RESUMEN
In this work, we identified a large number of structurally distinct isomers of midsized deprotonated water clusters, OH(-)(H2O)n=4-7, using first-principles methods. The temperature dependence of the structural variation in the solvation shell of OH(-) for these clusters was examined under the harmonic superposition approximation. We simulated the vibrational and photoelectron spectra based on these thermodynamic calculations. We found that the isomers with 3-coordinated hydroxide dominate the population in these midsized clusters. Furthermore, an increase in temperature causes a topological change from compact isomers with many intermolecular hydrogen bonds to open isomers with fewer but more directional intermolecular hydrogen bonds. We showed that this evolution in structure can be observed through the change in the vibrational spectra at 3200-3400 cm(-1). In addition, the increase in directional hydrogen bonded isomers, which have outer hydration shell with OH bonds pointing to the hydroxide, causes the vertical detachment energy to increase at higher temperatures. Lastly, we also performed studies to understand the variation in the aforementioned spectral quantities with the variation in the coordination number of the hydroxide.
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Theoretical and experimental methods were integrated to investigate the structures of H(+)(CH(3)OH)(m)(H(2)O)(n) clusters for m + n = 5 and 6. An effective theoretical approach is presented to search for extensive sets of structural isomers using an empirical model and substitution schemes. Stable isomers were then reoptimized by the B3LYP level of computations with the 6-31+G* basis set. Canonical averages of these structural isomers were analyzed by harmonic superposition approximation (HSA) to study their finite temperature behavior and enable quantitative comparisons with experimental results. Thermal energy is found to have a significant effect on the structure of these clusters. Our calculations show that cyclic isomers are preferred at low temperature, while linear and tree forms become more favorable at high temperature (>200 K). Furthermore, we found that proton can reside on both water and methanol ion cores and the proton switch is associated with morphology change. Experimental IR spectra in the free OH stretching region were also obtained and compared with calculated spectra.
Asunto(s)
Metanol/química , Protones , Agua/química , Simulación por Computador , Modelos Químicos , Estructura Molecular , Espectrofotometría Infrarroja , TemperaturaRESUMEN
We propose a multiscale method to explore the energy landscape of water clusters. An asynchronous genetic algorithm is employed to explore the potential energy surface (PES) of OSS2 and TTM2.1-F models. Local minimum structures are collected on the fly, and the ultrafast shape recognition algorithm was used to remove duplicate structures. These structures are then refined at the B3LYP/6-31+G* level. The number of distinct local minima we found (21, 76, 369, 1443, and 3563 isomers for n = 4-8, respectively) reflects the complexity of the PES of water clusters.
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Ground-state structures, vibrational frequencies, HOMO-LUMO energy gap, electron affinities, and cluster mixing energy of binary semiconductor clusters SimGen in the range s = m + n
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The energy landscape of protonated water clusters H(+)(H2O)n is thoroughly explored at the first-principle level using a hierarchical search methodology. In particular, the distinct configurational isomers of OSS2 empirical potential for n = 5-9 are uncovered and archived systematically using an asynchronous genetic algorithm and are subsequently refined with first-principle calculations. Using the OSS2 model, quantitative agreements in the thermal properties between Monte Carlo and harmonic superposition approximation (HSA) highlighted the reliability of the latter approach for the study of small- to medium-sized protonated water clusters. From the large sets of collected isomers, finite temperature behavior of the clusters can be efficiently examined at first-principle accuracy with the use of HSA. From the results obtained, evidence of structural changes from single-ring to treelike (n = 5-7) and multi-ring to single-ring structures (n = 7-9) is observed, as expected for the empirical model. Finally, the relevance of these findings to recent experimental data is discussed.