Effective core potential ab initio calculations on main group heptoxides and large silicate systems

The ab initio molecular structures for several main group heptoxides (X2O7n-, n=0,2,4) are calculated using effective core potentials at the HF and DFT (B3LYP) levels. Particular attention is given to the X-O-X bond angle, as this structural parameter is a key feature for the study of both heptoxides and larger derivatives, such as polysilicate systems. The extent of the p-d interactions, which was found to be the main factor governing the magnitude of the X-O-X angle in transition metal heptoxides, does not play a significant role in the main group analogues.http://www.sciencedirect.com/science/article/B6TGT-40V4F3D-3/1/f26d56c6395122fcba007ed20f55868

Evidence of C−H···O Hydrogen Bonds in Liquid 4-Ethoxybenzaldehyde by NMR and Vibrational Spectroscopies

Raman, FTIR, and NMR (both 13C and 17O) spectroscopies are used in a complementary way in order to study the occurrence of C−H···O intermolecular hydrogen bonds in liquid 4-ethoxybenzaldehyde (4EtOB). Additional information concerning the structure of the possible dimers is obtained through ab initio calculations, at the B3LYP/6-31G* level. The strongest evidences of the presence of C−H···O hydrogen bonds in the liquid phase arise from the temperature and solvent intensity dependence of the two bands observed in the νCO region of the vibrational spectra, as well as from the shift to low magnetic field detected for the carbonyl 17O NMR peak at higher dilutions. Further evidence is gathered from the changes observed in the νC-H vibrational modes, the 1JCH concentration dependence detected in the NMR spectra, and ab initio results. The experimental observations are consistent with the decrease of the C−H bond length upon hydrogen-bonding, as predicted for the nonstandard blue-shifting hydrogen bonds. Ab initio calculations predict several possible structures for the dimeric species, with nearly identical energies. The calculated dimerization energy is within the −5.1 to −6.5 kJ mol-1 range, considering both basis set superposition error and zero-point vibrational energy corrections, in agreement with the obtained experimental ΔH value of −5.7 ± 0.5 kJ mol-1

A Proposal for a Modified Moller-Plesset Perturbation Theory

A modified version of the Moller-Plesset approach for obtaining the correlation energy associated to a Hartree-Fock ground state is proposed. The method is tested in a model of interacting fermions that allows for an exact solution. Using up to third order terms improved results are obtained, even more accurate in the limit of loosely bound particles. This result suggests the possible convenience of the scheme for the study of chemical bound problems.Comment: 10 pages, 1 figur

Temperature, flow and eccentricity measurements in a journal bearing with a single axial groove at 90º to the load line

Parametric experiments have been conducted to analyse the influence of some supply conditions on the performance of a steadily loaded journal bearing. Temperature distribution on the internal surface, flowrate and bearing eccentricity were measured for different sets of operating conditions, at variable supply conditions. Quantitative information is provided which shows the effect of both shaft speed and applied load on maximum bush temperature and flowrate. It has been observed that flowrate was modestly affected by load and significantly affected by rotational speed, oil supply temperature and supply pressure. Maximum bush temperature was modestly affected by supply pressure, moderately affected by load and significantly affected by shaft speed. For low applied loads, the attitude angle was markedly affected by supply pressure. The experimental results also showed that for a small groove length there is a variation of bush temperature in axial direction in the groove region.Fundação para a Ciência e a Tecnologia (FCT) - PRAXIS XXI Programme/BD/13922/9

Pair formation in two electron correlated chains

We study two correlated electrons in a nearest neighbour tight- binding chain, with both on site and nearest neighbour interaction. Both the cases of parallel and antiparallel spins are considered. In addition to the free electron band for two electrons, there are correlated bands with positive or negative energy, depending on wheather the interaction parameters are repulsive or attractive. Electrons form bound states, with amplitudes that decay exponentially with separation. Conditions for such states to be filled at low temperatures are discussed.Comment: To appear in J. Phys: Condens. Matter 15 (2003

The role of lubricant feed temperature on the performance of twin groove journal bearings : an experimental study

An experimental assessment of the influence of lubricant feed temperature (Tf) on the behaviour of twin groove hydrodynamic journal bearings has been undertaken. Several loads, shaft speeds and bearing geometries were tested under constant lubricant feed pressure (pf), while Tf took four different values between 29ºC and 58ºC. It was found that the increase of Tf has an effect in bearing performance which is analogous in many ways to the effect of the increase in eccentricity: increase in lubricant flow rate (especially in the low eccentricity range), in outlet temperature (Tout) and in maximum bush temperature (Tmax). Nevertheless, the latter increase was lower than the corresponding increase in Tf. Also, in the high eccentricity range the increase in flow rate due to the increase of Tf could be deceiving in the sense that the additional flow was supplied mainly to the inactive region of the bearing (the unloaded land of the bearing).Fundação para a Ciência e a Tecnologia (FCT

Identification of microplastics using Raman spectroscopy: latest developments and future prospects

Widespread microplastic pollution is raising growing concerns as to its detrimental effects upon living organisms. A realistic risk assessment must stand on representative data on the abundance, size distribution and chemical composition of microplastics. Raman microscopy is an indispensable tool for the analysis of very small microplastics (<20 μm). Still, its use is far from widespread, in part due to drawbacks such as long measurement time and proneness to spectral distortion induced by fluorescence. This review discusses each drawback followed by a showcase of interesting and easily available solutions that contribute to faster and better identification of microplastics using Raman spectroscopy. Among discussed topics are: enhanced signal quality with better detectors and spectrum processing; automated particle selection for faster Raman mapping; comprehensive reference libraries for successful spectral matching. A last section introduces non-conventional Raman techniques (non-linear Raman, hyperspectral imaging, standoff Raman) which permit more advanced applications such as real-time Raman detection and imaging of microplastics.publishe

Bacterial cellulose as a raw material for food and food packaging applications.

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