Construction of vibrational basis spectra of multicomponent systems 1. Aerosil

Aerosil (pyrogenous dispersed silica) is considered as a three-component system consisting of a silica core, hydroxyl shell, and adsorbed water. Factoring was carried out for its amplitude-weighted vibrational state density (AWSD) spectra obtained using inelastic neutron scattering at 10 K into the spectra of the components comprising this system. The factoring method used is based on the postulate of lack of correlation of the basis spectra. The factoring was carried out by the construction of physically significant basis spectra satisfying the zero linear correlation coefficient criterion for a set of AWSD spectra of the starting and modified samples of Aerosil A380.Russian International Friendship University and Joint Institute for Nuclear Research, Dubna. Translated from Zhurnal Strukturnoi Khimii, Vol. 34, No. 1, pp. 54–63, January–February, 1993.     

Construction of vibrational basis spectra of multicomponent systems 2. Silica gel

Silica gel is considered a three-component system consisting of a silica core, hydroxyl shell, and pore-retained water. Inelastic neutron scattering was used to obtain a set of amplitude-weighted vibrational state density (AWSD) spectra of two silica gel families SG20 and SG100 with mean pore diameter of 20 and 100 Å, respectively. The AWSD spectra were factored into the components of this system. The factoring method used is based on the postulate of lack of correlation of the basis spectra. The factoring was carried out by the construction of physically significant basis spectra satisfying the zero linear correlation coefficient criterion.Russian International Friendship University and Joint Institute for Nuclear Research, Dubna. Translated from Zhurnal Strukturnoi Khimii, Vol. 34, No. 1, pp. 64–76, January–February, 1993.     

Comparative analysis of vibration spectra of dispersive silicas and their components

Comparative analysis has been carried out on a set of partial basis spectra of the doubly amplitude-weighted density of vibration states (AWDS) of four dispersive silica: aerosil A380, silica gels SG20 and SG100 (with pore sizes 20 and 100 Å), and aerogel AG200 with mass density 200 kg/m³. The set of basis spectra includes the vibration spectra of frameworks, and of the hydroxyl and aqueous components of the surface zone of the above compounds. Aerosils, silica gels, and aerogels have been found to be radically different structural forms of amorphous silica, both from the viewpoint of their frameworks and their surface zones. The basis spectra considered represent a collection of fingerprints of the structure-dynamic properties of the compounds studied, which allows us to formulate a qualitatively new approach to computational modeling of dispersive silicas.Russian University of People's Friendship, Institute of Surface Chemistry, Ukrainian Academy of Sciences, Unified Institute of Nuclear Research. Translated fromZhurnal Strukturnoi Khimii, Vol. 34, No. 4, pp. 39–51, July–August, 1993.Translated by L. Smolina     

Computational modeling of amorphous silica. 1. Modeling the starting structures. A general conception

This paper presents concepts underlying computational modeling of dispersive silicas. An object for modeling is polyatomic nanometer-sized clusters. A chemical software package adapted for the PC AT 386(486)/387 is utilized. A structure-sensitive method of vibration spectroscopy is suggested to verify the results. Ultradispersive materials are considered to be an adequate object to establish feedback between computational and real experiments. The program is realized on a set of dispersive silicas; an algorithm for construction of large clusters is proposed, which is based on the concept of radical difference of dispersive silicas. From analysis of vibration spectra we suggest that aerosils, silica gels, and aerogels should be regarded as amorphous tecto-, cyclo-, and polysilicas, respectively, in line with Liebau's classification of crystal silicates. The category of silica is determined by its production procedure. Institute of Surface Chemistry, Ukrainian Academy of Sciences. Russian University of Peoples' Friendship. Translated fromZhurnal Strukturnoi Khimii, Vol. 35, No. 2, pp. 74–84, March–April, 1994. Translated by O. Kharlamova     

NANOPACK: Parallel codes for semiempirical quantum chemical calculations of large systems in the sp‐ and spd‐basis

