Physical Chemistry Commons^™

Ferroelectric Hexagonal And Rhombic Monolayer Ice Phases, Wen-Hui Zhao, Jaeil Bai, Lan-Feng Yuan, Jinlong Yang, Xiao Cheng Zeng

Xiao Cheng Zeng Publications

Two new phases of water, the mid-density hexagonal monolayer ice and the high-density flat rhombic monolayer ice, are observed in our molecular dynamics simulations of monolayer water confined between two smooth hydrophobic walls. These are in addition to the two monolayer ices reported previously, namely, the low-density 4∙8² monolayer ice and the high-density puckered rhombic monolayer ice (HD-pRMI). Stabilities of the structures are confirmed by ab initio computation. Importantly, both new phases and the HD-pRMI are predicted to be ferroelectric. An in-plane external electric field can further stabilize these ferroelectric monolayer ices.

Mos2/Mx2 Heterobilayers: Bandgap Engineering Via Tensile Strain Or External Electrical Field, Ning Lu, Hongyan Guo, Lei Li, Jun Dai, Lu Wang, Wai-Ning Mei, Xiaojun Wu, Xiao Cheng Zeng

Xiao Cheng Zeng Publications

We have performed a comprehensive first-principles study of the electronic and magnetic properties of two-dimensional (2D) transition-metal dichalcogenide (TMD) heterobilayers MX₂/MoS₂ (M = Mo, Cr, W, Fe, V; X = S, Se). For M = Mo, Cr, W; X = S, Se, all heterobilayers show semiconducting characteristics with an indirect bandgap with the exception of the WSe2/MoS2 heterobilayer which retains the directbandgap character of the constituent monolayer. For M = Fe, V; X = S, Se, the MX₂/MoS₂ heterobilayers exhibit metallic characters. Particular attention of this study has been focused on engineering the bandgap …

Van Der Waals Trilayers And Superlattices: Modification Of Electronic Structures Of Mos2 By Intercalation, Ning Lu, Hongyan Guo, Lu Wang, Xiaojun Wu, Xiao Cheng Zeng

Xiao Cheng Zeng Publications

We perform a comprehensive first-principles study of the electronic properties of van der Waals (vdW) trilayers via intercalating a twodimensional (2D) monolayer (ML = BN, MoSe₂, WS₂, orWSe₂) between a MoS₂ bilayer to form various MoS₂/ML/MoS₂ sandwich trilayers. We find that the BN monolayer is themost effective sheet to decouple the interlayer vdW coupling of the MoS₂ bilayer, and the resulting sandwich trilayer can recover the electronic structures of the MoS₂ monolayer, particularly the direct-gap character. Further study of the MoS₂/BN superlattices confirms the effectiveness of …

Unraveling Crystalline Structure Of High-Pressure Phase Of Silicon Carbonate, Rulong Zhou, Bingyan Qu, Jun Dai, Xiao Cheng Zeng

Xiao Cheng Zeng Publications

Although CO₂ and SiO₂ both belong to group-IV oxides, they exhibit remarkably different bonding characteristics and phase behavior at ambient conditions. At room temperature, CO₂ is a gas, whereas SiO₂ is a covalent solid with rich polymorphs. A recent successful synthesis of the silicon-carbonate solid from the reaction between CO₂ and SiO₂ under high pressure [M. Santoro et al., Proc. Natl. Acad. Sci. U.S.A. 108, 7689 (2011)] has resolved a long-standing puzzle regarding whether a Si_xC_1−xO₂ compound between CO₂ and SiO₂ exists in nature. Nevertheless, the detailed …

Homogeneous Connectivity Of Potential Energy Network In A Solidlike State Of Water Cluster, Takuma Akimoto, Toshihiro Kaneko, Kenji Yasuoka, Xiao Cheng Zeng

Xiao Cheng Zeng Publications

A novel route to the exponential trapping-time distribution within a solidlike state in water clusters is described. We propose a simple homogeneous network (SHN) model to investigate dynamics on the potential energy networks of water clusters. In this model, it is shown that the trappingtime distribution in a solidlike state follows the exponential distribution, whereas the trapping-time distribution in local potential minima within the solidlike state is not exponential. To confirm the exponential trapping-time distribution in a solidlike state, we investigate water clusters, (H₂O)₆ and (H₂O)12, by molecular dynamics simulations. These clusters change dynamically from …

