A theoretical evidence for mutual influence between S···N(C) and hydrogen/lithium/halogen bonds: competition and interplay between π-hole and σ-hole interactions

Ab initio calculations were performed to investigate the cooperativity between the S···N(C) bond and the hydrogen/lithium/halogen bond interactions in O₂S···NCX···NCH and O₂S···CNX···CNH triads (X=H, Li, Cl, and Br). To understand the properties of the systems better, the corresponding dyads are also studied. It is evident that the lithium bond has a bigger influence on the chalcogen bond than vice versa. The results indicate that the enhanced interaction energies of the S···N(C) and X···N(C) interactions in the triad increase in the order NCCl < NCBr < NCH < NCLi and CNCl < CNBr < CNH < CNLi. This is the order of the increasing positive electrostatic potential V _S,max on the X atom. The nature of S···N(C) and X···N(C) interactions of the complexes is unveiled by energy decomposition analysis and natural bond orbital (NBO) theory. The cooperativity between both types of interaction is chiefly caused by the electrostatic effects.     

Molecular structure and bonding character of mono and divalent metal cations (Li+, Na+, K+, Be2+, Mg2+, and Ca2+) with substituted benzene derivatives: AIM,NBO, and NMR analyses

Cation–π complexes between several cations (Li⁺, Na⁺, K⁺, Be²⁺, Mg²⁺, and Ca²⁺) and different π-systems such as para-substituted (F, Cl, OH, SH, CH₃, and NH₂) benzene derivatives have been investigated by UB3LYP method using 6-311++G** basis set in the gas phase and the water solution. The ions have shown cation–π interaction with the aromatic motifs. Vibrational frequencies and physical properties such as dipole moment, chemical potential, and chemical hardness of these compounds have been systematically explored. The natural bond orbital analysis and the Bader’s quantum theory of atoms in molecules are also used to elucidate the interaction characteristics of the investigated complexes. The aromaticity is measured using several well-established indices of aromaticity such as NICS, HOMA, PDI, FLU, and FLUπ. The MEP is given the visual representation of the chemically active sites and comparative reactivity of atoms. Furthermore, the effects of interactions on NMR data have been used to more investigation of the studied compounds.     

Mild and convenient one pot synthesis of Schiff bases in the presence of P2O5/Al2O3 as new catalyst under solvent-free conditions

In this study P₂O₅/Al₂O₃ is found to catalyze the preparation of Schiff bases from the reaction of carbonyl compounds with primary amines efficiently under solvent-free conditions. The advantages of this friendly, environmentally and mild method are such as simplicity of the reaction procedure, the elimination of solvents, simple work-up, high product yields and short reaction times.     

A simple graphical approach to predict local residue conformation using NMR chemical shifts and density functional theory

The dependency of amino acid chemical shifts on φ and ψ torsion angle is, independently, studied using a five‐residue fragment of ubiquitin and ONIOM(DFT:HF) approach. The variation of absolute deviation of ¹³C^α chemical shifts relative to φ dihedral angle is specifically dependent on secondary structure of protein not on amino acid type and fragment sequence. This dependency is observed neither on any of ¹³C^β, and ¹H^α chemical shifts nor on the variation of absolute deviation of ¹³C^α chemical shifts relative to ψ dihedral angle. The ¹³C^α absolute deviation chemical shifts (ADCC) plots are found as a suitable and simple tool to predict secondary structure of protein with no requirement of highly accurate calculations, priori knowledge of protein structure and structural refinement. Comparison of Full‐DFT and ONIOM(DFT:HF) approaches illustrates that the trend of ¹³C^α ADCC plots are independent of computational method but not of basis set valence shell type. © 2016 Wiley Periodicals, Inc.     

