Nucleation and dissociation of methane clathrate embryo at the gas–water interface

Among natural energy resources, methane clathrate has attracted tremendous attention because of its strong relevance to current energy and environment issues. Yet little is known about how the clathrate starts to nucleate and disintegrate at the molecular level, because such microscopic processes are difficult to probe experimentally. Using surface-specific sum-frequency vibrational spectroscopy, we have studied in situ the nucleation and disintegration of methane clathrate embryos at the methane-gas-water interface under high pressure and different temperatures. Before appearance of macroscopic methane clathrate, the interfacial structure undergoes 3 stages as temperature varies, namely, dissolution of methane molecules into water interface, formation of cage-like methane-water complexes, and appearance of microscopic methane clathrate, while the bulk water structure remains unchanged. We find spectral features associated with methane-water complexes emerging in the induction time. The complexes are present over a wide temperature window and act as nuclei for clathrate growth. Their existence in the melt of clathrates explains why melted clathrates can be more readily recrystallized at higher temperature, the so-called "memory effect." Our findings here on the nucleation mechanism of clathrates could provide guidance for rational control of formation and disintegration of clathrates

溶液和熔化状态下分子间的能量传递

通过二维红外光谱研究了GdmSCN／KSCN=1／1，GdmSCN／KS^13CN=1／1和GdmSCN／KS^13C^15N=1／1三种混合晶体在熔融和溶液状态下的共振和非共振的分子间振动能量传递的性质．在这些样品中，给体／受体的能量差越大，能量传递越慢．而能量传递的快慢与拉曼光谱无关．非共振能量传递与给体／受体的能量差的关系不能用声子补偿的机理来描述．相反，它们的关系却可以用退相位机理来定量描述．在熔融状态下，共振和非共振能量速率与温度的依赖关系也与退相位机理的预测相符合．这一系列的结果表明只要分子的运动（平动和转动）远远快于非共振能量传递速率，那么退相位机理不仅在溶液中占主导，而且在熔融状态下（纯液体，不含溶剂）也占主导

The molecular rotational motion of liquid ethanol studied by ultrafast time resolved infrared spectroscopy

In this report, ultrafast time-resolved infrared spectroscopy is used to study the rotational motion of the liquid ethanol molecule. The results showed that the methyl, methylene, and CO groups have close rotational relaxation times, 1-2 ps, and the rotational relaxation time of the hydroxyl group (-OH) is 8.1 ps. The fast motion of the methyl, methylene and CO groups, and the slow motion of the hydroxyl group suggested that the ethanol molecules experience anisotropic motion in the liquid phase. The slow motion of the hydroxyl group also shows that the hydrogen bonded network could be considered as an effective molecule. The experimental data provided in this report are helpful for theorists to build models to understand the molecular rotational motion of liquid ethanol. Furthermore, our experimental method, which can provide more data concerning the rotational motion of sub groups of liquid molecules, will be useful for understanding the complicated molecular motion in the liquid phase

溶液和熔化状态下分子间的能量传递

通过二维红外光谱研究了GdmSCN／KSCN=1／1，GdmSCN／KS^13CN=1／1和GdmSCN／KS^13C^15N=1／1三种混合晶体在熔融和溶液状态下的共振和非共振的分子间振动能量传递的性质．在这些样品中，给体／受体的能量差越大，能量传递越慢．而能量传递的快慢与拉曼光谱无关．非共振能量传递与给体／受体的能量差的关系不能用声子补偿的机理来描述．相反，它们的关系却可以用退相位机理来定量描述．在熔融状态下，共振和非共振能量速率与温度的依赖关系也与退相位机理的预测相符合．这一系列的结果表明只要分子的运动（平动和转动）远远快于非共振能量传递速率，那么退相位机理不仅在溶液中占主导，而且在熔融状态下（纯液体，不含溶剂）也占主导

The Anion Effect on Li + Ion Coordination Structure in Ethylene Carbonate Solutions

