One-dimensional model for water and aqueous solutions. II. Solvation of inert solutes in water

The two one-dimensional models introduced in Part I are used to study the thermodynamics of solvation of inert solutes in water. It is shown that the anomalously large Gibbs energy of solvation of inert solutes in water, on one hand, and the large negative entropy of solvation, on the other hand, arise from different molecular sources. While the primitive model can give rise to a large positive solvation Gibbs energy, it fails to show large negative entropy and enthalpy of solvation. It is remarkable that the primitive cluster model can show both the large positive Gibbs energy and enthalpy of solvation.     

A convexity relation for the energies of certain polymers of unit cells of different sizes

A simple rule in the form of total energy inequalities is proposed for special polymers of very large unit cells, in terms of some related polymers of small unit cells. If energy data can be calculated for the latter regular polymers, these energy values lead to energy constraints on the polymers of very large unit cells. The actual evaluation of these constraints involves only a simple inspection of the nuclear charges and elementary calculations.     

Phenomenological predictions of cohesive energy and structural transition of nanoparticles

In this paper, it is shown that a liquid-drop model (LDM) can predict the size-dependent cohesive energy (SDCE) of large nanoparticles and clusters (particles with few atoms) quantitatively. The cohesive energy decreases linearly with the inverse of the particle size both for large nanoparticles and clusters though the slopes are different. This indicates that there are three different regions (I-III) of SDCE in the complete size range. Regions I and II represent the SDCE of large nanoparticles and clusters, respectively, while region II represents the intermediate region where the cohesive energy is almost size-independent. Different regions of SDCE correspond to different structures of nanoparticles, and structural transition associated with the particle size can easily be predicted from the SDCE. Analyzing the cohesive energy data on the basis of LDM, it is shown that the surface tension decreases with decreasing size for very small nanoparticles. The Tolman equation can account for the variation of surface tension by predicting the size dependency of the Tolman length.     

Delayed dissociation spectra of survivor ions from high-energy collisional activation

Collisional activation (CA) of large ions at kiloelectronvolt energies is accompanied by unexpectedly large losses of translational energy, which vary with the nature of the collision gas. Previous investigations have concentrated upon subsequent fragmentations occurring within a time window covering a few fis immediately following collision, using massanalyzed ion kinetic energy spectrometry. In the present work, survivor ions were selected for specified values of translational energy loss, and their internal energy contents assessed via their subsequent unimolecular fragmentation reactions within a later time window. Beam collimation was also applied when circumstances permitted to impose angular selection, thus minimizing cross talk between effects of collisional scattering and energy dispersion. It was shown that internal excitation of the reactant ion can account for only a small fraction of the observed loss of translational energy. The recoil energy of the target is thus the principal sink for the translational energy loss, since the latter was always chosen to be less than the lowest excitation energy of the target. This conclusion is shown to be consistent with theoretical models of the CA process. The practical implications of these conclusions for CA of large ions at kiloelectronvolt energies are discussed.     

The role of large conformational changes in efficient ultrafast internal conversion: deviations from the energy gap law

Conversion of electronic excitation energy into vibrational energy was investigated for photochromic spiropyran molecules, using femtosecond UV-mid-IR pump-probe spectroscopy. We observe a weaker energy gap dependence than demanded by the "energy gap law". We demonstrate that large conformational changes accompanying the optical excitation can explain the observed time scale and energy gap dependence of ultrafast S(1) --> S(0) internal conversion processes. The possibility of dramatic deviations from standard energy gap law behavior is predicted. We conclude that controlling molecular conformations by rigid environments can have a substantial impact on photophysical and (bio)chemical processes.     

The use of diatomic energies to analyze adsorption and coadsorption of CO and H on an Fe12 cluster

Semi-empirical molecular orbital calculations in the MINDO/SR formalism have been done for CO and H interacting with the (100) face of an Fe₁₂ cluster. Breakdown of the absorption energy into monoatomic and diatomic terms indicated a complex interaction in which a large diatomic energy term of 325 kcal/mole for CO in an on-top site is needed to obtain a net adsorption energy of 23 kcal/mole because of large destabilizing monoatomic energy terms and because adsorption destabilizes the Fe₁₂ cluster. Adsorption of H and CO in on-top and 4-fold sites gave similar behavior. The cluster d band is shifted by adsorbate-adsorbent charge transfer even though the d orbitals interact only very weakly with the adsorbate. While the interaction of CO with its nearest neighbor dominates the interaction energy, the interaction energy with the rest of the cluster is necessary for a quantitative account of adsorption and can be as large as the net adsorption energy. For coadsorption of CO and H on adjacent on-top sites, the diatomic energy terms indicate that H is not affected by the CO but the CO adsorption is destabilized by the H. For adjacent 4-fold sites both the CO and H adsorption strengths are decreased.     

