Zur Analyse der Baryumgruppe

Ohne Zusammenfassung     

Relative strength of hydrogen bond interaction in alcohol–water complexes

Hydrogen binding energies are calculated for the different isomers of 1:1 complexes of methanol, ethanol and water using ab initio methods from MP2 to CCSD(T). Zero-point energy vibration and counterpoise corrections are considered and electron correlation effects are analyzed. In methanol–water and ethanol–water the most stable heterodimer is the one where the water plays the role of proton donor. In methanol–ethanol the two isomers have essentially the same energy and no favorite heterodimer could be discerned. The interplay between the relative binding energy is briefly discussed in conjunction with the incomplete mixing of alcohol–water systems.     

Gas-phase electrophilic addition promoted by CH(3)S(+)=CH(2) ions on aromatic systems

The gas-phase methylenation reaction between CH(3)S(+)=CH(2) and alkylbenzenes, aniline, phenol and alkyl phenyl ethers, which yields [M + CH](+) and CH(3)SH, has been studied by Fourier transform ion cyclotron resonance (FT-ICR) techniques and computational chemistry at the DFT level. The methylthiomethyl cation is less reactive than methoxymethyl and, unlike the latter, is unreactive toward benzene. The calculations suggest that reaction with toluene should proceed primarily by addition at the para and ortho positions resulting in a benzyl-type ion. Reaction with aniline-2,3,4,5,6-d(5) reveals that elimination of CH(3)SD is kinetically favored by a factor of 5 over elimination of CH(3)SH. Experiments with C(6)H(6)ND(2) and theoretical calculations suggest that methylenation at the nitrogen atom is energetically favorable and likely, but the observed results may reflect some H/D scrambling, which occurs after attack at a ring position. By comparison, reaction with phenol-2,3,4,5,6-d(5) reveals that methylenation followed by elimination of CH(3)SD is kinetically favored by a factor of 3.8 over elimination of CH(3)SH. For phenol, the theoretical calculations suggest that attack by CH(3)S(+)=CH(2) at the para or ortho position is the only low-energy pathway for methylenation. However, a low-energy pathway for hydrogen scrambling is predicted by the calculations originating from the exit complex, [CH(3)SH(...) CH(2)=C(6)H(4)=OH](+), of reaction at a ring position.     

Neural network model predictive control of a styrene polymerization plant: online testing using an electronic worksheet

The batch styrene polymerization process presents transient and nonlinear temperature behavior. In this work, manual control and open loop experiments were carried out in order to build a process knowledge database. Initially, a cascade feedback control loop was implemented by manipulating the thyristor unit of the electrical heater in the thermal fluid tank. Aiming at the MPC development, algebraic equations of a neural network and its adjusted parameters were implemented in an electronic worksheet. Every five seconds, the worksheet was updated with measurements (reactor temperature, thermal fluid temperature and thyristor position) by means of the OLE for the Process Control protocol (OPC). The one-step-ahead temperature prediction was then employed in the objective function of the worksheet solver which used Visual Basic Applications programming. The manipulated variable action was then calculated and sent to the process. A hybrid controller (cascade feedback and MPC) outperformed the pure strategies since the time-integral performance indexes, IAE and ITAE, were reduced by around 22 % and 32 %, respectively. Methodology for the Model Predictive Control presented in this study was considered feasible because the solver of Microsoft Office Excel (2007) is very friendly, easy to understand and ready to implement using VBA.     

Ab initio study of the isomeric equilibrium of the HCN...H2O and H2O...HCN hydrogen-bonded clusters

An ab initio study of the stability, spectroscopic properties, and isomeric equilibrium of the hydrogen-bonded HCN...H2O and H2O...HCN isomers is presented. Density functional theory and perturbative second-order MP2 and coupled-cluster CCSD(T) calculations were carried out and binding energies obtained with correlation-consistent basis sets including extrapolation to the infinity basis set level. At the best theoretical level, CCSD(T), the H2O...HCN complex is more stable than the HCN...H2O complex by ca. 6.3 kJ mol(-1). Rotational and vibrational spectra, including anharmonic corrections, are calculated. These calculated spectroscopic data are used to obtain thermochemical contributions to the thermodynamic functions and hence the Gibbs free energy. The relative free energies are used to estimate the equilibrium constant for isomerism. We find that under typical conditions of supersonic expansion experiments (T < 150 K) H2O...HCN is essentially the only isomer present. Furthermore, our calculations indicate that the hydrogen-bonded cluster becomes favorable over the separated moieties at temperatures below 200 K.     

Structure and UV-vis spectrum of C(60) fullerene in ethanol: a sequential molecular dynamics/quantum mechanics study

A molecular dynamics simulation combined with semiempirical quantum mechanics calculations has been performed to investigate the structure, dynamical, and electronic properties of pure C60 in liquid ethanol. The behavior of the fullerene alcoholic solution was obtained by using the NPT ensemble under ambient conditions, including one C60 fullerene immersed in 1000 ethanol molecules. Our analyzed center-of-mass pairwise radial distribution function indicated that, on average, there are 32, 72, 132, and 187 ethanol molecules around, respectively, the first, second, third, and fourth solvation shells of the C60 molecule. To investigate the UV-vis transition energies of C60 in the presence of ethanol, we have considered constituents of the time uncorrelated supramolecular structures of the first solvation shell, i.e., clusters of C60@{EtOH}32 types. The semiempirical calculations were performed at the intermediate neglect of differential overlap level with configuration interaction singles (INDO/CIS). Our results have pointed out that the characteristic C60 UV-vis absorbance peaks are slightly shifted to longer wavelengths, as compared to the isolated molecule. These findings are in connection with the weak donor-acceptor character of the interactions involving electron lone pairs of oxygen atoms on the solvent and the fullerene surface.     

Ab initio analysis of monomers and dimers of trialkylphosphine oxides: Structural and thermodynamic stability

Structural and thermodynamic stabilities of monomers and dimers of trialkylphosphine oxides (TRPO) were studied using quantum chemistry calculations. Density functional theory calculations were carried out and the structures of four TRPO have been determined: TMPO (methyl; R = CH₃), TEPO (ethyl; R = CH₃CH₂), TBPO (n‐butyl; R = CH₃(CH₂)₃), and TOPO (n‐octyl; R = CH₃(CH₂)₇). TRPO homodimers were investigated considering two isomeric possibilities for each dimer. Relative binding energies and the enthalpic and entropic contributions to the Gibbs free energy were calculated for all dimers. The formation of dimers from the individual monomeric TRPO species as a function of temperature was also analyzed. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

Enhanced stability of the model mini‐protein in amino acid ionic liquids and their aqueous solutions

Using molecular dynamics simulations, the structure of model mini‐protein was thoroughly characterized in the imidazolium‐based amino acid ionic liquids and their aqueous solutions. Complete substitution of water by organic cations and anions further results in hindered conformational flexibility of the mini‐protein. This observation suggests that amino acid‐based ionic liquids are able to defend proteins from thermally induced denaturation. We show by means of radial distributions that the mini‐protein is efficiently solvated by both solvents due to a good mutual miscibility. Amino acid‐based anions prevail in the first coordination sphere of positively charged sites of the mini‐protein whereas water molecules prevail in the first coordination sphere of negatively charged sites of the mini‐protein. © 2015 Wiley Periodicals, Inc.