Department of Chemistry, Virginia Commonwealth University, Richmond, Virginia 23284-2006, USA.
J Chem Phys. 2013 Aug 28;139(8):084304. doi: 10.1063/1.4817327.
Equilibrium thermochemical measurements using an ion mobility drift cell technique have been utilized to investigate the binding energies and entropy changes associated with the stepwise hydration of the biologically significant ions pyrimidine radical cation and protonated pyrimidine. The binding energy of the hydrated pyrimidine radical cation is weaker than that of the proton-bound dimer pyrimidineH(+)(H2O) consistent with the formation of a weak carbon-based CH(δ+)··OH2 hydrogen bond (11.9 kcal/mol) and a stronger NH(+)··OH2 hydrogen bond (15.6 kcal/mol), respectively. Other proton-bound dimers such as pyrimidineH(+)(CH3OH) and pyrimidineH(+)(CH3CN) exhibit higher binding energies (18.2 kcal/mol and 22.8 kcal/mol, respectively) due to the higher proton affinities and dipole moments of acetonitrile and methanol as compared to water. The measured collisional cross sections of the proton-bound dimers provide experimental-based support for the DFT calculated structures at the M06-2x/6-311++G (d,p) level. The calculations show that the hydrated pyrimidine radical cation clusters form internally solvated structures in which the water molecules are bonded to the C4N2H4(●+) ion by weak CH(δ+)··OH2 hydrogen bonds. The hydrated protonated pyrimidine clusters form externally solvated structures where the water molecules are bonded to each other and the ion is external to the water cluster. Dissociative proton transfer reactions C4N2H4(●+)(H2O)(n-1) + H2O → C4N2H3(●) + (H2O)(n)H(+) and C4N2H5(+)(H2O)(n-1) + H2O → C4N2H4 + (H2O)(n)H(+) are observed for n ≥ 4 where the reactions become thermoneutral or exothermic. The absence of the dissociative proton transfer reaction within the C4N2H5(+)(CH3CN)n clusters results from the inability of acetonitrile molecules to form extended hydrogen bonding structures such as those formed by water and methanol due to the presence of the methyl groups which block the extension of hydrogen bonding networks.
使用离子淌度漂移池技术进行平衡热化学测量，研究了与生物相关的嘧啶自由基阳离子和质子化嘧啶逐步水合相关的结合能和熵变化。水合嘧啶自由基阳离子的结合能比质子结合二聚体嘧啶 H(+)(H2O)弱，这与形成弱碳基 CH(δ+)··OH2氢键（11.9 kcal/mol）和更强的 NH(+)··OH2氢键（15.6 kcal/mol）分别一致。其他质子结合二聚体，如嘧啶 H(+)(CH3OH)和嘧啶 H(+)(CH3CN)，由于乙腈和甲醇的质子亲和力和偶极矩高于水，因此表现出更高的结合能（分别为 18.2 kcal/mol 和 22.8 kcal/mol）。质子结合二聚体的测量碰撞截面为在 M06-2x/6-311++G（d，p）水平上用 DFT 计算的结构提供了实验支持。计算表明，水合嘧啶自由基阳离子簇形成内部溶剂化结构，其中水分子通过弱 CH(δ+)··OH2氢键与 C4N2H4(●+)离子键合。水合质子化嘧啶簇形成外部溶剂化结构，其中水分子相互键合，离子位于水分子簇之外。对于 n≥4，观察到 C4N2H4(●+)(H2O)(n-1) + H2O→C4N2H3(●)+(H2O)(n)H(+)和 C4N2H5(+)(H2O)(n-1) + H2O→C4N2H4 + (H2O)(n)H(+)的离解质子转移反应，其中反应变得热中性或放热。由于甲基的存在阻止了氢键网络的扩展，乙腈分子无法形成像水和甲醇那样的扩展氢键结构，因此在 C4N2H5(+)(CH3CN)n 簇中不存在离解质子转移反应。
