Investigation of the Precipitation Behavior of Asphaltenes in the Presence of Naphthenic Acids Using Light Scattering and Molecular Modeling Techniques

A delay in the onset of flocculation is observed for asphaltenes in the presence of several naphthenic acids: methyl abietate, hydrogenated methyl abietate, 5β-cholanic acid, and 5β-cholanic acid-3-one. This flocculation behavior is monitored as a function of the added precipitant (n-heptane) to solutions of suspended asphaltenes and naphthenic acids in model solutions of toluene/n-heptane, using a combination of dynamic light scattering (DLS) and near-infrared (NIR) spectroscopic techniques. DLS and NIR show very good correlation in indentifying the onsets of flocculation, which varied among the series of naphthenic acids. Specific interaction energies and equilibrium intermolecular distances of asphaltenes and naphthenic acids are calculated using molecular mechanics. The results from molecular mechanics calculations support the experimental results of the titrations, and structure–property relationships are defined. Structure–property relationships are established for naphthenic acids, defining the relative contributions and importance of various functional groups: C═C, C═O, COOR, and COOH. The additive effects of naphthenic acids, defined by an increase in the precipitation onset, increase in the order of 5β-cholanic acid-3-one \u3c hydrogenated methyl abietate \u3c methyl abietate \u3c 5β-cholanic acid, with experiments containing 5β-cholanic acid-3-one containing unexpected and interesting results

Inter-Ring Torsions in N-phenylmaleimide and its O-halo Derivatives: An Experimental and Computational Study

Structures of N-phenylmaleimide and its o-halophenyl derivatives have been determined in the solid state and show the angle between the phenyl and pyrolinyl ring planes to vary from 49.5 degrees to 83.9 degrees with increasing values for compounds with the larger ortho halophenyl substituents (H \u3c F less than or similar to Cl less than or similar to Br \u3c I). Experimental torsions and trends in the series are supported by semiempirical AMI and ab initio SCF, DFT, and MP2 calculations. Calculations (AM1) on N-phenylmaleimide modeling the torsional deformation between the rings show that the barrier to planarity has a lower energy than that through a perpendicular conformation. In its o-halo derivatives, molecular planarity is not possible, and torsional deformation proceeds through the perpendicular conformation with diminishing, possibly vanishing, barriers with increasing halogen size. For chloro, bromo, and iodo derivatives, twisted ground-state molecular conformations reside in broad minima essentially centered around the perpendicular conformations. The unusually strong, longer wavelength electronic bands observed in the solution spectra of the series were modeled by Zindo/S CIS computations at the optimum AM1 molecular geometries. The observed lower energy (285-305 nm) band for the parent through the o-bromo derivative appears to arise from a {n perpendicular to(O,N); pi (phenyl)} --\u3e pi*(maleimide) transition. The next higher energy (250-285 nm) band appears to be essentially a phenyl pi --\u3e pi* transition. In the o-iodo derivative, a phenyl pi --\u3e sigma* (C-I) transition appears to contribute to the longer wavelength band. Trends in the observed electronic spectra in acetonitrile within the series of compounds accord roughly with the results of the computations

Investigation of the Precipitation Behavior of Asphaltenes in the Presence of Naphthenic Acids Using Light Scattering and Molecular Modeling Techniques

A delay in the onset of flocculation is observed for asphaltenes in the presence of several naphthenic acids: methyl abietate, hydrogenated methyl abietate, 5β-cholanic acid, and 5β-cholanic acid-3-one. This flocculation behavior is monitored as a function of the added precipitant (<i>n</i>-heptane) to solutions of suspended asphaltenes and naphthenic acids in model solutions of toluene/<i>n</i>-heptane, using a combination of dynamic light scattering (DLS) and near-infrared (NIR) spectroscopic techniques. DLS and NIR show very good correlation in indentifying the onsets of flocculation, which varied among the series of naphthenic acids. Specific interaction energies and equilibrium intermolecular distances of asphaltenes and naphthenic acids are calculated using molecular mechanics. The results from molecular mechanics calculations support the experimental results of the titrations, and structure–property relationships are defined. Structure–property relationships are established for naphthenic acids, defining the relative contributions and importance of various functional groups: CC, CO, COOR, and COOH. The additive effects of naphthenic acids, defined by an increase in the precipitation onset, increase in the order of 5β-cholanic acid-3-one < hydrogenated methyl abietate < methyl abietate < 5β-cholanic acid, with experiments containing 5β-cholanic acid-3-one containing unexpected and interesting results

