Optimization of parameters for semiempirical methods VI: more modifications to the NDDO approximations and re-optimization of parameters

Modern semiempirical methods are of sufficient accuracy when used in the modeling of molecules of the same type as used as reference data in the parameterization. Outside that subset, however, there is an abundance of evidence that these methods are of very limited utility. In an attempt to expand the range of applicability, a new method called PM7 has been developed. PM7 was parameterized using experimental and high-level ab initio reference data, augmented by a new type of reference data intended to better define the structure of parameter space. The resulting method was tested by modeling crystal structures and heats of formation of solids. Two changes were made to the set of approximations: a modification was made to improve the description of noncovalent interactions, and two minor errors in the NDDO formalism were rectified. Average unsigned errors (AUEs) in geometry and ΔH(f) for PM7 were reduced relative to PM6; for simple gas-phase organic systems, the AUE in bond lengths decreased by about 5 % and the AUE in ΔH(f) decreased by about 10 %; for organic solids, the AUE in ΔH(f) dropped by 60 % and the reduction was 33.3 % for geometries. A two-step process (PM7-TS) for calculating the heights of activation barriers has been developed. Using PM7-TS, the AUE in the barrier heights for simple organic reactions was decreased from values of 12.6 kcal/mol(-1) in PM6 and 10.8 kcal/mol(-1) in PM7 to 3.8 kcal/mol(-1). The origins of the errors in NDDO methods have been examined, and were found to be attributable to inadequate and inaccurate reference data. This conclusion provides insight into how these methods can be improved. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1007/s00894-012-1667-x) contains supplementary material, which is available to authorized users

Theoretical Study on Highly Active Bifunctional Metalloporphyrin Catalysts for the Coupling Reaction of Epoxides with Carbon Dioxide

Highly active bifunctional metalloporphyrin catalysts were developed for the coupling reaction of epoxides with CO2 to produce cyclic carbonates. The bifunctional catalysts have both quaternary ammonium halide groups and a metal center. To elucidate the roles of these catalytic groups, DFT calculations were performed. Control reactions using tetrabutylammonium halide as a catalyst were also investigated for comparison. In the present article, the results of our computational studies are overviewed. The computational results are consistent with the experimental data and are useful for elucidating the structure-activity relationship. The key features responsible for the high catalytic activity of the bifunctional catalysts are as follows: 1) the cooperative action of the halide anion (nucleophile) and the metal center (Lewis acid); 2) the near-attack conformation, leading to the efficient opening of the epoxide ring in the rate-determining step; and 3) the conformational change of the quaternary ammonium cation to stabilize various anionic species generated during catalysis, in addition to the robustness (thermostability) of the catalysts

Integrated photonics modular arithmetic processor

Integrated photonics computing has emerged as a promising approach to overcome the limitations of electronic processors in the post-Moore era, capitalizing on the superiority of photonic systems. However, present integrated photonics computing systems face challenges in achieving high-precision calculations, consequently limiting their potential applications, and their heavy reliance on analog-to-digital (AD) and digital-to-analog (DA) conversion interfaces undermines their performance. Here we propose an innovative photonic computing architecture featuring scalable calculation precision and a novel photonic conversion interface. By leveraging Residue Number System (RNS) theory, the high-precision calculation is decomposed into multiple low-precision modular arithmetic operations executed through optical phase manipulation. Those operations directly interact with the digital system via our proposed optical digital-to-phase converter (ODPC) and phase-to-digital converter (OPDC). Through experimental demonstrations, we showcase a calculation precision of 9 bits and verify the feasibility of the ODPC/OPDC photonic interface. This approach paves the path towards liberating photonic computing from the constraints imposed by limited precision and AD/DA converters.Comment: 23 pages, 9 figure

Understanding the Effect of Cation and Solvation on the Structure and Reactivity of Nitrile Anions

This Ph.D. dissertation is focused on the investigation the structure of nitrile anion containing molecules and how the structure and reactivity of those molecules are affected by solvation and counter ion. A systematic approach was employed in this investigation, beginning with an evaluation of the accuracy of three commonly used model chemistries (Hartree-Fock (HF), Second-order Møller-Plesset perturbation theory (MP2), the Becke three-parameter exchange functional coupled with the nonlocal correlation functional of Lee, Yang, and Parr (B3LYP), all paired with the 6-31+G(d) basis set). A series of complexes of various cations with a number of explicit molecules of tetrahydrofuran (THF) and dimethyl ether (DME) were studied with these model chemistries and the results were compared, where possible, with experimental results. From this work, it was determined that the B3LYP models gave the most accurate results for the complexes in question. This work was then extended to acetonitrile anion containing complexes of solvent and cation. Based on the results of that extension, it was determined that cation size and charge density on the cation were critical factors in determining the structure of the acetonitrile anion molecule and in determining if the anion was metalated at the nitrogen or α-carbon position, with larger cations favoring carbon metalation and more significant deformation of the α-carbon from the expected sp2 hybridization. The final aspect of this dissertation was the determination of reaction coordinate energy profiles for a pair of substitution reactions involving nitrile anion containing cycloaliphatic molecules. The results of this study showed that, due to steric and kinetic factors, the axial products and transitions states associated with these reactions were favored, and that the degree of preference was kinetically controlled