International audienc

Nonresonant Vibrational Energy Transfer on Metal Nanoparticle/Liquid Interface

Knowledge of vibrational energy transfer on a metal nanoparticle/liquid interface is essential for understanding the energy conversion process involved in many heterogeneous nanocatalyses. In this study, we investigate mode-specific vibrational energy transfer between CO molecules on different adsorbate sites on a 1 nm platinum metal nanoparticle/liquid interface by using ultrafast two-dimensional IR spectroscopy. The vibrational energy transport is found to be induced by vibration/vibration coupling with very little surface electron/vibration mediation. The energy transfer rate is determined to be about 1/140 ps^–1 from the atop site CO to the bridge site CO, and the specific rate is around 1/400 ps^–1 between the two nearest adsorbates. The energy transfer between different adsorbate sites can be described by the dephasing mechanism reasonably well. The mechanical coupling may contribute to the transfer, but analyses suggest that the role of dipole/dipole interaction is a more important factor for the energy transfer

Comparison Studies on Sub-Nanometer-Sized Ion Clusters in Aqueous Solutions: Vibrational Energy Transfers, MD Simulations, and Neutron Scattering

In this work, MD simulations with two different force fields, vibrational energy relaxation and resonant energy transfer experiments, and neutron scattering data are used to investigate ion pairing and clustering in a series of GdmSCN aqueous solutions. The MD simulations reproduce the major features of neutron scattering experimental data very well. Although no information about ion pairing or clustering can be obtained from the neutron scattering data, MD calculations clearly demonstrate that substantial amounts of ion pairs and small ion clusters (subnanometers to a few nanometers) do exist in the solutions of concentrations 0.5 M*, 3 M*, and 5 M* (M* denotes mole of GdmSCN per. 55.55 mole of water). Vibrational relaxation experiments suggest that significant amounts of ion pairs form in the solutions. Experiments measuring the resonant energy transfers among the thiocyanate anions in the solutions suggest that the ions form clusters and in the clusters the average anion distance is 5.6 angstrom (5.4 angstrom) in the 3 M* (5 M*) Gdm(-D)SCN/D2O solution

Negligible Isotopic Effect on Dissociation of Hydrogen Bonds

Isotopic effects on the formation and dissociation kinetics of hydrogen bonds are studied in real time with ultrafast chemical exchange spectroscopy. The dissociation time of hydrogen bond between phenol-OH and <i>p</i>-xylene (or mesitylene) is found to be identical to that between phenol-OD and <i>p</i>-xylene (or mesitylene) in the same solvents. The experimental results demonstrate that the isotope substitution (D for H) has negligible effects on the hydrogen bond kinetics. DFT calculations show that the isotope substitution does not significantly change the frequencies of vibrational modes that may be along the hydrogen bond formation and dissociation coordinate. The zero point energy differences of these modes between hydrogen bonds with OH and OD are too small to affect the activation energy of the hydrogen bond dissociation in a detectible way at room temperature

The Anion Effect on Li⁺ Ion Coordination Structure in Ethylene Carbonate Solutions

Rechargeable lithium ion batteries are an attractive alternative power source for a wide variety of applications. To optimize their performances, a complete description of the solvation properties of the ion in the electrolyte is crucial. A comprehensive understanding at the nanoscale of the solvation structure of lithium ions in nonaqueous carbonate electrolytes is, however, still unclear. We have measured by femtosecond vibrational spectroscopy the orientational correlation time of the CO stretching mode of Li⁺-bound and Li⁺-unbound ethylene carbonate molecules, in LiBF₄, LiPF₆, and LiClO₄ ethylene carbonate solutions with different concentrations. Surprisingly, we have found that the coordination number of ethylene carbonate in the first solvation shell of Li⁺ is only two, in all solutions with concentrations higher than 0.5 M. Density functional theory calculations indicate that the presence of anions in the first coordination shell modifies the generally accepted tetrahedral structure of the complex, allowing only two EC molecules to coordinate to Li⁺ directly. Our results demonstrate for the first time, to the best of our knowledge, the anion influence on the overall structure of the first solvation shell of the Li⁺ ion. The formation of such a cation/solvent/anion complex provides a rational explanation for the ionic conductivity drop of lithium/carbonate electrolyte solutions at high concentrations