Polymorphism And Polyamorphism In Bilayer Water Confined To Slit Nanopore Under High Pressure, Jaeil Bai, Xiao Cheng Zeng

Xiao Cheng Zeng Publications

A distinctive physical property of bulk water is its rich solid-state phase behavior, which includes 15 crystalline (ice I–ice XIV) and at least 3 glassy forms ofwater, namely, low-density amorphous, highdensity amorphous, and very-high-density amorphous (VHDA). Nanoscale confinement adds a new physical variable that can result in a wealth of new quasi-2D phases of ice and amorphous ice. Previous computer simulations have revealed that when water is confined between two flat hydrophobic plates about 7–9 Å apart, numerous bilayer (BL) ices (or polymorphs) can arise [e.g., BL-hexagonal ice (BL-ice I)]. Indeed, growth of the BL-ice I through vapor deposition on …

Long-Range Ordered Carbon Clusters: A Crystalline Material With Amorphous Building Blocks, Lin Wang, Bingbing Liu, Hui Li, Wenge Yang, Yang Ding, Stanislav V. Sinogeikin, Yue Meng, Zhenxian Liu, Xiao Cheng Zeng, Wendy L. Mao

Xiao Cheng Zeng Publications

Solid-state materials can be categorized by their structures into crystalline (having periodic translation symmetry), amorphous (no periodic and orientational symmetry), and quasi-crystalline (having orientational but not periodic translation symmetry) phases. Hybridization of crystalline and amorphous structures at the atomic level has not been experimentally observed. We report the discovery of a long-range ordered material constructed from units of amorphous carbon clusters that was synthesized by compressing solvated fullerenes. Using x-ray diffraction, Raman spectroscopy, and quantum molecular dynamics simulation, we observed that, although carbon-60 cages were crushed and became amorphous, the solvent molecules remained intact, playing a crucial role in maintaining …

Experimental And Theoretical Studies In Nuclear Magnetic Resonance, John D. Persons

Department of Chemistry: Dissertations, Theses, and Student Research

Nuclear Magnetic Resonance (NMR) has proven to be an excellent tool to probe the structure of molecules by gaining insight into nuclear interactions. Used in conjunction with theoretical calculations, NMR has the potential to elucidate these nuclear interactions and how they affect structure, bonding and dynamics on a molecular scale. The first two projects presented herein study the peroxide explosive hexamethylene triperoxide diamine (HMTD). In the first project, we use the characteristics of the solid-state NMR lineshape to determine the ¹⁴N quadrupole coupling constant (C_Q). The second project uses multi-dimensional solution-state NMR and chiral shift reagents …

Experimental And Theoretical Studies In Solid-State Nuclear Magnetic Resonance, Monica N. Kinde

Department of Chemistry: Dissertations, Theses, and Student Research

Solid-state nuclear magnetic resonance (SSNMR) has proven to be a powerful tool for probing molecular structure and dynamics. Deuterium SSNMR is particularly useful due to the presence of anisotropic interactions whose motional averaging contributes structural and dynamical insight. The magnitude and type of molecular motion can be determined from analysis of solid echo deuterium lineshapes and/or relaxation studies. This work uses various applications of solid-state NMR as well as ab initio and density functional computational methods to study three different areas of physical chemistry: biophysical chemistry, materials science and fundamental concepts of physical chemistry. The first project addressed the dynamics …

Ƴ-Selective Directed Catalytic Asymmetric Hydroboration Of 1,1-Disubstituted Alkenes, Sean M Smith, Gia L. Hoang, Rhitankar Pal, Mohammad O. Bani Khaled, Liberty S. W. Pelter, Xiao Cheng Zeng, James M. Takacs

Xiao Cheng Zeng Publications

Directed catalytic asymmetric hydroborations of 1,1-disubstituted alkenes afford ƴ

-dioxaborato amides and esters in high enantiomeric purity (90–95% ee).

