Department of Physics, Faculty of Science, The University of Ngaoundere, P.O. BOX 454, Ngaoundere, Cameroon.
Phys Chem Chem Phys. 2018 Nov 28;20(46):29184-29206. doi: 10.1039/c8cp05823g.
We report in this work the absolute solvation enthalpies and the absolute solvation free energies of the proton in methanol at temperatures ranging from 20 to 340 K and an extrapolation to a desired temperature. To achieve this, we thoroughly investigated the structures of neutral methanol clusters (MeOH)n=2-10 and those of the protonated methanol decamer H+(MeOH)n=10 at the M06-2X/6-31++g(d,p) level of theory. As a result, we noted that up to the octamer, the population of the neutral methanol clusters is constituted by cyclic isomers. For nonamers and decamers, both cyclic and branched cyclic isomers contribute to the population of the clusters. Moreover, folded or distorted cyclic isomers are the most favored at low temperatures, while higher temperatures favored the flat cyclic isomers for n = 7-9. For the methanol decamer, a branched cyclic isomer is found to be the most favored at low temperatures. Elsewhere, the infrared spectra of all the investigated structures are provided and compared against experiment. The binding energy of neutral methanol is calculated at the X/6-31++g(d,p) levels of theory, where X represents the DFT functionals M062X, APFD, MN15, ωB97XD and M08HX. It is observed that these functionals provide results in good agreement with the experimental vaporization enthalpy. However, the APFD functional shows the best performance followed by the other functionals in the order of M062X, MN15 and ωB97XD. Furthermore, the calculated solvation energies of the proton in methanol at these various levels of theory and at MP2/6-31++g(d,p) show that the ωB97XD functional shows the best performance in evaluating the solvation enthalpy and the solvation free energy of the proton in methanol and the calculated values are respectively -1140.5 kJ mol-1 and -1100.7 kJ mol-1 at room temperature. Elsewhere, we noted that the absolute solvation enthalpy of the proton in methanol is less affected by a change in temperature. However, the absolute solvation free energy of the proton in methanol remains constant only at temperatures lower than 180 K. For higher temperatures, the absolute solvation free energy of the proton in methanol increases as a linear function of the temperature and can be approximated by ΔGm(H+,T) = 0.200T - 1161.4.
我们在这项工作中报告了质子在甲醇中的绝对溶剂化焓和绝对溶剂化自由能，温度范围从 20 到 340 K，并进行了所需温度的外推。为了实现这一目标，我们在 M06-2X/6-31++g(d,p)理论水平上彻底研究了中性甲醇团簇(MeOH)n=2-10 和质子化甲醇十聚体 H+(MeOH)n=10 的结构。结果表明，在八聚体之前，中性甲醇团簇的组成由环状异构体组成。对于非九聚体和十聚体，环状和支链环状异构体都有助于团簇的组成。此外，折叠或扭曲的环状异构体在低温下最受欢迎，而在 7-9 时，较高的温度有利于平坦的环状异构体。对于甲醇十聚体，发现支链环状异构体在低温下最受欢迎。此外，还提供了所有研究结构的红外光谱，并与实验进行了比较。在 X/6-31++g(d,p)理论水平上计算了中性甲醇的结合能，其中 X 代表 DFT 函数 M062X、APFD、MN15、ωB97XD 和 M08HX。结果表明，这些函数在与实验汽化焓很好地吻合。然而，APFD 函数表现最好，其次是其他函数，顺序为 M062X、MN15 和 ωB97XD。此外，在这些不同理论水平上计算的质子在甲醇中的溶剂化能和在 MP2/6-31++g(d,p)水平上计算的质子在甲醇中的溶剂化自由能表明，ωB97XD 函数在评估质子在甲醇中的溶剂化焓和溶剂化自由能方面表现最佳，计算值分别为-1140.5 kJ/mol 和-1100.7 kJ/mol，室温下。此外，我们注意到甲醇中质子的绝对溶剂化焓受温度变化的影响较小。然而，只有在温度低于 180 K 时，甲醇中质子的绝对溶剂化自由能才保持不变。对于较高的温度，甲醇中质子的绝对溶剂化自由能随温度呈线性增加，可以近似为ΔGm(H+,T)=0.200T-1161.4。