Application of a Bayesian algorithm for the Statistical Energy model updating of a railway coach

The classical statistical energy analysis (SEA) theory is a common approach for vibroacoustic analysis of coupled complex structures, being efficient to predict high-frequency noise and vibration of engineering systems. There are however some limitations in applying the conventional SEA. The presence of possible strong coupling between subsystems and the lack of diffuseness result in a significant uncertainty. This is the main motivation for the present study, where a procedure to update SEA models is proposed. The proposed procedure is the combination of the classical SEA method and a Bayesian technique. Due to reasons such as finding a limited number of important parameters, using a limited search range, avoiding matrix inversion and taking the effect of noise into account, the proposed strategy can be considered as a proper alternative to the experimental SEA approach. To investigate the performance of the proposed strategy, the SEA model updating of a railway passenger coach is carried out. First, a sensitivity analysis is carried out to select the most sensitive parameters of the SEA model. For the selected parameters of the model, prior probability density functions are then taken into account based on published data on comparison between experimental and theoretical results, so that the variance of the theory is estimated. The Monte Carlo Metropolis Hastings algorithm is employed to estimate the modified values of the parameters. It is shown that the algorithm can be efficiently used to update the SEA models with a high number of unknown parameters.     

Retention time prediction of polycyclic aromatic hydrocarbons in gas chromatography–mass spectrometry using QSPR based on random forests and artificial neural network

Structural Chemistry - In this study, a quantitative structure–property relationship (QSPR) was proposed using the random forests (RF) and artificial neural network (ANN) for determining the...     

Modified Lyapunov exponent,new measure of dynamics

Lyapunov exponents, defined as exponential divergent or convergent rate of initially infinitely close solution trajectories, have been widely used for diagnosing chaotic systems, as well for stability analysis of nonlinear systems. Although calculated from the evolution of disturbance vectors associated with the flow, Lyapunov exponents are not associated with any specific directions, and such evolutions are driven by the dynamics in all directions in the state space. It is desirable to explore the asymptotic behaviors of the dynamic systems along certain specific directions and the specific dynamics driving such behaviors. In this paper, the Lyapunov exponents are modified. The modified Lyapunov exponents can indicate the exponential divergent or convergent rates in certain directions, which are driven by the dynamics in the same directions. The existence and the invariance to the initial conditions of the proposed modified exponents are proven mathematically. The algorithm for calculating the modified Lyapunov exponents from mathematical models is also developed. A wide range of case studies, from classical nonlinear dynamic systems to engineering systems, are presented to demonstrate the proposed modified Lyapunov exponents, and the indications of the modified exponents are also discussed. The proposed modified Lyapunov exponents can reveal additional insights into the system dynamics to the conventional Lyapunov exponents. Such information can be instrumental for stability control design.     

Two new special flavones from Uvaria accuminata

From the stem bark of Uvaria accuminata two new compounds, 5-methoxy-6,8,8-trimethyl-2-phenylchromene-4,7-dione (1) and a sesquiterpene-fused flavone, named acuminavone (2), were isolated together with a known flavone, desmosdomutin or dasytrichone, (5-hydroxy-6,8,8-trimethyl-2-phenylchromene-4,7-dione). The structure elucidation of compounds 1 and 2 in spectroscopic studies is described.     

Theoretical study on the atmospheric formation of sulfur trioxide as the primary agent for acid rain

The reaction mechanism of SO₂ with O₃ on the singlet potential energy surface has been investigated theoretically at the G3MP2B3//B3LYP/6-311+G(3df) level of theory. The reactants are initially associated with adducts IN1(O₂S–OOO) and IN2(OS-cyclic O₄) in a barrier-less process. Subsequently, these adducts undergo isomerization and dissociation processes to produce cis-OSOO + ³O₂, SO₃(C _s ) + ³O₂ and SO₃(D _3h ) + ³O₂ products. The SO₃(D _3h ) + ³O₂ is major product and the cis-OSOO + ³O₂ and SO₃(C _s ) + ³O₂ are minor products. No stable pathway has been found for the formation of trans-OSOO and cyclic-SOOO isomers in the reaction of SO₂ + O₃. For major product, the rate constant of SO₂ + O₃ reaction is 2.30 × 10⁻²³ cm³ molecule⁻¹ s⁻¹, at room temperature and atmospheric pressure.