String method in collective variables: minimum free energy paths and isocommittor surfaces

A computational technique is proposed which combines the string method with a sampling technique to determine minimum free energy paths. The technique only requires to compute the mean force and another conditional expectation locally along the string, and therefore can be applied even if the number of collective variables kept in the free energy calculation is large. This is in contrast with other free energy sampling techniques which aim at mapping the full free energy landscape and whose cost increases exponentially with the number of collective variables kept in the free energy. Provided that the number of collective variables is large enough, the new technique captures the mechanism of transition in that it allows to determine the committor function for the reaction and, in particular, the transition state region. The new technique is illustrated on the example of alanine dipeptide, in which we compute the minimum free energy path for the isomerization transition using either two or four dihedral angles as collective variables. It is shown that the mechanism of transition can be captured using the four dihedral angles, but it cannot be captured using only two of them.     

X-ray emission and absorption spectra of Cr(CO)<Subscript>6</Subscript>: A comparison of quantum-chemical and full multiple scattering calculations with experiment

A simple method of forming an energy matrix is suggested for solving the problem on the electronic states of arbitrarily large molecular systems consisting of large separate fragments that are quite stable in their characteristics. It is shown that the appropriate data on fragments can be recorded in a form suitable for storage in databanks. As a result, the determination of the eigenfunctions and energy levels corresponding to the electronic motions is logically similar to the conventional linking technique that forms a molecule from fragments.     

换热器与相变材料的兼容性研究进展

相变材料是一类以潜热实现能量存储释放的储能材料,由于其在相变温度附近具有很大的储热密度,相变材料可以被用于建筑控温、太阳能热发电和高温传热蓄热等应用中。 换热器是相变储能设备的重要组成部分,可以将热量在供需两端进行传递和转移,保障需求一方的使用,随着相变材料研究的不断深入及其工程应用的广泛普及,换热器已在众多相变储能项目中发挥了重要的枢纽作用。 为了保证换热器的使用性能,需要对换热器在相变材料中的防腐蚀性进行全面的分析。 本文总结了大量国内外的文献,分析不同成分的相变材料对换热器材料的腐蚀性,为换热器材料的选择提供了参考。     

Prediction of peptide conformation using a scale-transformed entropy sampling algorithm

An algorithm for predicting the lowest energy structure of a peptide has been developed. High-energy barriers on an energy surface can be easily overcome by logarithmically transforming the energy space. For efficient optimizations, the energy space is searched using a scale-transformed entropy sampling method, and conformations specific to a primary structure of the peptide are sampled with large weights. The efficiency of the present method is demonstrated by calculations on cholecystokinin octapeptide (CCK-8).     

蛋白质β-折叠二聚体结合能的理论研究

使用B3LYP和MP2方法研究了17个由甘氨酸多肽构成的具有反平行结构和平行结构的肛折叠二聚体．估算了在反平行结构中存在的H…H,O…O二级静电排斥作用的大小．并以LR,SR,MR为结构单元探讨了肽链长度的增加对体系的结合能的影响．结果表明在反平行的肛折叠二聚体中,由于二级静电作用的影响,随着肽链长度的增加,体系的结合能是以折线形式增强的,而在平行结构中,体系的结合能随着肽链长度的增加呈线性增强关系．     

Monte Carlo temperature basin paving with effective fragment potential: an efficient and fast method for finding low-energy structures of water clusters (H2O)20 and (H2O)25

Determining low-energy structures of large water clusters is a challenge for any optimization algorithm. In this work, we have developed a new Monte Carlo (MC)-based method, temperature basin paving (TBP), which is related to the well-known basin hopping method. In the TBP method, the Boltzmann weight factor used in MC methods is dynamically modified based on the history of the simulation. The states that are visited more are given a lower probability by increasing their temperatures and vice versa. This allows faster escapes from the states frequently visited in the simulation. We have used the TBP method to find a large number of low-energy minima of water clusters of size 20 and 25. We have found structures energetically same to the global minimum structures known for these two clusters. We have compared the efficiency of this method to the basin-hopping method and found that it can locate the minima faster. Statistical efficiency of the new method has been investigated by running a large number of trajectories. The new method can locate low-energy structures of both the clusters faster than some of the reported algorithms for water clusters and can switch between high energy and low-energy structures multiple times in a simulation illustrating its efficiency. The large number of minima obtained from the simulations is used to get both general and specific features of the minima. The distribution of minima for these two clusters based on the similarity of their oxygen frames shows that the (H(2)O)(20) can have different variety of structures, but for (H(2)O)(25), low-energy structures are mostly cagelike. Several (H(2)O)(25) structures are found with similar energy but with different cage architectures. Noncage structures of (H(2)O)(25) are also found but they are 6-7 kcal/mol higher in energy from the global minimum. The TBP method is likely to play an important role for exploring the complex energy landscape of large molecules.     

