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Abstract This paper reports an experimental and theoretical study of the standard (p degrees = 0.1 MPa) molar enthalpies of formation at T = 298.15 K of the sulfur-containing amino acids L-cysteine [CAS 52-90-4] and L-cystine [CAS 56-89-3]. The standard (p degrees = 0.1 MPa) molar enthalpies of formation of crystalline L-cysteine and L-cystine were calculated from the standard molar energies of combustion, in oxygen, to yield CO(2)(g) and H(2)SO(4)center dot 115H(2)O, measured by rotating-bomb combustion calorimetry at T = 298.15 K. The vapor pressures of L-cysteine were measured as function of temperature by the Knudsen effusion mass-loss technique. The standard molar enthalpy of sublimation, at T = 298.15 K, was derived from the Clausius-Clapeyron equation. The experimental values were used to calculate the standard (p degrees = 0.1 MPa) enthalpy of formation of L-cysteine in the gaseous phase, Delta(f)H(m)degrees(g) = -382.6 +/- 1.8 kJ.mol(-1). Due to the low vapor pressures of L-cystine and since this compound decomposes at the temperature range required for a possible sublimation, it was not possible to determine its enthalpy of sublimation. Standard ab initio molecular orbital calculations at the G3(MP2)//B3LYP and/or G3 levels were performed. Enthalpies of formation, using atomization and isodesmic reactions, were calculated and compared with experimental data. A value of -755 +/- 10 kJ.mol(-1) was estimated for the enthalpy of formation of cystine. Detailed inspections of the molecular and electronic structures of the compounds studied were carried out. Finally, bond dissociation enthalpies (BDE) of S-H, S-S, and C-S bonds, and enthalpies of formation of L-cysteine-derived radicals, were also computed.

Abstract By employing H-1 NMR spectroscopy and molecular simulations, we provide an explanation for recent observations that the aqueous Solubilities of ionic liquids exhibit salting-out to salting-in regimes upon addition of distinct inorganic salt ions. Using a typical ionic liquid [1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide], we observed the existence of preferential specific interactions between the low electrical charge density ("apolar moiety") parts of the ionic liquid cation and the inorganic salts. These a priori unexpected interactions become increasingly favorable as one moves from salting-out to salting-in effects. More specifically, this interpretation is validated by distinct aqueous solution H-1 NMR data shifts in the ionic liquid cation upon inorganic salt addition. These shifts, which are well noted in the terminal and preterminal hydrogens of the alkyl chain appended to the imidazolium ring, correlate quantitatively with solubility data, both for cases where the nature of inorganic salt is changed, at constant concentration, and for those where the concentration of a given inorganic salt is varied. Molecular simulations have also been performed permitting us to garner a broader picture of the underlying mechanism and structure of this complex solvation phenomenon. These findings can now be profitably used to anticipate solution behavior upon inorganic salt addition well beyond the specificity of the ionic liquid solutions, i.e., for a diversity of distinct solutes differing in chemical nature.

Abstract A qualitative and quantitative energetic and structural study of dibenzyl ketone (DBK) and benzyl ethyl ketone (BEK) was carried out in order to obtain insights into the type and magnitude of aromatic interactions that these systems present in their different phases. The crystal structure of DBK was obtained by X-ray crystallography, and it shows that the conformation adopted in the crystalline state is governed by the intermolecular interactions. The standard (p(0) = 10(5) Pa) molar enthalpy of formation in the gaseous state at T = 298.15 K was derived by Calvet and combustion calorimetry. Using a homodesmic reaction scheme, the first calorimetric evaluation of the interaction enthalpy between two stacked phenyl rings is presented. A stabilizing enthalpic effect of (12.9 +/- 4.9) kJ mol(-1) associated with the intramolecular pi-pi interaction in DBK was found. The gas phase intramolecular pi ... pi interaction in DBK is in agreement with quantum chemical calculations at B3LYP/6-311++ G(d, p) and MP2 with various basis-sets. An intramolecular pi ... pi interaction in DBK and a weak C-H ... pi interaction in BEK were found by variable-temperature (1)H-NMR spectroscopy in MeOD. These observations are consistent with a hindered rotor interpretation, supported by ab initio calculations for the gas phase at the MP2/cc-pVDZ level. The global results indicate a distinct molecular structure on going from crystalline DBK to liquid, gas, and solution phases, ruled by the overall contribution of the intra- and intermolecular interactions.