Development of Li0.33La0.56TiO3 based Solid Electrolyte Materials

As the increasing need for electronic products like smartphones, lithium-ion batteries have been a vital topic in this era. For developing batteries with higher electrochemical performance and safety, solid electrolytes play a significant role in increasing safety owing to less risk of electrolytes leakage and a wider electrochemical window for higher energy density. Among various materials, inorganic ceramic solid electrolytes lead the intention because of their high electrochemical performance, such as ionic conductivity. However, compared to traditional liquid electrolytes, the ionic conductivity of ceramic solid electrolytes is still below current requirements. To optimize ionic conductivity, strategies of decreasing grain and grain boundary resistance are required. In this thesis, three different strategies for optimizing Li0.33La0.56TiO3 materials’ ionic conductivity were developed. Lithium lanthanum titanate (LLTO) powder was prepared using a modified sol-gel process with three chelating agents. After initial structural characteristics, LLTO pellets were prepared by spark plasma sintering. To investigate alternation of grain and grain boundary resistance of the LLTO, Ag dopants were introduced to LLTO, and composite pellets of LLTO and silver nanowires were also fabricated. The LLTO synthesized by acetic acid was found to have the strongest intensity, minor impurity, and the biggest crystallite size from XRD patterns and Rietveld refinement compared with LLTO synthesized by citric acid and a mix of citric acid and glucose. According to the XRD patterns, Ag doping of LLTO (Li0.33-xLa0.56AgxTiO3) was positively proved by shifting away from the original target Li0.33La0.56TiO3. The composite pellet (LLTO/AgNWs) was successfully fabricated by spark plasma sintering, and the LLTO/AgNWs pellet showed a more apparent grain boundary from the SEM image. The modified sol-gel method has been proved that it is an efficient way to synthesize LLTO with low reaction temperature and short reaction time compared with the traditional physical reaction method. The pure LLTO pellet was fabricated with a 10(-4) S/cm grain conductivity via spark plasma sintering (SPS). Due to successful chemical composition alteration, the Ag-doped LLTO pellet reached higher grain conductivity by 2 *10(-5) S/cm than the pure LLTO pellet. The composite LLTO/AgNWs pellet was also efficiently fabricated through SPS

Dynamical Instabilities and High Temperature Phase Stability in Ionic Crystals.

A large class of high-temperature phases become dynamically unstable at low temperatures and transform to lower symmetry phases upon cooling. In this thesis we seek to understand the energetics and vibrational thermodynamic properties associated with these transformation mechanisms in a variety of technologically important materials, including a newly discovered battery solid electrolyte, oxide phases in nuclear rod cladding, and thermal barrier coatings. Using first-principles phonon calculations, we examine the dynamical stability and vibrational properties of Li3OCl, a solid electrolyte material. We show that it is dynamically unstable with respect to octahedral rotations. Further examination of the anharmonic energy landscapes resulting from these rotations revealed that while rotations can lead to lower symmetry structures, the energy gained by these rotations are small. At low temperatures, the cubic form should persist due to anharmonic vibrational excitations. We also find that Li3OCl is entropically stabilized with respect to LiCl and Li2O at temperatures above 480 K. Zirconium alloys used in nuclear fuel rod cladding experience corrosive and oxidizing environments. Understanding the phase stability of these oxide phases at high temperatures is crucial to designing corrosion-resistant materials. Vibrational free energies for several Zr-O compounds were calculated and incorporated into a previously calculated temperature composition phase diagram [1] to identify the temperature stability limit of the recently identified delta-prime-ZrO phase. We show that this phase is stable well beyond typical nuclear reactor temperatures. Instabilities observed in cubic, tetragonal, and monoclinic ZrO2 are also studied. The cubic instability leads to a transformation into the tetragonal phase. A volume-induced instability in the tetragonal phase results in a transformation into a new orthorhombic phase. This instability has implications for the finite temperature stability of tetragonal ZrO2 and the role of anharmonicity in high-temperature materials. Strain is shown to affect stabilities of the three tetragonal variants, as well as the relative stabilities of the tetragonal and monoclinic phases. These results suggest that strain can stabilize the high-temperature tetragonal phase, which is preferable for epitaxial thin films used in high-k dielectrics and for ferroelastic toughening in thermal barrier coatings.PhDMaterials Science and EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/116700/1/mhichen_1.pd