The Cohesive Interactions in Phenylimidazoles.

This work presents a comprehensive study exploring the thermodynamics of the solid phase of a series of phenylimidazoles, encompassing experimental measurements of heat capacity, volatility, and thermal behavior. The influence of successive phenyl group insertions on the imidazole ring on thermodynamic properties and supramolecular behavior was thoroughly examined through the evaluation of 2-phenylimidazole (2-PhI), 4-phenylimidazole (4-PhI), 4,5-diphenylimidazole (4,5-DPhI), and 2,4,5-triphenylimidazole (2,4,5-TPhI). Structural correlations between molecular structure and thermodynamic properties were established. Furthermore, the investigation employed UV-vis spectroscopy and quantum chemical calculations. Additive effects arising from the introduction of phenyl groups were found through the analysis of the solid-liquid and solid-gas equilibria, as well as heat capacities. A good correlation emerged between the thermodynamic properties of sublimation and the molar volume of the unit cell, evident across 2-PhI, 4,5-DPhI, and 2,4,5-TPhI. In contrast to its isomer 2-PhI, 4-PhI exhibited greater cohesive energy due to the stronger N-H···N intermolecular interactions, leading to the disruption of coplanar geometry in the 4-PhI molecules. The observed higher entropies of phase transition (fusion and sublimation) are consistent with the higher structural order observed in the crystalline lattice of 4-PhI.

Experimental Evidence for Azeotrope Formation from Protic Ionic Liquids.

Herein, we present experimental evidence that protic ionic liquids (PILs), derived from 1 : 1 liquid mixtures of the organic superbases 1,5-diazabicyclo[4.3.0]non-5-ene (DBN) and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) with carboxylic acids, form azeotropic mixtures with acid/base molar fractions different from 1 : 1. The ability of the carboxylic acids to form strong hydrogen bonds with the PIL ion pair leads to an azeotropic composition richer in the acid component. The results show that the azeotropic composition is ruled by the extent of acid-base equilibrium and the relative volatility of the neutral species in the PIL medium. The PILs show marked negative deviations from Raoult's Law with the stronger superbase (DBU) leading to an azeotropic composition closer to the equimolar 1 : 1 ratio.

Vaporization of protic ionic liquids derived from organic superbases and short carboxylic acids.

This work presents a comprehensive evaluation of the phase behaviour and cohesive enthalpy of protic ionic liquids (PILs) composed of 1,5-diazabicyclo[4.3.0]non-5-ene (DBN) or 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) organic superbases with short-chain length (acetic, propionic and butyric) carboxylic acids. Glass transition temperatures, Tg, and enthalpies of vaporization, ΔHvap, were measured for six [BH][A] (1 : 1) PILs (B = DBN, DBU; A = MeCOO, EtCOO, nPrCOO), revealing more significant changes upon increasing the number of -CH2- groups in the base than in the acid. The magnitude of ΔHvap evidences that liquid PILs have a high proportion of ions, although the results also indicate that in DBN PILs the concentration of neutral species is not negligible. In the gas phase, these PILs exist as a distribution of ion pairs and isolated neutral species, with speciation being dependent on the temperature and pressure conditions - at high temperatures and low pressures the separated neutral species dominate. The higher Tg and ΔHvap of the DBU PILs are explained by the stronger basicity of DBU (as supported by NMR and computational calculations), which increases the extent of proton exchange and the ionic character of the corresponding PILs, resulting in stronger intermolecular interactions in condensed phases.

Effect of Confined Hindrance in Polyphenylbenzenes.

A comprehensive thermodynamic study of the whole ortho-polyphenylbenzenes series from biphenyl (n = 1) to hexaphenylbenzene (n = 6) is presented. Combustion calorimetry and phase equilibria measurements for 1,2,3,4-tetraphenylbenzene (n = 4) and pentaphenylbenzene (n = 5) together with literature data were used to understand and quantify the constraint effect of ortho-substitution on the molecular energetics and phase stability of polyaromatic compounds. All of the derived thermodynamic properties (enthalpy of sublimation, entropy of sublimation, and gas phase molecular energetics) show a marked trend shift at n = 4 to 5, which is related to the change of the degree of molecular flexibility after 1,2,3,4-tetraphenylbenzene (n = 4). The greater intramolecular constraint in the more crowded members of the series (n = 5 and 6) leads to a significant change in the molecular properties and cohesive energy. The trend shift in the molecular properties is related with the decrease in molecular flexibility, which leads to lower molecular entropy and destabilization of the intramolecular interaction potential due to the increased hindrance in a confined molecular space.

Nature of the C2-methylation effect on the properties of imidazolium ionic liquids.