Efficient algorithm for conformational search of macrocyclic molecules

A new algorithm, complementarity, is developed for conformational search of macrocyclic molecules. The algorithm scans a large number of candidate conformations and energy-minimizes only the promising ones. These candidates can be generated by two operators that construct new conformations from known minima. The candidates have similar bonded-interaction energy as the known minima and possibly lower nonbonded interaction energy. This algorithm is 9 to 11 times faster than the existing methods when tested on two large rings, cycloheptadecane and rifamycin SV. © 1997 by John Wiley & Sons, Inc.     

Regulation of excitation transit ons by molecular design endowing full-color-tunable emissions with unexpected high quantum yields for bioimaging application

Organic fluorophores are indispensible in chemical/biological imaging. The conjugated fluorescent molecules simultaneously possessing highly tunable emission, high quantum yield in solvents of different polarities, and large Stokes shift are quite rare. Herein, we report a new category of fluorophores based on diarylated thieno[3,4-b]thiophenes efficiently synthesized by direct C-H arylation reaction. TbT-Fluors showed full-color-tunable emissions with large Stokes shifts. Intriguingly,the fluorescence quantum yields of TbT-Fluors are barely sensitive to solvent polarities, approaching 100%. Based on photophysical and theoretical investigations, we found that the enhanced oscillator strength of the S_1-S_0 transition and increased T2-S1 energy difference can sufficiently compensate the negative effect from the decreased energy gap and increased reorganization energy in dimethyl sulfoxide(DMSO). Bioimaging applications revealed that some TbT-Fluors can penetrate the cell membrane and are superior for imaging in terms of high photochemical stability and low cytotoxicity. Furthermore, TbT-PhF exhibits specific colocalization with mitochondria in living cells.     

Theoretical studies on the binding energy of β-sheet models

In this paper, B3LYP and MP2 methods are used to investigate the binding energy of seventeen antiparallel and parallel β-sheet models. The results indicate that the binding energy obtained from B3LYP calculations is weaker than that obtained from MP2 calculations but the relative binding energy yielded by B3LYP is almost the same as that by MP2. For the antiparallel β-sheets in which two N-H⋯O=C hydrogen bonds can form either a large hydrogen-bonded ring or a small hydrogen-bonded ring, the binding energy increases obviously when one large ring unit is added, whereas it only changes slightly when one small ring unit is added because of the secondary electrostatic repulsive interaction existing in the small ring unit which is estimated to be about 20 kJ/mol. For the parallel β-sheet models, the binding energy increases almost exactly linearly with the increase of the chain length.     

基本不变量神经网络解析梯度方法的研究

提升势能面的运行速度对于动力学模拟至关重要. 相对于计算简单、 但耗时更长的数值梯度计算, 直接求解势能面梯度的解析公式能够大幅提高势能面的运行效率. 本文发展了基本不变量神经网络解析梯度的生成方法. 计算解析梯度的代码可以通过程序自动生成. 对大量数据点进行测试后, 证明了该方法可以得到正确的势能面梯度输出结果. 通过测试不同势能面的调用时间, 发现采用解析梯度方法能够带来10倍以上的性能提升. 随着体系的增大, 这种性能提升也会越明显.     

High selectivity of oxime-modified ZIFs to uranium

Journal of Radioanalytical and Nuclear Chemistry - Seawater contains a large amount of uranium resources, which can meet the needs of current nuclear energy development. We designed and synthesized...     

Very Low Energy Electrons Transform the Cyclobutane‐Pyrimidine Dimer into a Highly Reactive Intermediate

Electrons with virtually no kinetic energy (close to 0 eV) trigger the decomposition of cytotoxic cyclobutane‐pyrimidine dimer (CPD) into a surprisingly large variety of fragment ions plus their neutral counterparts. The response of CPD to low energy electrons is thus comparable to that of explosives like trinitrotoluene (TNT). The dominant unimolecular reaction is the splitting into two thymine like units, which can be considered as the essential molecular step in the photolyase of CPD. We find that CPD is significantly more sensitive towards low energy electrons than its thymine building blocks. It is proposed that electron attachment at very low energy proceeds via dipole bound states, supported by the large dipole moment of the molecule (6.2 D). These states act as effective doorways to dissociative electron attachment (DEA).     

Artificial interphases enable dendrite-free Li-metal anodes

Li-metal is an ideal anode that can provide rechargeable batteries with high energy density,but its application in large scale is restricted by its high activit...     

Energy deposition in the plasma-facing components of Ignitor

The large fluxes of fusion neutrons produced in the Ignitor experiment can cause severe damage to the most exposed components of the tokamak or of the diagnostic system. In a simplified geometry model, the FLUKA and the MCNP-4B Monte Carlo codes are used to evaluate the influence of the neutron source energy and the material composition on the energy deposition in these components.