State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210093, People's Republic of China.
J Phys Chem A. 2010 Dec 16;114(49):12914-7. doi: 10.1021/jp106359v. Epub 2010 Nov 19.
In order to explore the aqueous acid chemistry of carbonic acid, we employ a constrained ab initio molecular dynamics (AIMD) technique to study acid dissociations of its three conformers including CC (cis-cis), CT (cis-trans), and TT (trans-trans). The simulations of reagent states reveal similar hydration characteristics for them: the hydroxyls donate H-bonds to solvating waters but no obvious H-bonding exists between hydroxyl oxygen atoms and waters. It is found that the CC conformer dissociates spontaneously to bicarbonate within picoseconds whereas the other two can stay for relatively long simulation times. This suggests that CC has the strongest acidity among the three conformers and it is not stable in water. The simulations indicate that the symmetrical hydroxyls of TT conformer have a pKa value of 3.11 and the two asymmetrical hydroxyls of CT show different pKa values: 2.60 and 3.75, respectively. Overall, these results confirm the recent experimental measurement: about 4.0 for deuterated carbonic acid. By analyzing the dissociation processes, it is revealed that the differences of the acid constants stem from the initial steps of hydroxyls stretches. This simulation study provides a quantitative and microscopic basis for better understanding the reactivity of aqueous carbonate species.
为了探索碳酸的水合酸化学，我们采用约束性从头算分子动力学（AIMD）技术研究了其三种构象（CC（顺式-顺式）、CT（顺式-反式）和 TT（反式-反式））的酸离解。试剂态的模拟揭示了它们相似的水合特征：羟基向溶剂水提供氢键，但羟基氧原子与水之间没有明显的氢键存在。结果发现 CC 构象在皮秒内自发离解为碳酸氢盐，而另外两种构象可以在较长的模拟时间内保持稳定。这表明 CC 在三种构象中具有最强的酸性，在水中不稳定。模拟表明 TT 构象的对称羟基的 pKa 值为 3.11，CT 构象的两个不对称羟基具有不同的 pKa 值：分别为 2.60 和 3.75。总的来说，这些结果证实了最近的实验测量值：氘代碳酸的约为 4.0。通过分析离解过程，揭示了酸常数的差异源于羟基拉伸的初始步骤。这项模拟研究为更好地理解水合碳酸物种的反应性提供了定量和微观基础。
