水溶液中铵离子pKa值的理论计算

在HF/6-31+G*和B3LYP/6-31+G*水平上, 采用导体极化连续模型(CPCM)及UAKS孔穴计算了11种铵离子在水溶剂中的溶剂化自由能, 与实验值相比较, 平均误差和标准偏差分别为0.17, 12.04和0.96, 10.96 kJ/mol. 结合B3LYP/6-31+G*水平上的11种铵离子气相质子转移反应自由能, 得到了水溶剂中的绝对pKa值, 计算结果与实验数据吻合得很好, 相应的平均误差和标准偏差分别为0.05, 1.50和0.45, 1.40 pKa单位. 可见, 采用CPCM-UAKS模型能够较为精确地计算铵离子型化合物的绝对pKa值.

Theoretical Calculation of pKa for Ammonium Ions in Aqueous Solution

The conductor-like polarizable continuum model(CPCM) combined with UAKS cavity was applied to calculating the solvation free energy of 11 ammonium ions in aqueous solution at HF/6-31 G* and B3LYP/6-31 G* levels. The computational values were consistent with the experimental values, the mean errors and standard deviations were 0.17, 12.04 and 0.96, 10.96 kJ/mol, respectively. Using the calculated gas-phase proton-transfer free energies of the ammonium ions at the B3LYP/6-31 G* level of theory, the absolute pKa values in aqueous solution were obtained and agreed well with the experimental values, the relevant mean errors and standard deviations were only 0.05, 1.50 and 0.45, 1.40 pKa units, respectively. In order to validate the practicability of CPCM-UAKS model, the absolute pKa value of noradrenalin was also calculated. The result shows that CPCM-UAKS method can be used to calculate the absolute pKa values of ammonium ions compounds in aqueous solution accurately.