Vibrational properties of polysiloxanes: from dimer to oligomers and polymers. 1. Structural and vibrational properties of hexamethyldisiloxane (CH3)3SiOSi(CH3)3

The hexamethyldisiloxane (HMDS)(CH₃)₃SiOSi(CH₃)₃ molecule is one of the basic building blocks of silicones and polysiloxanes, as it is used for many chain terminations. Far‐infrared, mid‐infrared, and polarized Raman spectroscopic studies, combined with quantum chemical calculations and vibrational normal mode analyses, were performed for the HMDS molecule. The internal rotation of the trimethylsilyl group was calculated to be nearly free. The large‐amplitude bending motion was found very anharmonic with a barrier to linearity below 4 kJ/mol. Exhaustive assignments of observed wavenumbers have been performed on the basis of calculated potential energy distributions (PED) and atomic displacements. By isotopic ¹⁶O ¹⁸O substitution, the Si O Si symmetric and antisymmetric stretching modes shift from 521 and 1074 cm⁻¹ to 514 and 1028 cm⁻¹, respectively. This spectroscopic observation provides convincing evidence that the molecule is bent with an angle estimated at around 150°. The comparison of HMDS vibrational spectra with the vibrational spectra of some siloxane derivatives reveals strong effects of silicon substituents on the Si O Si symmetric and antisymmetric stretchings. The Si O Si siloxane bridge group plays a key role in the properties of the HMDS molecule and may also account for some important silicone polymer properties such as their very low glass transition, their high compressibility, and their low surface tension. Copyright © 2009 John Wiley & Sons, Ltd.     

Infrared spectral changes with crystallization in poly(vinylchloride): Correlations with X-ray and density data

The influence of crystallinity and stereoregularity on the infrared (IR) spectrum of atactic PVC in the solid state has been studied by many researchers [1-12]. Although the molecules in commercial PVC consist of both syndiotactic and isotactic sequences, the bulk polymer is not highly stereoregular, having approximately 50% syndiotacticity. Its infrared spectrum is different from that of highly syndiotactic PVC [3,5,7,9,10-12], particularly in the carbon-to-chlorine stretching region where there are three bands located at 610(615), 635, and 690 cm^?1. These three bands are known to be of complex origin, since each band consists of more than one absorption frequency and its relative intensity depends on the physical state or history of the specimen [3,5,7,9,10-12]. The spectrum in this region is most rigorously interpreted in terms of chain conformational structure, the spatial arrangement of the atoms around the C-C1 bond. Thus, while changes in absorbance intensities for the bands with history do not necessarily reflect changes in crystallinity, their history dependence renders these bands potentially useful as crystallinity indicators.     

Raman Scattering Tensors in Biological Molecules and Their Assemblies

INTRODUCTION

The tensor associated with a Raman band plays an important role in determining the band intensity and its structural significance. Each Raman tensor interrelates two electric vectors, that of the exciting radiation (i.e. laser photon) and that of the Raman scattered radiation (i.e. the inelastically scattered photon which results from the exchange of a vibrational quantum between the exciting photon and the molecule). The Raman tensor is obtained formally as the first derivative of the molecular polarizability tensor, the derivative being taken with respect to the vibrational normal coordinate. In other words, the Raman tensor associated with a vibrational Raman band is an indicator of how the polarizability of the molecule oscillates with the molecular normal mode of vibration.     

