Montero-Cabrera Luis A, Pérez-Badell Y, Mora-Fonz M J
Laboratorio de Química Computacional y Teórica, Facultad de Química, Universidad de La Habana, 10400 Havana, Cuba. luis.montero@ fq.uh.cu
J Phys Chem A. 2008 Apr 3;112(13):2880-7. doi: 10.1021/jp710583n. Epub 2008 Feb 28.
The influence of molecular water on the structure and formation of silica clusters is modeled with the use of the MMH (multiple minima hypersurfaces) approach. It combines quantum chemical Hamiltonians for the calculation of the internal energy with statistical modeling and formulas for the calculation of thermodynamic functions of association. The structures of the most probable clusters of hydration and some properties of the association with water are proposed. Different simple structures are calculated to consider the entropy of association in place of the simpler approach of a single "global minimum". Ab initio, DFT, and semiempirical calculations of the structures and relevant reactions of silica fragments are also reported, confirming the reliability of the results on very different grounded quantum mechanical methods. Particularly, it has been shown that semiempirical PM3 Hamiltonian is reliable for silica cluster calculations of this kind, in comparison with accurate ab initio SCF and other DFT calculations. Apparently, the well-known systematic failures of this Hamiltonian are absent in this kind of structures and interactions. The increasing hydrophobic character of neutral silica clusters appears as a result of the free energies of association, and therefore, it originates on entropy. It is remarkable that the simple global minimum approach currently used in works of molecular modeling on this kind of compounds, where several hydrogen bridges have a place to exist with several different conformations, must be taken with caution given the remarkable entropy of association of such systems.
利用MMH（多重极小值超曲面）方法对分子水对二氧化硅团簇结构和形成的影响进行了建模。它将用于计算内能的量子化学哈密顿量与统计建模以及用于计算缔合热力学函数的公式相结合。提出了最可能的水合团簇结构以及与水缔合的一些性质。计算了不同的简单结构，以考虑缔合熵，而不是采用单一“全局最小值”这种更简单的方法。还报道了二氧化硅片段结构及相关反应的从头算、密度泛函理论（DFT）和半经验计算，证实了基于非常不同的量子力学方法得到的结果的可靠性。特别是，与精确的从头算自洽场（SCF）和其他DFT计算相比，已表明半经验PM3哈密顿量对于此类二氧化硅团簇计算是可靠的。显然，在这种结构和相互作用中不存在该哈密顿量众所周知的系统性误差。中性二氧化硅团簇疏水性的增加是缔合自由能的结果，因此，它源于熵。值得注意的是，鉴于此类系统缔合熵显著，目前在这类化合物的分子建模工作中使用的简单全局最小值方法必须谨慎对待，因为这类化合物中存在几个具有几种不同构象的氢键。
