Ab initio investigation of benzene clusters: molecular tailoring approach

An exhaustive study on the clusters of benzene (Bz)(n), n = 2-8, at MP2/6-31++G(??) level of theory is reported. The relative strengths of CH-π and π-π interactions in these aggregates are examined, which eventually govern the pattern of cluster formation. A linear scaling method, viz., molecular tailoring approach (MTA), is efficiently employed for studying the energetics and growth patterns of benzene clusters consisting up to eight benzene (Bz) units. Accuracy of MTA-based calculations is appraised by performing the corresponding standard calculations wherever possible, i.e., up to tetramers. For benzene tetramers, the error introduced in energy is of the order of 0.1 mH (～0.06 kcal/mol). Although for higher clusters the error may build up, further corrections based on many-body interaction energy analysis substantially reduce the error in the MTA-estimate. This is demonstrated for a prototypical case of benzene hexamer. A systematic way of building up a cluster of n monomers (n-mer) which employs molecular electrostatic potential of an (n-1)-mer is illustrated. The trends obtained using MTA method are essentially identical to those of the standard methods in terms of structure and energy. In summary, this study clearly brings out the possibility of effecting such large calculations, which are not possible conventionally, by the use of MTA without a significant loss of accuracy.     

Dielectric and Thermodynamic Studies on the Hydrogen Bonded Binary System of Isopropyl Alcohol and Aniline

The molecular interactions between the polar systems isopropyl alcohol (IPA) and aniline for various mole fractions at different temperatures were studied by determining the dielectric permittivity using an HP-LF impedance analyzer at radio frequencies, the plunger method in the microwave frequency range and Abbe’s refractometer in optical region. Kirkwood effective correlation factors, corrective Kirkwood correlation factors, excess permittivities, Bruggeman parameters, excess Helmholtz free energy, relaxation time, dipole moment and excessive dipole moment were calculated using the experimental data. Optimized geometries were calculated using Spartan Modeling software for both pure and equimolar systems of isopropyl alcohol and aniline for Hamiltonian quantum mechanical calculations. Conformational analysis of the formation of hydrogen bond between the two systems is supported by the FT-IR spectra.     

Steric factors override thermodynamic driving force in regioselectivity of proline hydroxylation by prolyl-4-hydroxylase enzymes

Prolyl-4-hydroxylase is an important nonheme iron-containing dioxygenase in humans involved in the regioselective hydroxylation of a proline residue in a peptide chain on the C(4) position. In biosystems this process is important to create collagen cross-linking and cellular responses to hypoxia. We have performed a series of density functional theory (DFT) studies into the origin of the regioselectivity of proline hydroxylation by P4H enzymes using a minimal active site model (where substrate is unhindered in the binding site) and a larger active site model that incorporates steric hindrance of the substrate by several secondary sphere aromatic residues. Our studies show that thermodynamically the most favorable hydrogen atom abstraction position of proline is from the C(5) position; hence, the small model gives a low reaction barrier and large exothermicity for this process. However, stereochemical repulsions of the substrate with aromatic residues of Tyr(140) and Trp(243) in the second coordination sphere prevent C(5) hydroxylation and make C(4) hydroxylation the dominant mechanism, despite a lesser driving force for the reaction. These studies explain the remarkable regioselectivity of proline hydroxylation by P4H enzymes and show that the regioselectivity is kinetically controlled but not thermodynamically. In addition, we calculated spectroscopic parameters and found good agreement with experimental data.     

Glass physics: from fundamentals to applications

The European Physical Journal E -     

Surrogate-based modeling and dimension reduction techniques for multi-scale mechanics problems

Successful modeling and/or design of engineering systems often requires one to address the impact of multiple "design variables" on the prescribed outcome.There are often multiple,competing objectives based on which we assess the outcome of optimization.Since accurate,high fidelity models are typically time consuming and computationally expensive,comprehensive evaluations can be conducted only if an efficient framework is available.Furthermore,informed decisions of the model/hardware’s overall performance rely on an adequate understanding of the global,not local,sensitivity of the individual design variables on the objectives.The surrogate-based approach,which involves approximating the objectives as continuous functions of design variables from limited data,offers a rational framework to reduce the number of important input variables,i.e.,the dimension of a design or modeling space.In this paper,we review the fundamental issues that arise in surrogate-based analysis and optimization,highlighting concepts,methods,techniques,as well as modeling implications for mechanics problems.To aid the discussions of the issues involved,we summarize recent efforts in investigating cryogenic cavitating flows,active flow control based on dielectric barrier discharge concepts,and lithium(Li)-ion batteries.It is also stressed that many multi-scale mechanics problems can naturally benefit from the surrogate approach for "scale bridging."     

Electron spin resonance and optical absorption studies on copper-doped lithium hydrazinium sulphate single crystals

ESR and optical absorption studies have been carried out on Cu²⁺-doped lithium hydrazinium sulphate single crystals at 303 K. The spin-Hamiltonian parameters evaluated indicate a N₂O₂ square planar environment for Cu²⁺ ion in this lattice. The correlation ofESR and crystal structure data leads us to conclude that Cu²⁺ ion enters the lattice interstitially. Charge compensation is achieved by the release of protons. Using the optical absorption andESR data, bonding parameters and orbital reduction factors are also evaluated.     

Electron paramagnetic resonance studies of photorefractive crystals II: Fe3+ doped Bi12SiO20 with copper vapour laser illumination in 10–100 K range

Photo-induced charge transfer and its kinetics were investigated in Bi₁₂SiO₂₀ in 10–300 K temperature range, using EPR of Fe³⁺ centre, underin situ illumination with copper vapour laser (CVL). The decay kinetics was found to follow double exponential behaviour. Relaxation of the photo-induced electron transfer to the preillumination condition occurred even at 10 K. Shallow traps were, therefore, associated with the electron trapping, leading to a better understanding of the fast photorefractive response of BSO.     

A new type of electrohydrodynamic instability in nematic liquid crystals with positive dielectric anisotropy

A new type of electrohydrodynamic instability originally reported in nematic liquid crystal mixtures with positive dielectric anisotropy and as moderately thick samples is further studied. The ability of homogeneously aligned nematics with positive dielectric anisotropy, in the presence of a magnetic field, to exhibit Williams domains as a threshold effect is numerically investigated. The variation of the threshold voltage for domain formation and dielectric alignment with dielectric anisotropy is calculated theoretically and compared with the experimental results as moderately thick and thin samples.