Computation of gas-phase enthalpies of formation with chemical accuracy: The curious case of 3-nitroaniline

Using a variety of density functional, post-Hartree-Fock and composite methods, in conjunction with extended basis sets, and a homodesmotic reaction, we conservatively propose a gas-phase enthalpy of formation for 3-nitroaniline of 17 ± 1 kcal mol-1, a value that significantly differs from experimental values suggested in compendia of thermochemical data. Assuming that the reported experimental solid-state enthalpy of formation by Nishiyama et al. is reliable at 8.2 ± 0.3 kcal mol-1, an enthalpy of sublimation for 3-nitroaniline of 25 ± 1 kcal mol-1 is estimated. © 2006 Elsevier B.V. All rights reserved

Comparison of Small Molecule and Polymeric Urethanes, Thiourethanes, and Dithiourethanes: Hydrogen Bonding and Thermal, Physical, and Mechanical Properties

The hydrogen bonding behavior of a homologous series of small molecule and polymeric urethanes, thiourethanes. and dithiourethanes was investigated in solution, melt, and solid states. The relative hydrogen bonding strengths in both small molecule and polymer systems were evaluated, and the results were compared to theoretical calculations of hydrogen bonding strength. The results for NMR and FTIR analysis of the small molecule models indicated that the NH protons on the carbamate and thiocarbamates have greater hydrogen bonding strengths than the NH protons of the dithiocarbamate. The polyurethane and polythiourethanes were found to have approximately equivalent physical and mechanical properties as a result of a similar extent of hydrogen bonding, whereas the polydithiourethane, due to a lower degree of hydrogen bonding, has reduced thermal and mechanical transition temperatures as well as lower hardness values. The polythiourethane and polydithiourethane networks exhibit narrower glass transitions compared to polyurethane networks., apparently the result of an efficient isocyanate/isothiocyanate-thiol reaction with little or no side products. Because of weakness of the C-S bond compared to the C-O bond, thiourethanes and dithiourethanes have lower thermal stability than the corresponding urethanes. Finally, the polythiourethanes and polydithiourethane have higher refractive index values than their polyurethane Counterpart

Noncovalent Interactions in Microsolvated Networks of Trimethylamine <i>N</i>‑Oxide

The effects of the formation of hydrogen-bonded networks on the important osmolyte trimethylamine N-oxide (TMAO) are explored in a joint Raman spectroscopic and electronic structure theory study. Spectral shifts in the experimental Raman spectra of TMAO and deuterated TMAO microsolvated with water, methanol, ethanol, and ethylene glycol are compared with the results of electronic structure calculations on explicit hydrogen-bonded molecular clusters. Very good agreement between experiment and theory suggests that it is the local hydrogen-bonded geometry at TMAO’s oxygen atom that dominates the structure of the extended hydrogen-bonded networks and that TMAO’s unique stabilizing abilities are a result of the “indirect effect” model. Natural bonding orbital (NBO) calculations further reveal that hyperconjugation results in vibrational blue shifts in TMAO’s C–H stretching region when solvated and a red shift in methanol’s C–H stretching region when hydrogen bonding with TMAO

Characterization and Photopolymerization of Divinyl Fumarate

A complete characterization of the electron density distribution of divinyl fumarate and its effect on various properties has been performed by using a combination of UV−vis spectroscopy, cyclic voltammetry, theoretical calculations, and a diagnostic Michael addition reaction involving an aliphatic thiol and the fumarate carbon−carbon double bond. The results show that the presence of the conjugation between the two vinyl ester double bonds and the fumarate carbon−carbon double bond significantly changes the electron density in both; that is, the vinyl ester double bonds of divinyl fumarate are more electron rich and the fumarate double bonds are more electron poor compared to nonconjugated analogues. This electron density distribution greatly influences the copolymerization behavior of divinyl fumarate. Divinyl fumarate also acts as both a monomer and photoinitiator in the photopolymerization of 1,6-hexanediol diacrylate. Because of the larger electron density deficiency of the fumarate group on divinyl fumarate compared to its saturated analogue, diethyl fumarate, there is a reduced propensity of the fumarate group to copolymerize with electron-deficient acrylate groups. Finally, the fundamental photocleavage reaction of vinyl fumarate that leads to initiating radicals was determined by chemical trapping (2,2,6,6-tetramethyl-1-piperidinyloxy free radical, TEMPO) to be the primary α-cleavage process between the carbonyl carbon and the vinyl ester oxygen