Ab initio modelling of calcium phosphate clusters and their vibrational spectra

Calcium phosphate and hydroxylcalcium phosphate clusters that model amorphous phase elementary unit cells as well as their vibrational spectra were calculated by ab initio quantum chemical method using GAMESS code. Normal coordinate analysis was accomplished for phosphate anion, tricalcium phosphate and hydroxylapatite. Calculated IR-spectra and spectra of inelastic neutron scattering were defined in comparison with experimental data. It was shown that within the suggested approach, vibrational spectroscopy appears to be a reliable method of verifying the quantum chemical calculated structure versus the experimental data.Кластери кальцiй-фосфату та гiдроксикальцiй-фосфату, якi моделюють елементарнi одиницi аморфної фази, а також їх вiбрацiйнi спектри були розрахованi за допомогою ab initio квантово-хiмiчного методу з використанням програмного коду GAMESS. Було проведено нормально-координатний аналiз для фосфатного анiону, трикальцiйфосфату та гiдроксилапатиту. Розрахованi IЧ-спектри та спектри непружного розсiювання нейтронiв були спiвставленi з експериментальними даними. Показано, що у рамках запропонованого пiдходу вiбрацiйна спектроскопiя є надiйним методом для верифiкацiї структур, що знайденi методом квантово-хiмiчних розрахункiв

Nanomaterials: Reality and computational modelling

A problem of computer modelling of real nanomaterials is considered. An algorithmic approach based on nanomaterial classification and particular properties of every classification mode, including recently observed technological polymorphism, is suggested. The concept is put in practice using the DYQUAMOD program system, based on semi-empirical quantum-chemical methods, which allows the study of structure and vibrational spectra of nano-sized objects. The computing facilities are demonstrated by a few examples concerning some technologically important materials. © 1995

Quantum technology applied to alpha-C:H film modeling aspects

The application of quantum technology on alpha-C:H film modeling was discussed. The possibility of atomic-level manipulation on solid surfaces in the field of the tip of a scanning tunnelling microscope (STM) as a formation of nanosize protuberances on the Ge (111) surface was demonstrated. The existence of the local chemical reaction on the alpha-C:H film surface, which was caused by a local character of interatomic interaction, is a consequence of a strongly localized distribution of the electron density over surface atoms of the sp3-component. The results show more feature which is important in the context of further applications

Quantum technology applied to alpha-C:H film modeling aspects

The application of quantum technology on alpha-C:H film modeling was discussed. The possibility of atomic-level manipulation on solid surfaces in the field of the tip of a scanning tunnelling microscope (STM) as a formation of nanosize protuberances on the Ge (111) surface was demonstrated. The existence of the local chemical reaction on the alpha-C:H film surface, which was caused by a local character of interatomic interaction, is a consequence of a strongly localized distribution of the electron density over surface atoms of the sp3-component. The results show more feature which is important in the context of further applications

VIBRATIONS OF DISPERSE SILICAS - A COMPARATIVE-STUDY

Four families of disperse amorphous silica representing different solid species modifications have been studied by inelastic neutron scattering at 10, 80 and 290K. Doubly amplitude-weighted density of vibrational states (AWDS) have been obtained from the experimental time-of-flight spectra

VIBRATIONS OF DISPERSE SILICAS - A COMPARATIVE-STUDY

Four families of disperse amorphous silica representing different solid species modifications have been studied by inelastic neutron scattering at 10, 80 and 290K. Doubly amplitude-weighted density of vibrational states (AWDS) have been obtained from the experimental time-of-flight spectra

Deformation of poly(dimethylsiloxane) oligomers under uniaxial tension: Quantum chemical view

The response of a silicone polymer fragment to external stresses is considered in terms of a mechanochemical reaction. The quantum chemical realization of the approach is based on a coordinate-of-reaction concept for the purpose of introducing a mechanochemical internal coordinate (MIC) that specifies a deformational mode. The related force of response is calculated as the energy gradient along the MIC, while the atomic configuration is optimized over all of the other coordinates under the MIC constant-pitch elongation. The approach is applied to a set of linear silicone oligomers Sin with n = 4, 5, and 10 subjected to uniaxial tension, followed by the molecule breaking and a postfracture relaxation. Three stages of deformation, differing by structural transformation, have been detected. The observed peculiarities of the oligomer mechanical behavior are well attributed to the characteristic modes of vibrational spectra. The oligomer strength and the related Young's moduli are obtained. A cooperative radical-driven mechanism of silicone polymer fracture is suggested. © 1999 American Chemical Society

Fumed Silica Particles Polydimethylsiloxane Oligomers Interface. 2. Quantum Chemical Modelling of Particle Surface. Tetragonal Packing

Two model configurations of a silica supercluster have been considered using two identical quantum-chemical techniques, AM1 and PM3. Differences in the technique parameters allowed to examine the impact of the straightened (AM1) and bent (PM3) configurations of the siloxane chains on the vibrational spectra of the models. Comparing the calculated spectra with the experimental ones, a bent configuration of the siloxane chains of the fumed silica particles is suggested to be a reality. Quantitative characteristics of the supercluster irregularity are presented in the internal coordinates and the reasons for the fumed silica amorphicity are discussed

Fumed silica particles/polydimethylsiloxane oligomers interface. I. Quantum-chemical modelling of particle surface. Cubic packing

The paper opens a series of publications devoted to a microscopic approach to the intermolecular interaction between fumed silica particles and polydimethylsiloxanes of different structure. The paper presents results on atomic characterization of the surface of both hydroxylated and silylated particles in a supercluster approach. A rhomb-like-four-layer supercluster containing 222 atoms is proposed as the basic unit simulating an individual surface facet of the hydroxylated silica particle with a (111)-like packing of the siloxane elements. The total-energy-minimization procedure for obtaining equilibrated cluster structures has been performed at the level of semiempirical quantum-chemical considerations by using the AM1 method. The optimized basic cluster served as a basis for a microscopic examination of the silylation process. The supercluster characteristics are presented by distribution functions related to atomic charges, bond lengths, and bond angles

Fumed Silica Particles Polydimethylsiloxane Oligomers Interface. 2. Quantum Chemical Modelling of Particle Surface. Tetragonal Packing

Two model configurations of a silica supercluster have been considered using two identical quantum-chemical techniques, AM1 and PM3. Differences in the technique parameters allowed to examine the impact of the straightened (AM1) and bent (PM3) configurations of the siloxane chains on the vibrational spectra of the models. Comparing the calculated spectra with the experimental ones, a bent configuration of the siloxane chains of the fumed silica particles is suggested to be a reality. Quantitative characteristics of the supercluster irregularity are presented in the internal coordinates and the reasons for the fumed silica amorphicity are discussed