Abstract The energetic effects caused by replacing one of the methylene groups in the 9,10-dihydroanthracene by ether or ketone functional groups yielding xanthene and anthrone species, respectively, were determined from direct comparison of the standard (p degrees = 0 1 MPa) molar enthalpies of formation in the gaseous phase, at T = 298.15 K. of these compounds. The experimental static-bomb combustion calorimetry and Calvet microcalorimetry and the computational G3(MP2)//B3LYP method were used to get the standard molar gas-phase enthalpies of formation of xanthene. (41 8 +/- 3 5) kJ mol(-1), and anthrone, (31 4 +/- 3 2) kJ mol(-1) The enthalpic increments for the substitution of methylene by ether and ketone in the parent polycyclic compound (9,10-dihydroanthracene) are -(1179 +/- 5 5) kJ mol(-1) and -(1283 +/- 5.4) kJ mol(-1), respectively

Abstract In the title compound, C(21)H(18)N(4)O(4), there is an intramolecular N-H center dot center dot center dot O hydrogen bond between the amino H atom and an O atom of the 2-nitro group of the adjacent benzene ring. The central benzene ring forms dihedral angles of 79.98 (7) and 82.88 (7)degrees with the two phenyl rings. In the crystal structure, molecules are linked into a three-dimensional network by weak C-H center dot center dot center dot N, C-H center dot center dot center dot O and C-H center dot center dot center dot pi interactions.

Abstract The standard molar enthalpies of formation, at T = 298.15 K, of all possible single methylated derivatives of benzothiophene and dibenzothiophene were calculated by means of the G3(MP2)//B3LYP approach employing several different working reactions (homodesmotic and atomization). The most stable compounds are the 7-methylbenzothiophene and 4-methyldibenzothiophene while the least stable are the 5-methylbenzothiophene and 1-methyldibenzothiophene compounds. Calculated enthalpic increments for the reactions of methylation are in the range -29.5 and - 39.1 kJ mol(-1).

Abstract The title compound, C17H11N, crystallizes with two molecules in the asymmetric unit which are linked by a weak C-H center dot center dot center dot N hydrogen bond. The dihedral angles between the benzene ring and the naphthalene ring system in the two molecules are 60.28 (3) and 60.79 (3)degrees. In the crystal, molecules are linked into a three-dimensional network by weak C-H center dot center dot center dot pi interactions.

Abstract Thermochemical and thermophysical studies have been carried out for crystalline 3,4,4'-trichlorocarbanilide. The standard (p degrees = 0.1 MPa) molar enthalpy of formation, at T = 298.15 K, for the crystalline 3,4,4'-trichlorocarbanilide (TCC) was experimentally determined using rotating-bomb combustion calorimetry, as -(234.6 +/- 8.3) kJ . mol(-1). The standard enthalpy of sublimation, at the reference temperature of 298.15 K, was measured by the vacuum drop microcalorimetric technique, using a High Temperature Calvet Microcalorimeter as (182.1 +/- 1.7) kJ . mol(-1). These two thermochemical parameters yielded the standard molar enthalpy of formation of the studied compound, in the gaseous phase, at T = 298.15 K, as -(52.5 +/- 8.5) kJ . mol(-1). This parameter was also calculated by computational thermochemistry at M05-2X/6-311++G and B3LYP/6-311++G(3df, 2p) levels, with a deviation less than 4.5 kJ . mol(-1) from experimental value. Moreover, the thermophysical study was made by differential scanning calorimetry. DSC, over the temperature interval between T = 263K and its onset fusion temperature, T = (527.5 +/- 0.4) K. A solid-solid phase transition was found at T = (428 +/- 1) K, with the enthalpy of transition of (6.1 +/- 0.1) kJ . mol(-1). The X-ray crystal structure of TCC was determined and the three-centred N-H center dot center dot center dot O=C hydrogen bonds present analyzed.

Abstract We have carried out a study of the energetics, structural, and physical properties of o-, m-, and p-hydroxybenzophenone neutral molecules, C(13)H(10)O(2), and their corresponding anions. In particular, the standard enthalpies of formation in the gas phase at 298.15 K for all of these species were determined. A reliable experimental estimation of the enthalpy associated with intramolecular hydrogen bonding in chelated species was experimentally obtained. The gas-phase acidities (GA) of benzophenones, substituted phenols, and several aliphatic alcohols are compared with the corresponding aqueous acidities (pK(a)), covering a range of 278 kJ.mol(-1) in GA and 11.4 in pK(a). A computational study of the various species shed light on structural effects and further confirmed the self-consistency of the experimental results.