Li Ping, Bu Yuxiang
Institute of Theoretical Chemistry, Shandong University, Jinan 250100, People's Republic of China.
J Chem Phys. 2004 Nov 22;121(20):9971-81. doi: 10.1063/1.1792111.
The behavior of double proton transfer occurring in a representative glycinamide-formic acid complex has been investigated at the B3LYP/6-311 + + G( * *) level of theory. Thermodynamic and, especially, kinetic parameters, such as tautomeric energy, equilibrium constant, and barrier heights have been discussed, respectively. The relevant quantities involved in the double proton transfer process, such as geometrical changes, interaction energies, and intrinsic reaction coordinate calculations have also been studied. Computational results show that the participation of a formic acid molecule favors the proceeding of the proton transfer for glycinamide compared with that without mediate-assisted case. The double proton transfer process proceeds with a concerted mechanism rather than a stepwise one since no ion-pair complexes have been located during the proton transfer process. The calculated barrier heights are 11.48 and 0.85 kcal/mol for the forward and reverse directions, respectively. However, both of them have been reduced by 2.95 and 2.61 kcal/mol to 8.53 and -1.76 kcal/mol if further inclusion of zero-point vibrational energy corrections, where the negative barrier height implies that the reverse reaction should proceed with barrierless spontaneously, analogous to that occurring between glycinamide and formamide. Furthermore, solvent effects on the thermodynamic and kinetic processes have also been predicted qualitatively employing the isodensity surface polarized continuum model within the framework of the self-consistent reaction field theory. Additionally, the oxidation process for the double H-bonded glycinamide-formic acid complex has also been investigated. Contrary to that neutral form possessing a pair of two parallel intermolecular H bonds, only a single H bond with a comparable strength has been found in its ionized form. The vertical and adiabatic ionization potentials for the neutral complex have been determined to be about 9.40 and 8.69 eV, respectively, where ionization is mainly localized on the glycinamide fragment. Like that ionized glycinamide-formamide complex, the proton transfer in the ionized complex is characterized by a single-well potential, implying that the proton initially attached to amide N4 in the glycinamide fragment cannot be transferred to carbonyl O13 in the formic acid fragment at the geometry of the optimized complex.
在B3LYP/6 - 311++G(**)理论水平下，研究了具有代表性的甘氨酰胺 - 甲酸络合物中双质子转移的行为。分别讨论了热力学参数，特别是动力学参数，如互变异构能、平衡常数和势垒高度。还研究了双质子转移过程中涉及的相关量，如几何变化、相互作用能和内禀反应坐标计算。计算结果表明，与无介导辅助的情况相比，甲酸分子的参与有利于甘氨酰胺质子转移的进行。双质子转移过程以协同机制而非分步机制进行，因为在质子转移过程中未发现离子对络合物。正向和反向方向计算得到的势垒高度分别为11.48和0.85 kcal/mol。然而，如果进一步包含零点振动能校正，两者分别降低了2.95和2.61 kcal/mol，变为8.53和 - 1.76 kcal/mol，其中负的势垒高度意味着反向反应应自发无势垒进行，类似于甘氨酰胺和甲酰胺之间发生的情况。此外，还在自洽反应场理论框架内，采用等密度面极化连续介质模型定性预测了溶剂对热力学和动力学过程的影响。此外，还研究了双氢键合的甘氨酰胺 - 甲酸络合物的氧化过程。与具有一对两个平行分子间氢键的中性形式相反，在其离子化形式中仅发现了一个强度相当的单氢键。中性络合物的垂直和绝热电离势分别确定为约9.40和8.69 eV，其中电离主要定域在甘氨酰胺片段上。与离子化的甘氨酰胺 - 甲酰胺络合物一样，离子化络合物中的质子转移以单阱势为特征，这意味着在优化络合物的几何结构下，最初附着在甘氨酰胺片段中酰胺N4上的质子不能转移到甲酸片段中的羰基O13上。
