoretically determined CCSs. To our knowledge, these are the first first-principles determinations of CCS for hydrated ions. Apolar cosolvents, particularly acetonitrile, have been postulated to inhibit proton transfer by blocking the Grotthuss mechanism, but our data suggest that acetonitrile simply stabilizes the ammonium ion.
在这里，低温离子淌度-质谱（cryo-IM-MS）被用于研究 4-氨基苯甲酸在从溶液向气相转变过程中的簇内质子转移反应。先前的研究表明，4-氨基苯甲酸（4-ABAH）的胺基质子化在溶液中是有利的（N-质子化形式），而在气相中羧酸基团的质子化是有利的（O-质子化形式）。低温 IM-MS（80 K）对水合 4-ABAH 离子（4-ABAH（HO））的研究结果被解释为证据，表明质子转移反应通过连接-NH 和-COOH 基团的水桥在 = 6 处发生，即质子转移反应通过 Grotthuss 机制发生。水分子的弱结合能限制了获得水合离子碰撞截面（CCS）的第一性原理计算；因此，通过将实验到达时间分布与理论确定的 CCS 相关联，推导出了 4-ABAH（HO）离子的候选结构。据我们所知，这些是第一个对水合离子进行第一性原理 CCS 测定的结果。非极性共溶剂，特别是乙腈，被假定通过阻断 Grotthuss 机制来抑制质子转移，但我们的数据表明乙腈只是稳定了铵离子。
