Dong Haitao, Du Hongbo, Qian Xianghong
Department of Mechanical Engineering, Campus Delivery 1374, Colorado State University, Fort Collins, Colorado 80523, USA.
J Phys Chem A. 2008 Dec 11;112(49):12687-94. doi: 10.1021/jp807315p.
The acidity constant pKa for polymeric organic acid is expected to be different from its corresponding monomer value due to the change of chemical environment upon polymerization. Thermodynamic cycles were used to determine the free-energy changes for the proton dissociation processes in aqueous solution and the corresponding pKa values for monomer methacrylic acid and several similar carboxylic acids. First-principles calculations and continuum solvation model were used to determine the gas-phase and solvation free energies, respectively. A protocol was developed to use the efficient density functional calculations with B3LYP functional instead of the demanding CBS-QB3 method to determine the gas-phase free energies with relative high accuracy, thus enabling the determination of pKa values for the short oligomers of methacrylic acid. The predicted pKa values for the dimer and trimer of methacrylic acid are higher by about 0.8 pKa units than the predicted monomer value.
由于聚合过程中化学环境的变化，预计聚合有机酸的酸度常数pKa与其相应单体的值不同。利用热力学循环来确定水溶液中质子解离过程的自由能变化以及单体甲基丙烯酸和几种类似羧酸的相应pKa值。分别使用第一性原理计算和连续介质溶剂化模型来确定气相和溶剂化自由能。开发了一种方案，使用具有B3LYP泛函的高效密度泛函计算，而不是要求苛刻的CBS-QB3方法，以相对高精度确定气相自由能，从而能够确定甲基丙烯酸短寡聚物的pKa值。甲基丙烯酸二聚体和三聚体的预测pKa值比预测的单体值高约0.8个pKa单位。
