Fatty Acid Methyl Esters as Biosolvents of Epoxy Resins: A Physicochemical Study

The C8 to C18 fatty acid methyl esters (FAME) have been compared as solvents for two epoxy resin pre-polymers, bisphenol A diglycidyl ether (DGEBA) and triglycidyl paminophenol ether (TGPA). It was found that the solubilization limits vary according to the ester and that methyl caprylate is the best solvent of both resins. To explain these solubility performances, physical and chemical properties of FAME were studied, such as the Hansen parameters, viscosity, binary diffusion coefficient and vaporization enthalpy. Determination of the physicochemical parameters of FAME was carried out by laboratory experimentations and by calculation from bibliographic data. The Hansen parameters of FAME and epoxy resins pre-polymers were theoretically and experimentally determined. The FAME chain length showed a long dependence on the binary diffusion parameters and kinematic viscosity, which are mass and momentum transport properties. Moreover, the vaporization enthalpy of these compounds was directly correlated with the solubilization limits

Competition between decay and dissociation of core-excited OCS studied by X-ray scattering

We show the first evidence of dissociation during resonant inelastic soft X-ray scattering. Carbon and oxygen K-shell and sulfur L-shell resonant and non-resonant X-ray emission spectra were measured using monochromatic synchrotron radiation for excitation and ionization. After sulfur, L2,3 -> {\pi}*, {\sigma}* excitation, atomic lines are observed in the emission spectra as a consequence of competition between de-excitation and dissociation. In contrast the carbon and oxygen spectra show weaker line shape variations and no atomic lines. The spectra are compared to results from ab initio calculations and the discussion of the dissociation paths is based on calculated potential energy surfaces and atomic transition energies.Comment: 12 pages, 6 pictures, 2 tables, http://link.aps.org/doi/10.1103/PhysRevA.59.428

Carbon clusters near the crossover to fullerene stability

The thermodynamic stability of structural isomers of $\mathrm{C}_{24}$, $\mathrm{C}_{26}$, $\mathrm{C}_{28}$ and $\mathrm{C}_{32}$, including fullerenes, is studied using density functional and quantum Monte Carlo methods. The energetic ordering of the different isomers depends sensitively on the treatment of electron correlation. Fixed-node diffusion quantum Monte Carlo calculations predict that a $\mathrm{C}_{24}$ isomer is the smallest stable graphitic fragment and that the smallest stable fullerenes are the $\mathrm{C}_{26}$ and $\mathrm{C}_{28}$ clusters with $\mathrm{C}_{2v}$ and $\mathrm{T}_{d}$ symmetry, respectively. These results support proposals that a $\mathrm{C}_{28}$ solid could be synthesized by cluster deposition.Comment: 4 pages, includes 4 figures. For additional graphics, online paper and related information see http://www.tcm.phy.cam.ac.uk/~prck

Correlation effects in ionic crystals: I. The cohesive energy of MgO

High-level quantum-chemical calculations, using the coupled-cluster approach and extended one-particle basis sets, have been performed for (Mg2+)n (O2-)m clusters embedded in a Madelung potential. The results of these calculations are used for setting up an incremental expansion for the correlation energy of bulk MgO. This way, 96% of the experimental cohesive energy of the MgO crystal is recovered. It is shown that only 60% of the correlation contribution to the cohesive energy is of intra-ionic origin, the remaining part being caused by van der Waals-like inter-ionic excitations.Comment: LaTeX, 20 pages, no figure

Radiative cooling of carbon cluster anions C2n+1− (n = 3–5)