Self-Assembling Subnanometer Pores With Unusual Mass-Transport Properties, Xibin Zhou, Guande Liu, Kazuhiro Yamato, Yi Shen, Ruixian Cheng, Xiaoxi Wei, Wanli Bai, Yi Gao, Hui Li, Yi Liu, Futao Liu, Daniel M. Czajkowsky, Jingfang Wang, Michael J. Dabney, Zhonghou Cai, Jun Hu, Frank V. Bright, Lan He, Xiao Cheng Zeng, Zhifeng Shao, Bing Gong

Xiao Cheng Zeng Publications

A long-standing aim in molecular self-assembly is the development of synthetic nanopores capable of mimicking the mass-transport characteristics of biological channels and pores. Here we report a strategy for enforcing the nanotubular assembly of rigid macrocycles in both the solid state and solution based on the interplay of multiple hydrogen-bonding and aromatic π − π stacking interactions. The resultant nanotubes have modifiable surfaces and inner pores of a uniform diameter defined by the constituent macrocycles. The self-assembling hydrophobic nanopores can mediate not only highly selective transmembrane ion transport, unprecedented for a synthetic nanopore, but also highly efficient transmembrane water permeability. …

First-Principles Studies On Physical And Chemical Properties Of Nanostructures, Menghao Wu

Department of Physics and Astronomy: Dissertations, Theses, and Student Research

The physical and chemical properties of decorated graphene and graphene ribbons, single-layer III-V systems, three-dimensional carbon and BN foam, and transition-metal-molecular sandwich nanowires have been investigated by first-principle calculations and their potential applications have been predicted. First, it is shown that zigzag graphene nanoribbons (ZGNRs) can be converted into half metal when their edges are decorated by some chemical functional groups, and the half-metalicity is induced by chemical potential difference between two edges when one edge is decorated by electron-donating group like –OH and the other edge is decorated by electron-accepting group like –F, -NH₂, -N(CH₃) …

Quantum Mechanical And Molecular Mechanical Study Of Solvent Effects, Dejun Si

Department of Chemistry: Dissertations, Theses, and Student Research

Intermolecular interaction and solvent effects play important roles in determining physical and chemical properties of molecular systems, and must be considered in relevant quantum mechanical (QM) calculations. Due to the high computational cost, full and rigorous QM treatment of both solute and solvent molecules is impractical. Computationally efficient molecular mechanical (MM) methods can be used to describe solvent effects, and combined into QM methods to formulate QM/MM methods. Classical force field method and reaction-field method are the two most popular MM methods. However, the issue of effectively combining MM methods with high-level QM methods remains unsolved. This thesis presents several …

Ultrafast Rempi In Benzene And The Monohalobenzenes Without The Focal Volume Effect, Timothy D. Scarborough, James Strohaber, David B. Foote, Collin J. Mcacy, Cornelis J. Uiterwaal

C.J.G.J. Uiterwaal Publications

We report on the photoionization and photofragmentation of benzene (C₆H₆) and of the monohalobenzenes C₆H₅–X (X = F, Cl, Br, I) under intense-field, single-molecule conditions. We focus 50-fs, 804-nm pulses from a Ti:sapphire laser source, and record ion mass spectra as a function of intensity in the range ~10¹³ W/cm² to ~10¹⁵ W/cm². We count ions that were created in the central, most intense part of the focal area; ions from other regions are rejected. For all targets, stable parent ions (C₆H₅X+) are …

Elastic Properties Of Poly(Vinyldene Fluoride) (Pvdf) Crystals: A Density Functional Theory Study, Yong Pei, Xiao Cheng Zeng

Xiao Cheng Zeng Publications

We computed structural and elastic properties of totally nine phases of poly(vinyldene fluoride) (PVDF) crystals using the density-functional theory (DFT) method with and without inclusion of the dispersion corrections. In addition to the four known crystalline forms, mechanic properties of five theoretically predicted crystalline forms of PVDF are also investigated. The all-trans form I_p exhibits the largest cohesive energy, bulk, and Young’s modulus among the nine crystalline forms. The DFT calculations suggest that the δ crystalline forms (III_au, III_pu, III_pd, and III_ad) possess poor chain rigidity among the nine PVDF …

Structure Evolution Of Gold Cluster Anions Between The Planar And Cage Structures By Isoelectronic Substitution: AuN− (N = 13–15) And MauN− (N = 12–14; M = Ag, Cu), Rhitankar Pal, Lei-Ming Wang, Wei Huang, Lai-Sheng Wang, Xiao Cheng Zeng

Xiao Cheng Zeng Publications

The structural and electronic effects of isoelectronic substitution by Ag and Cu atoms on gold cluster anions in the size range between 13 and 15 atoms are studied using a combination of photoelectron spectroscopy and first-principles density functional calculations. The most stable structures of the doped clusters are compared with those of the undoped Au clusters in the same size range. The joint experimental and theoretical study reveals a new C_3v symmetric isomer for Au₁₃ ⁻, which is present in the experiment, but has hitherto not been recognized. The global minima of Au₁₄ ⁻ and Au …

