Partay Lívia, Jedlovszky Pal, Jancsó Gabor
Department of Colloid Chemistry, Eötvös Lorand University, Pazmany Péter stny. 1/a, H-1117 Budapest, Hungary.
J Phys Chem B. 2005 Apr 28;109(16):8097-102. doi: 10.1021/jp044390d.
The difference of the hydration free energy of pyridine and its methyl- and symmetrically dimethyl-substituted derivatives has been calculated by the method of free energy perturbation. To check the precision of the results obtained, we have repeated the calculations using thermodynamic integration over different paths. Besides the hydration free energy, the difference in the energy and entropy of hydration between pyridine and monomethyl- and dimethylpyridines has also been determined. The obtained results clearly show that the hydration free energy of the pyridine derivatives becomes more negative with each additional methyl group. However, the accuracy of the calculation does not allow us to draw any conclusion about the dependence of the hydration free energy on the location of the methyl group. The analysis of the Coulomb and Lennard-Jones contributions to the hydration free energy differences has shown the dominance of the latter term. The comparison of the hydration energy and free energy values has shown that there is a strong compensation effect between the energetic and entropic terms of the free energy. The hydration energy of the solute becomes considerably more negative with each additional methyl group due to the dispersion attraction between the methyl group and the surrounding water molecules. The introduction of a methyl group results in an approximately 30 J/(mol K) decrease of the entropy of hydration, and hence, at 300 K, the entropic contribution to the hydration free energy increases by about 9 kJ/mol. Due to their opposite signs, the entropic and energetic contributions largely cancel each other, resulting in approximately an order of magnitude smaller value for the free energy.
采用自由能微扰法计算了吡啶及其甲基取代和对称二甲基取代衍生物的水合自由能差异。为检验所得结果的精度，我们通过在不同路径上进行热力学积分重复了计算。除水合自由能外，还测定了吡啶与一甲基吡啶和二甲基吡啶之间水合能和水合熵的差异。所得结果清楚地表明，吡啶衍生物的水合自由能随着每个额外甲基的增加而变得更负。然而，计算精度不允许我们得出关于水合自由能对甲基位置依赖性的任何结论。对库仑和 Lennard-Jones 对水合自由能差异的贡献分析表明，后一项占主导地位。水合能和自由能值的比较表明，自由能的能量项和熵项之间存在强烈的补偿效应。由于甲基与周围水分子之间的色散吸引力，溶质的水合能随着每个额外甲基的增加而显著变得更负。引入一个甲基导致水合熵大约降低 30 J/(mol K)，因此，在 300 K 时，水合自由能的熵贡献增加约 9 kJ/mol。由于它们的符号相反，熵贡献和能量贡献在很大程度上相互抵消，导致自由能的值大约小一个数量级。
