Entropic trends in aqueous solutions of the common functional groups
Entropic trends in aqueous solutions of the common functional groups
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DOI：
10.1039/b907383c
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发表时间：
2010-01-01
影响因子：
3.4
通讯作者：
Henchman, Richard H.
中科院分区：
文献类型：
--
作者：
Irudayam, Sheeba Jem;Plumb, Richard D.;Henchman, Richard H.
虽然人们对水溶液中有机小分子的性质了解很多，但有一个量却没有得到详细的了解，那就是它们的熵。不同的官能团以不同的方式与水分子相互作用，从而影响分子的溶解度、构象和缔合行为。实验可以获得溶剂化的总熵，但很难给出熵分量的更详细的分解。建立的理论和计算方法的基础上微扰洞察溶质-溶剂和溶剂-溶剂熵分量的变化，但这些多体条款，代表的变化是不直观的解释，可以是昂贵的或难以评估。另一方面，配分函数方法具有确定系统中每个分子的每个自由度的熵的能力。他们通过确定每个自由度的有效势，并从有效势的配分函数中计算相关的熵分量来做到这一点。这种方法的障碍一直是找到一种可靠的方法来定义和推导这些有效的潜力。我们从两个方面克服了这一问题：首先，从分子动力学模拟中测得的力和力矩的大小推导出每个分子的有效势的形状，它与受限分子的振动和振动运动有关;其次，每个有效势的最小值的数量，它与解中的位置和方向的数量有关，是由周围的溶剂分子的平移和旋转离散。这种方法已被证明是成功地再现了液态水的熵，并检查周围的水的熵损失的稀有气体溶质。在这项工作中，我们扩展的方法，以揭示水的熵周围的小有机分子与一系列的官能团的性质。溶质和水的振动熵和自由熵随着极性原子的增加而减小，这是可以预料的。溶质取向的数量取决于溶质的大小，而水取向的数量取决于溶质中极性原子的数量。溶质是根据它们的供体和受体如何影响水的取向熵来分类的。标准吉布斯自由能的计算值与实验值吻合得很好，平均无符号误差为2.5 kJ mol（-1），但熵和熵值不够负，可以用更好的力场来改善。
While much is known about the properties of small organic molecules in aqueous solution, one quantity that has eluded a detailed understanding is their entropy. Different functional groups interact in diverse ways with water molecules and thereby influence a molecule's solubility, conformation and association behaviour. Experiment can access the total entropy of solvation but struggles to give a more detailed break-down of the entropic components. Established theoretical and computational methods based on perturbation give insight into changes in solute-solvent and solvent-solvent entropy components, but these multibody terms that represent changes are not intuitive to interpret and can be expensive or difficult to evaluate. Partition-function methods, on the other hand, have the capability of determining the entropy of every degree of freedom of every molecule in the system. They do this by determining effective potentials for each degree of freedom, and evaluating the associated entropy component from the partition function of the effective potential. The obstacle to such an approach has been finding a reliable way to define and derive these effective potentials. This we have overcome in a two-fold manner: firstly, the shape of the effective potential for each molecule, which relates to vibrational and librational motion of the confined molecule, is derived from the magnitudes of the forces and torques measured in a molecular dynamics simulation of the solution; secondly, the number of minima for each effective potential, which relates to the number of positions and orientations in solution, is derived from the translational and rotational discretisations by the surrounding solvent molecules. This method has been shown to successfully reproduce the entropy of liquid water and to examine the entropy loss of water around noble-gas solutes. In this work, we extend the approach to reveal the nature of water's entropy around small organic molecules with a range of functional groups. The vibrational and librational entropies of solutes and water decrease for solutes with more polar atoms, as would be expected. The number of solute orientations depends on solute size, whereas the number of water orientations depends on the number of polar atoms in the solute. Solutes are classified according to how their donors and acceptors affect water's orientational entropy. Agreement of the calculated standard Gibbs free energy of solvation with experiment is very good with a mean-unsigned error of 2.5 kJ mol(-1), but the entropies and enthalpies, not being negative enough, could be improved with better force fields.