Synthesis And Catalytic Activity Of Nanostructured Cerium Oxide, Neil J. Lawrence

Department of Chemistry: Dissertations, Theses, and Student Research

Cerium oxide (ceria, CeO_2-x where x is 0 to 0.5) has been one of the most widely used heterogeneous catalysts particularly in three way catalytic converters. Most of the catalytic traits can be attributed to two properties of ceria: first, the high mobility and storage capacity of oxygen within the lattice; second, the ease with which cerium changes between Ce³⁺ and Ce⁴⁺ states. These properties, combined with the abundance of cerium on earth, make ceria a low-cost highly effective alternative to noble metal catalysts. Recent research has been focused on the nanoscale properties of ceria.

The effect …

Characteristics And Stability Of Oxide Films On Plutonium Surfaces, Harry Guillermo García Flores

Department of Chemistry: Dissertations, Theses, and Student Research

The oxidation of plutonium (Pu) metal continues to be an area of considerable activity. The reaction characteristics have significant implications for production use, storage, and disposition of this radiological material. Developing an accurate physical model of the structures, oxidation states, and oxygen concentration gradients present during oxidation are essential to understanding this process. Traditionally, the stable oxides of Pu have been thought to be plutonium sesquioxide (Pu₂O₃, O/Pu = 1.5, Pu³⁺) and plutonium dioxide (PuO₂, O/Pu = 2.0, Pu⁴⁺), existing in a layered structure on Pu metal. Many of the …

Theoretical And Experimental Studies In Nuclear Magnetic Resonance, Adrienne M. Roehrich

Department of Chemistry: Dissertations, Theses, and Student Research

Nuclear magnetic resonance (NMR) is a tool used to probe the physical and chemical environments of specific atoms in molecules. This research explored small molecule analogues to biological materials to determine NMR parameters using ab initio computations, comparing the results with solid-state NMR measurements. Models, such as dimethyl phosphate (DMP) for oligonucleotides or CuCl for the active site of the protein azurin, represented computationally unwieldy macromolecules. ³¹P chemical shielding tensors were calculated for DMP as a function of torsion angles, as well as for the phosphate salts, ammonium dihydrogen phosphate (ADHP), diammonium hydrogen phosphate, and magnesium dihydrogen phosphate. The …

Two-Dimensional To Three-Dimensional Structural Transition Of Gold Cluster Au 10 During Soft Landing On Tio 2 Surface And Its Effect On Co Oxidation, Hui Li, Yong Pei, Xiao Cheng Zeng

Xiao Cheng Zeng Publications

We investigate the possible structural transition of a planar Au₁₀ cluster during its soft landing on a TiO₂ (110) surface with or with no oxygen defects. The collision between the gold cluster and the oxide surface is simulated using the Car–Parrinello quantum molecular dynamics method. Both high-speed and low-speed conditions typically implemented in soft-landing experiments are simulated. It is found that under a high-speed condition, the gold cluster Au₁₀ can undergo a sequence of structural transitions after colliding with a defect-free TiO₂ (110) surface. When the TiO₂ (110) surface possesses oxygen vacancies, however, chemical bonds …

Icosahedral B12-Containing Core–Shell Structures Of B80, Hui Li, Nan Shao, Bo Shang, Lan-Feng Yuan, Jinlong Yang, Xiao Cheng Zeng

Xiao Cheng Zeng Publications

Low-lying icosahedral (I_h) B₁₂-containing structures of B₈₀ are explored, and a number of core–shell isomers are found to have lower energy than the previous predicted B₈₀ fullerene. The structural transformation of boron clusters from tubular structure to core–shell structure may occur at a critical size less than B₈₀.

Graphene-Like Bilayer Hexagonal Silicon Polymorph, Jaeil Bai, Hideki Tanaka, Xiao Cheng Zeng

Xiao Cheng Zeng Publications

We present molecular dynamics simulation evidence for a freezing transition from liquid silicon to quasi-twodimensional (quasi-2D) bilayer silicon in a slit nanopore. This new quasi-2D polymorph of silicon exhibits a bilayer hexagonal structure in which the covalent coordination number of every silicon atom is four. Quantum molecular dynamics simulations show that the stand-alone bilayer silicon (without the confinement) is still stable at 400 K. Electronic band-structure calculations suggest that the bilayer hexagonal silicon is a quasi-2D semimetal, similar to a graphene monolayer, but with an indirect zero band gap.