Radiative cooling of carbon cluster anions C2n+1− (n = 3–5) is investigated using the cryogenic electrostatic ion storage ring DESIREE. Two different strategies are applied to infer infrared emission on slow (milliseconds to seconds) and ultraslow (seconds to minutes) timescales. Initial cooling of the ions over the millisecond timescale is probed indirectly by monitoring the decay in the yield of spontaneous neutralization by thermionic emission. The observed cooling rates are consistent with a statistical model of thermionic electron emission in competition with infrared photon emission due to vibrational de-excitation. Slower cooling over the seconds to minutes timescale associated with infrared emission from low-frequency vibrational modes is probed using time-dependent action spectroscopy. For C9− and C11−, cooling is evidenced by the time-evolution of the yield of photo-induced neutralization following resonant excitation of electronic transitions near the detachment threshold. The cross-section for resonant photo-excitation is at least two orders of magnitude greater than for direct photodetachment. In contrast, C7− lacks electronic transitions near the detachment threshold

The dissociation energies of NF(X 3Σ-) and NCl(X 3Σ-)

We have computed potential energy functions for the ground states (X 3Σ-) of NF and NCl using a series of correlation consistent basis sets ranging from double to sextuple zeta quality and including core-valence correlation effects in conjunction with coupled-cluster single and double excitations with perturbative treatment of triple excitations [CCSD(T)] and large internally contracted multireference configuration interaction (icMRCI) wave functions. The best estimates for the dissociation energies (De's) are 76.6±1.3 kcal/mol for NF and 64.6±1.3 kcal/mol for NCl, respectively. Our results suggest that previous experimental estimates for the dissociation energy of NCl are in error by as much as 15 kcal/mol. The calculated spectroscopic constants for NF and NCl are in good agreement with the measured constants. © 1997 American Institute of Physics

The electronic structure of vanadium carbide, VC

Within an energy range of 2.4 eV, we have explored 29 of the 36 states of the diatomic molecule VC that arise from the atoms in their ground state, V (4 s2 3 d3; F4) +C (2 s2 2 p2; P3). We use multireference methods with large atomic natural orbital basis sets. The ground state is of Δ2 symmetry with the first two excited states, Δ4 and Σ+2, located 4.2 and 7.0 kcalmol above the X state. All the states examined in this work are relatively strongly bound and show significant charge transfer from V to C. The binding energy of the X Δ2 state is estimated to be 95.3 kcalmol in good agreement with the experimental value. © 2005 American Institute of Physics

SiH2, a critical study

The first four spectroscopic states of the silylene molecule SiH 2, namely, X̃1 A1, ã3 B1, Ã1 B1 and B̃1A 1 were examined theoretically using multireference methods coupled with very large correlation consistent basis sets. Our aim is understanding why SiH2 has a singlet ground state (X̃1A1) as opposed to the X̃3B1 state of the isovalent carbene CH2, as well as the rationalization of its geometric and bonding characteristics. The interpretational philosophy followed is based on strictly calculable quantities in an effort to reduce to a minimum the always present but not well-defined 'chemical intuitionism'. All of our calculated quantities are in excellent agreement with existing experimental results

Ab initio study of the electronic structure of manganese carbide

We report electronic structure calculations on 13 states of the experimentally unknown manganese carbide (MnC) using standard multireference configuration interaction (MRCI) methods coupled with high quality basis sets. For all states considered we have constructed full potential energy curves and calculated zero point energies. The X state, correlating to ground state atoms, is of -4 symmetry featuring three bonds, with a recommended dissociation energy of D0 =70.0 kcalmol and re =1.640 Å. The first and second excited states, which also correlate to ground state atoms, are of -6 and -8 symmetry, respectively, and lie 17.7 and 28.2 kcalmol above the X state at the MRCI level of theory. © 2006 American Institute of Physics

On symmetry breaking in BNB: Real or artifactual?

The X̃∑+u state of the linear BNB molecule was examined using several ab inito methods methods based on multireference wave functions. The methods included higher-order correlation effects without affecting the equivalency of the boron atoms. The four lowest-lying states of BN, all of which can interact strongly along the BN-B coordinate, were included in the state averaging procedure. The state-averaged multireference-based correlated method (SA-CASSCF+PT2) revealed no symmetry breaking along the antisymmetric stretching mode of the molecule