Methylation at the C2 position of 1,3-disubstituted imidazolium-based ionic liquids (ILs) is one of the structural features that has gained attention due to its drastic impact on thermophysical and transport properties. Several hypotheses have been proposed to explain this effect but there is still much discrepancy. Aiming for the rationalization of the effects of these structural features on the properties of imidazolium ILs, we present a thermodynamic and computational study of two methylated ILs at the C2 position of imidazolium, [1C42C13C1im][NTf2] and [1C32C13C1im][NTf2]. The phase behaviour (glass transition and vaporization equilibrium) and computational studies of the anion rotation around the cation and ion pair interaction energies for both ILs were explored. The results have shown that C2-methylation has no impact on the enthalpy of vaporization. However, it decreases the entropy of vaporization, which is a consequence of the change in the ion pair dynamics that affects both the liquid and gas phases. In addition, the more hindered dynamics of the ion pair are also reflected in the increase in the glass transition temperature, Tg. The entropic contribution of anion-around-cation rotation in the imidazolium [NTf2] ILs was quantified experimentally by the comparative analysis of the entropy of vaporization, and computationally by the calculation of the entropies of hindered internal rotation. The global results exclude the existence of significant H-bonding in the C2-protonated (non-methylated) ILs and explain the C2-methylation effect in terms of reduced entropy of the ion pair in the liquid and gas phases. In light of these results, the C2-methylation effect is intrinsically entropic and originates from the more hindered anion-around-cation rotation as a consequence of the substitution of the -H with a bulkier -CH3 group.

Understanding M-ligand bonding and mer-/fac-isomerism in tris(8-hydroxyquinolinate) metallic complexes.

Tris(8-hydroxyquinolinate) metallic complexes, Mq3, are one of the most important classes of organic semiconductor materials. Herein, the nature of the chemical bond in Mq3 complexes and its implications on their molecular properties were investigated by a combined experimental and computational approach. Various Mq3 complexes, resulting from the alteration of the metal and substitution of the 8-hydroxyquinoline ligand in different positions, were prepared. The mer-/fac-isomerism in Mq3 was explored by FTIR and NMR spectroscopy, evidencing that, irrespective of the substituent, mer- and fac-are the most stable molecular configurations of Al(iii) and In(iii) complexes, respectively. The relative M-ligand bond dissociation energies were evaluated experimentally by electrospray ionization tandem mass spectrometry (ESI-MS-MS), showing a non-monotonous variation along the group (Al > In > Ga). The results reveal a strong covalent character in M-ligand bonding, which allows for through-ligand electron delocalization, and explain the preferred molecular structures of Mq3 complexes as resulting from the interplay between bonding and steric factors. The mer-isomer reduces intraligand repulsions, being preferred for smaller metals, while the fac-isomer is favoured for larger metals where stronger covalent M-ligand bonds can be formed due to more extensive through-ligand conjugation mediated by metal "d" orbitals.

Stochastic detection and characterisation of individual ferrocene derivative tagged graphene nanoplatelets.

Graphene nanoplatelets (GNPs) are 'tagged' with 1-(biphen-4-yl)ferrocene. Chronoamperometry is then utilised to observe single particle impacts when GNPs suspended in solution collide with a carbon fibre micro wire electrode held at an oxidising potential, resulting in current/time transient "spikes". The impacts are associated with two types of charge transfer: Faradaic due to oxidation of the 'tag' and capacitative due to disruption of the double layer. Analysis of the spikes suggests approximate monolayer coverage of 1-(biphen-4-yl)ferrocene on the GNP surfaces, with a surface coverage of (2.2 ± 0.3) × 10(-10) mol cm(-2). In contrast non-derivatised ferrocene does not exhibit any significant adsorption on the GNP material.

Diarylferrocene tweezers for cation binding.

The host-guest chemistry of ferrocene derivatives was explored by a combined experimental and theoretical study. Several 1-arylferrocenes and 1,1'-diarylferrocenes were synthesized by the Suzuki-Miyaura cross-coupling reaction. The ability of these compounds to bind small cations in the gas phase was investigated experimentally by electrospray ionization mass spectrometry (ESI-MS). The results evidenced a noticeable ability of all 1,1'-diarylferrocenes studied to bind cations, while the same was not observed for the corresponding 1-arylferrocenes nor ferrocene. The 1,1'-diarylferrocenecation relative interaction energies were evaluated by ESI-MS and quantum chemical calculations and showed that cation binding in these systems follows electrostatic trends. It was found that, due to their unique molecular shape and smooth torsional potentials, 1,1'-diarylferrocenes can act as molecular tweezers of small-sized cations in the gas phase.

Effect of Self-Association on the Phase Stability of Triphenylamine Derivatives.

The self-association equilibrium, i.e. formation of noncovalent dimers, in two triphenylamine derivatives, TPD (N,N'-bis(3-methylphenyl)-N,N'-diphenylbenzidine) and mMTDAB (1,3,5-tris[(3-methylphenyl)phenylamino]benzene), in solution was evaluated by (1)H NMR spectroscopy. The gas-phase energetics of the respective dimerization processes was explored by computational quantum chemistry. The results indicate that self-association is significantly more extensive in TPB than in TDAB. It is proposed that this fact helps to explain why TPB presents a stability higher than expected in the liquid phase, which is reflected in a lower melting temperature, a less volatile liquid, and possibly a higher tendency to form a glass. These results highlight the influence of self-association on the phase equilibria and thermodynamic properties of pure organic substances.