Investigating the mechanisms of diamond polishing using Raman spectroscopy

Recent research has shown that a phase transformation of diamond to a different form of carbon is involved when diamonds are polished in the traditional fashion. The question as to how this phase transformation is activated and maintained to produce high wear rates is of great technological interest since it may radically change the way we view the processing of diamond. This paper describes the use of Raman spectroscopy to examine debris produced on the diamond polishing wheel, both during its preparation and during polishing. In addition, polished diamond surfaces were examined for the possible existence of non-diamond surface layers in an attempt to identify material removal mechanisms. Raman spectroscopy proves ideal for these analyses because its relatively high spatial resolution is well suited to the analysis of small wear features and debris particles, and because of the wealth of information it reveals about chemical structure. This level of structural information has been lacking in previous analyses of diamond polishing debris. In addition to the non-diamond carbon found in the wear debris, significant quantities of two iron oxides, magnetite (Fe₃O₄) and haematite (α-Fe₂O₃), were also found. An interesting observation was that a transformation from magnetite to haematite could be induced either by using high power laser excitation or by frictional heating during polishing. It is suggested that some of the Raman peaks previously attributed to lonsdaleite might better be explained by the presence of these oxides.     

Benchmark calculations with correlated molecular wavefunctions. XIII. Potential energy curves for He2, Ne2 and Ar2 using correlation consistent basis sets through augmented sextuple zeta

The potential energy curves of the rare gas dimers He2, Ne2, and Ar2 have been computed using correlation consistent basis sets ranging from singly augmented aug-cc-pVDZ sets through triply augmented t-aug-cc-pV6Z sets, with the augmented sextuple basis sets being reported herein. Several methods for including electron correlation were investigated, namely Møller—Plesset perturbation theory (MP2, MP3 and MP4) and coupled cluster theory [CCSD and CCSD(T)]. For He2 CCSD(T)/d-aug-cc-pV6Z calculations yield a well depth of 7.35 cm-1 (10.58 K), with an estimated complete basis set (CBS) limit of 7.40 cm-1 (10.65 K). The latter is smaller than the 'exact' well depth (Aziz, R. A., Janzen, A. R., and Moldover, M. R., 1995, Phys. Rev. Lett., 74, 1586) by about 0.2 cm-1 (0.35 K). The Ne2 well depth, computed with the CCSD(T)/d-aug-cc-pV6Z method, is 28.31 cm-1 and the estimated CBS limit is 28.4 cm-1, approximately 1 cm-1 smaller than the empirical potential of Aziz, R. A., and Slaman, M., J., 1989, Chem. Phys., 130, 187. Inclusion of core and core—valence correlation effects has a negligible effect on the Ne2 well depth, decreasing it by only 0.04 cm-1. For Ar2, CCSD(T)/d-aug-cc-pV6Z calculations yield a well depth of 96.2 cm-1. The corresponding HFDID potential of Aziz, R. A., 1993, J. chem. Phys., 99, 4518 predicts of De of 99.7 cm-1. Inclusion of core and core-valence effects in Ar2 increases the well depth and decreases the discrepancy by approximately 1 cm-1.     

Polymerizable derivatives of long-chain fatty acids. IV. Vinyl esters

Contrary to some published reports, the vinyl esters of saturated fatty acids polymerize readily and rapidly. Vinyl oleate, when present in excess of 5%, and oxygen exert marked retarding effects. Techniques are described for the free-radical-initiated polymerization of the vinyl esters of caprylic, capric, lauric, myristic, palmitic, and stearic acids in bulk, dispersion, solution, and emulsion. Some data are given for polymerization in the presence of chain-transfer agents, such as carbon tetrachloride, dodecylmercaptan, and ethylbenzene. Conditions are reported for obtaining degrees of polymerization from about 2 (when chain-transfer agents are employed) to 10,000 (weight average). The weight average degree of polymerization increases markedly as the conversion increases, particularly above 80%. Even up to extremely high conversions, soluble polymers are obtained in most cases. Solubility characteristics, transition point data, molecular weights (osmometric and light-scattering), and isolation and purification techniques are also reported.     

Adsorption of gases in carbon nanotubes: are defect interstitial sites important?