Molecular Dynamics Studies Of Simple Model Fluids And Water Confined In Carbon Nanotube, Jun Wang

Department of Chemistry: Dissertations, Theses, and Student Research

Molecular Dynamics (MD) simulation is one of the most important computational techniques with broad applications in physics, chemistry, chemical engineering, materials design and biological science. Traditional computational chemistry refers to quantum calculations based on solving Schrodinger equations. Later developed Density Functional Theory (DFT) based on solving Kohn-Sham equations became the more popular ab initio calculation technique which could deal with ~1000 atoms by explicitly considering electron interactions. In contrast, MD simulation based on solving classical mechanics equations of motion is a totally different technique in the field of computational chemistry. Electron interactions were implicitly included in the empirical atom-based potential …

Planar-To-Tubular Structural Transition In Boron Clusters: B20 As The Embryo Of Single-Walled Boron Nanotubes, Boggavarapu Kiran, Satya S. Bulusu, Hua-Jin Zhai, Soohaeng Yoo, Xiao Cheng Zeng, Lai-Sheng Wang

Xiao Cheng Zeng Publications

Experimental and computational simulations revealed that boron clusters, which favor planar (2D) structures up to 18 atoms, prefer 3D structures beginning at 20 atoms. Using global optimization methods, we found that the B₂₀ neutral cluster has a double-ring tubular structure with a diameter of 5.2 Å. For the B₂₀^- anion, the tubular structure is shown to be isoenergetic to 2D structures, which were observed and confirmed by photoelectron spectroscopy. The 2D-to-3D structural transition observed at B₂₀, reminiscent of the ring-to-fullerene transition at C₂₀ in carbon clusters, suggests it may be considered as the embryo …

Creating Nanocavities Of Tunable Sizes: Hollow Helices, Bing Gong, Huaqiang Zeng, Jin Zhu, Lihua Yuan, Yaohua Han, Shizhi Cheng, Mako Furukawa, Ruben D. Parra, Audrey Y. Kovalevsky, Jeffrey L. Mills, Ewa Skrzypczak-Jankun, Suzana Martinovic, Richard D. Smith, Chong Zheng, Thomas Szyperski, Xiao Cheng Zeng

Xiao Cheng Zeng Publications

A general strategy for creating nanocavities with tunable sizes based on the folding of unnatural oligomers is presented. The backbones of these oligomers are rigidified by localized, threecenter intramolecular hydrogen bonds, which lead to well-defined hollow helical conformations. Changing the curvature of the oligomer backbone leads to the adjustment of the interior cavity size. Helices with interior cavities of 10 Å to >30 Å across, the largest thus far formed by the folding of unnatural foldamers, are generated. Cavities of these sizes are usually seen at the tertiary and quaternary structural levels of proteins. The ability to tune molecular dimensions …

Effects Of External Electric Field On The Interfacial Properties Of Weakly Dipolar Fluid, V. B. Warshavsky, T. V. Bykov, Xiao Cheng Zeng

Xiao Cheng Zeng Publications

In the framework of modified mean-field density-functional theory, effects of a uniform electric field on the interfacial properties of a model dipolar fluid are studied. Both density and orientational order-parameter profiles of the planar vapor-liquid interface are obtained as a function of the field strength. For the dipolar fluids with reduced dipole moment μ₀^*0^*E*/T*!1) can shift the surface tension by few percent. We also find that the electric field actually reduces the thermodynamical surface tension but enhances the mechanical surface tension at the equimolar dividing surface. To detect these field effects on …

Viscosities Of Vegetable Oils And Fatty Acids, Hossein Noureddini, B. C. Teoh, L. Davis Clements

Papers in Biomaterials

Data for viscosity as a function of temperature from 24 to 110°C (75 to 230°F) have been measured for a number of vegetable oils (crambe, rapeseed, corn, soybean, milk- coconut, lesquerella) and eight fatty acids in the from Cg to CZZ The viscosity measurements were performed according to ASTM test methods D 445 and D 446. Several correlations were fitted to the experimental data. Correlation constants for the best fit are presented. The range of temperature in which the correlations are valid is from 24°C (75°F), or the melting point of the substance, to 110°C (230°F). The correlation constants are …