216 research outputs found
Obtaining the lattice energy of the anthracene crystal by modern yet affordable first-principles methods
The non-covalent interactions in organic molecules are known to drive their self-assembly to form molecular crystals. We compare, in the case of anthracene and against experimental (electronic-only) sublimation energy, how modern quantum-chemical methods are able to calculate this cohesive energy taking into account all the interactions between occurring dimers in both first-and second-shells. These include both O(N 6)- and O(N 5)-scaling methods, Local Pair Natural Orbital-parameterized Coupled-Cluster Single and Double, and Spin-Component-Scaled-MĂžller-Plesset perturbation theory at second-order, respectively, as well as the most modern family of conceived density functionals: double-hybrid expressions in several variants (B2-PLYP, mPW2-PLYP, PWPB95) with customized dispersion corrections (âD3 and âNL). All-in-all, it is shown that these methods behave very accurately producing errors in the 1â2 kJ/mol range with respect to the experimental value taken into account the experimental uncertainty. These methods are thus confirmed as excellent tools for studying all kinds of interactions in chemical systems.Financial support by the âMinisterio de EconomĂa y Competitividadâ of Spain and the âEuropean Regional Development Fundâ through projects CTQ2011-27253, CTQ2012-31914, and Consolider-Ingenio CSD2007-00010 in Molecular Nanoscience, and by the Generalitat Valenciana (ISIC 2012/008 and PROMETEO/2012/053) is acknowledged. The work in Mons is supported by the Belgian National Fund for Scientific Research (FNRS). Y.O. is a FNRS Post-doctoral Research Fellow. J.C.S.G. is a FNRS Visiting Professor
A thermochemical and theoretical study of the phenylpyridine isomers
The standard (p° = 0.1 MPa) molar enthalpies of formation for 2-, 3-, and 4-phenylpyridine in the gas phase were derived from the standard molar enthalpies of combustion, in oxygen, at 298.15 K, measured by static bomb combustion calorimetry. The standard molar enthalpies of vaporization for 2-, 3-, and 4-phenylpyridine at T = 298.15 K were measured by correlation-gas chromatography. The enthalpy of sublimation of 4-phenylpyridine was obtained as a weighted mean of the value derived from the vaporization and fusion enthalpy values and the value measured directly by Calvet microcalorimetry. The following enthalpies of formation were then derived:â 2-phenylpyridine, g = 228.3 ± 5.8 kJ·mol-1; 3-phenylpyridine, g = 240.9 ± 5.5 kJ·mol-1; 4-phenylpyridine, g = 240.0 ± 3.3 kJ·mol-1. The most stable geometries of all phenylpyridine isomers were obtained using both restricted HartreeâFock (RHF) and density functional theory (DFT/B3LYP) methods. The resulting geometries were then used to obtain estimates of enthalpies of formation of the three isomers of phenylpyridine, which are in good agreement with the experimental values. A theoretical interpretation of the effect of the phenyl ring has on the relative stabilities of the three molecules is presented
The vaporization enthalpy and vapor pressure of (d)-amphetamine and of several primary amines used as standards at T /K = 298 as evaluated by correlation gas chromatography and transpiration
The vapor pressures of several aliphatic and phenyl substituted primary amines at T/K = 298.15 are measured by transpiration studies, and their vaporization enthalpies are calculated. The results were combined with compatible literature values to evaluate both the vaporization enthalpy and vapor pressure of (d)-amphetamine by correlation gas chromatography. The results are compared to existing values either estimated or measured for racemic amphetamine. Vaporization enthalpies and vapor pressures at T/K = 298.15 of the following were measured by transpiration (kJ·mol-1, p/Pa): 1-heptanamine, (49.75 ± 0.38, 291); 1-octanamine, (55.05 ± 0.29, 108); 1-decanamine, (64.94 ± 0.32, 12); benzylamine, (54.32 ± 0.32, 88); (dl)-α-methylbenzylamine, (55.26 ± 0.33, 82); 2-phenethylamine (57.51 ± 0.35, 43). The use of several of these materials as standards resulted in a vaporization enthalpy and vapor pressure for (d)-amphetamine at T/K = 298.15 of (58.2 ± 2.7) kJ mol-1 and (38 ± 12) Pa. © 2013 American Chemical Society
Laboratory investigations of the interaction between benzene and bare silicate grain surfaces
Experimental results on the thermal desorption of benzene (C6H6) from
amorphous silica (SiO2) are presented. The amorphous SiO2 substrate was imaged
using atomic force microscopy (AFM), revealing a surface morphology reminiscent
of that of interplanetary dust particles (IDPs). Temperature programmed
desorption (TPD) experiments were conducted for a wide range of C6H6 exposures,
yielding information on both C6H6-SiO2 interactions and the C6H6-C6H6
interactions present in the bulk C6H6 ice. The low coverage experiments reveal
complicated desorption behaviour that results both from porosity and roughness
in the SiO2 substrate, and repulsive interactions between C6H6 molecules.
Kinetic parameters were obtained through a combination of direct analysis of
the TPD traces and kinetic modelling, demonstrating the coverage dependence of
both desorption energy and pre-exponential factor. Experiments were also
performed whereby the pores were blocked by pre-exposure of the SiO2 to water
vapour. C6H6 was observed to be adsorbed preferentially on the SiO2 film not
covered by H2O at the temperature at which these experiments were performed.
This observation means that intermolecular repulsion likely becomes important
at smaller C6H6 exposures on grains with a H2O mantle. Kinetic modelling of
C6H6 multilayer desorption yields kinetic parameters in good agreement with
previous studies, with the SiO2 having little impact on the desorption beyond
the first few layers.Comment: 23 pages, including 6 figures and 1 table ; Submitted to MNRA
Application of the PM6 method to modeling the solid state
The applicability of the recently developed PM6 method for modeling various properties of a wide range of organic and inorganic crystalline solids has been investigated. Although the geometries of most systems examined were reproduced with good accuracy, severe errors were found in the predicted structures of a small number of solids. The origin of these errors was investigated, and a strategy for improving the method proposed
Thermodynamics of Mixtures Containing Amines. XV. LiquidâLiquid Equilibria for Benzylamine + CH3(CH2)nCH3 (n = 8, 9, 10, 12, 14)
Coexistence curves for the liquidâliquid equilibria (LLE) of 1-phenylmethanamine
(benzylamine) + CH3(CH2)nCH3 (n = 8, 9, 10, 12, 14) have been
determined using the critical opalescence method by means of a laser scattering technique.
All of the LLE curves show an upper critical solution temperature (UCST), which increases
with increasing n. For systems including a given n-alkane, the UCST decreases in the
sequence aniline > 2-methylaniline (o-toluidine) > benzylamine > N-methylaniline >
pyridine. This means that amineâamine interactions become weaker in the same order.
Most of the DISQUAC interaction parameters for the aliphatic/amine (a,n) and aromatic/
amine (b,n) contacts previously determined for solutions with aniline, o-toluidine, or
N-methylaniline have been used for the representation of the LLE data. Only the first
dispersive interaction parameter of the (a,n) contact has been modified. The coordinates of
the critical points are correctly represented by the model
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