Molecular simulations are used to shed light on an ongoing controversy over where gases adsorb on single walled carbon nanotube bundles. We have performed simulations using models of carbon nanotube bundles composed of tubes of all the same diameter (homogeneous) and tubes of different diameters (heterogeneous). Simulation data are compared with experimental data in an effort to identify the best model for describing experimental data. Adsorption isotherms, isosteric heats of adsorption, and specific surface areas have been computed for Ar, CH 4, and Xe on closed, open, and partially opened homogeneous and heterogeneous nanotube bundles. Experimental data from nanotubes prepared from two different methods, electric arc and HiPco, were examined. Experimental adsorption isotherms and isosteric heats for nanotubes prepared by the electric arc method are in best agreement with simulations for heterogeneous bundles of closed nanotubes. Models including adsorption in defect interstitial channels are required to achieve good agreement with experiments. Experimental isosteric heats and specific surface areas on HiPco nanotubes are best described by a model consisting of heterogeneous bundles with approximately 11% of the nanotubes opened.     

OPTIMAL FISH HARVESTING FOR A POPULATION MODELED BY A NONLINEAR PARABOLIC PARTIAL DIFFERENTIAL EQUATION

As the human population continues to grow, there is a need for better management of our natural resources in order for our planet to be able to produce enough to sustain us. One important resource we must consider is marine fish populations. We use the tool of optimal control to investigate harvesting strategies for maximizing yield of a fish population in a heterogeneous, finite domain. We determine whether these solutions include no‐take marine reserves as part of the optimal solution. The fishery stock is modeled using a nonlinear, parabolic partial differential equation with logistic growth, movement by diffusion and advection, and with Robin boundary conditions. The objective for the problem is to find the harvest rate that maximizes the discounted yield. Optimal harvesting strategies are found numerically.     

An ab initio and DFT study of structure and vibrational spectra of disiloxane H3SiOSiH3 conformers. Comparison to experimental data

The structural and vibrational properties of siloxane monomers may account in the physical and chemical properties of silicone polymers. Because disiloxane (H(3)SiOSiH(3)) is the smallest molecule in the set which runs through small siloxanes like hexamethyldisiloxane (CH(3))3SiOSi(CH(3))3 to silicone polymers, its energetic, structural and vibrational features have been investigated in detail using density functional theory (B3LYP), post Hartree-Fock methods (MP2 and CCSD(T)) and basis sets up to spdfg quality. Five conformations were considered: three bent structures with C2v (double staggered, SS, and double eclipsed, EE, conformations) and Cs symmetries, and two linear forms with D3d and D3h symmetries. At all levels of theory, the relative stability was C2v(SS) approximately C2v(EE)>Cs>D3h>D3d. The difference of energy between the two C2v conformers is lower than 0.04 kcal/mol. At the highest level of theory (CCSD(T)/cc-pVQZ), the barrier to linearisation from C(2v) to D(3h) conformers was calculated at 0.43 kcal/mol, which is extremely low. Most of the structural and vibrational features of the disiloxane do not depend on the conformation of the molecule but are strongly influenced by the SiOSi angle. Anharmonic calculations allowed, without any scaling factor, an exhaustive reinvestigation of the assignments of observed wavenumbers in the infrared and Raman spectra of gaseous disiloxane. Particularly, in the gas phase spectrum, the SiOSi symmetric and antisymmetric stretches have been assigned at 599 and 1105, 596 and 1060, 527 and 1093 cm(-1) for H(3)SiOSiH(3), H(3)Si(18)OSiH(3) and D(3)SiOSiD(3), respectively. The experimental wavenumber splitting of SiOSi symmetric and antisymmetric stretches of H(3)SiOSiH(3) gave an estimation of the SiOSi angle at around 145 degrees . Ab initio methods were revealed more accurate for structural parameters, when DFT/B3LYP was enough for spectral assignments, even at the harmonic level using a single scaling factor.     

连续波锁模Pr3+:YLF激光器

本文报导了采用氩离子激光器来泵浦Pr³⁺:YLF晶体,应用声光调制器实现了主动锁模;同时应用振动─高反射平面镜也实现了被动锁模,两种锁模均得到了ps光脉冲.据作者了解这是这种晶体材料的第一次锁模运转.