A parallel implementation of the conventionally used NDDO (MNDO, AM1, PM3, CLUSTER‐Z1) and modified NDDO‐WF (CLUSTER‐Z2) techniques for semiempirical quantum chemical calculations of large molecular systems in the sp‐ and spd‐basis, respectively, is described. The atom‐pair distribution of data over processors forms the basis of the parallelization. The technological aspects of designing scalable parallel calculations on supercomputers (using ScaLAPACK and MPI libraries) are discussed. The scaling of individual algorithms and the entire package was carried out for model systems with 894, 1920, and 2014 atomic orbitals. The package speed‐up provided by different multiprocessor systems involving a cluster of Intel PIII processors, Alpha‐21264‐processor‐built machine MBC‐1000M, and Cray‐T3E is analyzed. The effect of computer characteristics on the package performance is discussed. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002     

From Molecules to Particles: Quantum-chemical View Applied to Fumed Silica

An extended quantum-chemical study has been performed to examine the flame-generated silica formation at the atomic level. Starting from a set of free molecules, condensation was shown to be a non-barrier and energetically favorable. The coalescence of the formed bare-surface protoparticles can be prevented by particle surface passivation in the course of the hydroxylation reaction. The protoparticle size is determined by a balance of the fusion and hydroxylation/dehydroxylation processes. The main factors responsible for the inherent amorphicity of the fumed silica have been determined.     

Shpolski effect in optical spectra of frozen solutions of the organic C60 fullerene derivative in toluene

Physics of the Solid State - The optical spectra of a solution of the C60 ethyl acetate (C60-EA) in toluene are investigated. The absorption spectra measured at T = 80 K reveal a combination of an...     

Electronic structure and spectra of N-methylfullerenepyrrolidine

A quantum-chemical analysis of the donor-acceptor properties of a C₆₀ + dimethylenemethylamine binary system is performed. It is shown that the term of the intermolecular interaction is two-well, because of which the optimization of the system structure at the initial distances between the molecules of 0.21 nm or smaller leads to the formation of N-methylfullerenepyrrolidine C₆₀. At initial distances exceeding 0.25 nm, a weakly bound charge transfer complex with almost complete retention of the properties of the individual molecules is formed. The low-temperature emission and absorption vibronic spectra of N-methylfullerenepyrrolidine C₆₀ in the crystalline toluene matrix are studied experimentally. It is found that the absorption spectra of the fulleroid and pure fullerene in the region corresponding to the excitation of the lowest electronic states are generally similar. The difference observed between the low-frequency spectra agrees completely with the quantum-chemical analysis performed.     

Fullerene nanoclusters as enhancers in linear spectroscopy and nonlinear optics

The enhancement of nonlinear optical phenomena in fullerene solutions is treated in terms of the model of amplification of local electric field due to resonance excitation of clusters, which are extended chargetransfer complexes. Excitation light is transformed in the clusters into local charge-transfer excitons, inducing polarization of the medium sufficient for amplification of the local electric field of incident and exiting light. The intensity of the observed linear optical phenomena has been directly correlated with the magnitude of the nonlinear characteristics of the medium on one hand and the efficiency of clustering on the other hand. The gain coefficients have been determined. Experimental and computational testing procedures that make it possible to determine suitability of the fullerene-doped matrix for nonlinear-optics applications are proposed.     

Study of nonbonding orbitals of phosphoryl oxygen by probing with ions with open d-shells

To evaluate the complex-forming properties of the oxygen in the phosphoryl group in phosphine oxide derivatives as a function of the hydrocarbon substituent, it is proposed to study the absorption spectra in the visible region of colored complexes of phosphine oxide derivatives with metal salts possessing unfilled d- or f-shells. When cobalt (II) chloride was used for this purpose, it was found that the hyperchromic effect-the increase in band intensity, which corresponds to ⁴A₂ ⁴T₁ (P) transition-may be used for a comparative characteristic of the covalent bond of the bond between metal and phosphoryl oxygen. The cobalt ion serves in this case as a probe in the study of reactivity and character of the nonbonding orbitals of the phosphoryl oxygen.