Self-association of oligothiophenes in isotropic systems.

The self-association equilibrium constants, Kass, for the dimerization of some small oligothiophenes in acetone, acetonitrile and chloroform were measured by (1)H NMR spectroscopy. The gas phase interaction energies for some oligothiophene dimers were determined by computational quantum chemistry. The (1)H NMR results indicate that Kass generally increases with the chain length (the number of thienyl rings, n) and solvent polarity; however, Kass for thiophene (n = 1) was found to be higher than for the bithiophenes (n = 2). The linear oligothiophenes 2,2'-bithiophene and 2,2',5',2''-terthiophene were found to self-associate less than their corresponding nonlinear isomers 3,3'-bithiophene and 3,2',5',3''-terthiophene in solution and in the gas phase. For α-quaterthiophene (n = 4) Kass in solution was found to be smaller than expected. The non-linear dependence of the standard molar Gibbs energy of self-association, ΔassG, on the chain length in solution could be nicely reproduced and related to the conformational entropy change of dimerization. It was observed that the melting properties of oligothiophenes correlate well with their tendency to self-associate, with more self-association leading to increased liquid stability, and thus lower melting temperatures. These results highlight the relevance of self-association in isotropic systems for the correct molecular interpretation of phase equilibria.

Elucidating the role of aromatic interactions in rotational barriers involving aromatic systems.

The measurement of aryl-naphthyl rotational barriers, ΔG(⧧), in various solvents for two substituted 1,8-diarylnaphthalenes by dynamic (1)H NMR showed that ΔG(‡) trends in aromatic systems can be fully rationalized only when considering the different types of aromatic interactions that can be established in the ground and transition states, namely, intramolecular interactions involving the aromatic rings and specific solvation interactions.

Experimental support for the role of dispersion forces in aromatic interactions.

Herein a core scaffold of 1-phenylnaphthalenes and 1,8-diphenylnaphthalenes with different substituents on the phenyl rings was used to study substituent effects on parallel-displaced aromatic π⋅⋅⋅π interactions. The energetics of the interaction was evaluated in gas phase based on the standard molar enthalpies of formation, at T=298.15 K, for the compounds studied; these values were derived from the combination of the results obtained by combustion calorimetry and Knudsen/Quartz crystal effusion. A homodesmotic gas-phase reaction scheme was used to quantify and compare the intramolecular interaction enthalpies in various substituted 1,8-diphenylnaphthalenes. The application of this methodology allowed a direct evaluation of aromatic interactions, and showed that substituent effects on the interaction enthalpy cannot be rationalized solely on classical electrostatic grounds, because no correlation with the σ(meta) or σ(para) Hammett constants was observed. Moreover, the results obtained indicate that aromatic π⋅⋅⋅π interactions are significantly enhanced by substitution, in a way that correlates with the ability of the interacting aryl rings to establish dispersive interactions. A combined experimental and computational approach for calculation of the true aromatic π⋅⋅⋅π interaction energies in these systems, free of secondary effects, was employed, and corroborates the rationale derived from the experimental results. These findings clearly emphasize the role of dispersion and dilute the importance of electrostatic forces on this type of interactions.

Phenylnaphthalenes: sublimation equilibrium, conjugation, and aromatic interactions.

In this work, the interplay between structure and energetics in some representative phenylnaphthalenes is discussed from an experimental and theoretical perspective. For the compounds studied, the standard molar enthalpies, entropies and Gibbs energies of sublimation, at T = 298.15 K, were determined by the measurement of the vapor pressures as a function of T, using a Knudsen/quartz crystal effusion apparatus. The standard molar enthalpies of formation in the crystalline state were determined by static bomb combustion calorimetry. From these results, the standard molar enthalpies of formation in the gaseous phase were derived and, altogether with computational chemistry at the B3LYP/6-311++G(d,p) and MP2/cc-pVDZ levels of theory, used to deduce the relative molecular stabilities in various phenylnaphthalenes. X-ray crystallographic structures were obtained for some selected compounds in order to provide structural insights, and relate them to energetics. The thermodynamic quantities for sublimation suggest that molecular symmetry and torsional freedom are major factors affecting entropic differentiation in these molecules, and that cohesive forces are significantly influenced by molecular surface area. The global results obtained support the lack of significant conjugation between aromatic moieties in the α position of naphthalene but indicate the existence of significant electron delocalization when the aromatic groups are in the ß position. Evidence for the existence of a quasi T-shaped intramolecular aromatic interaction between the two outer phenyl rings in 1,8-di([1,1'-biphenyl]-4-yl)naphthalene was found, and the enthalpy of this interaction quantified on pure experimental grounds as -(11.9 ± 4.8) kJ·mol(-1), in excellent agreement with the literature CCSD(T) theoretical results for the benzene dimer.

Structural and thermodynamic characterization of polyphenylbenzenes.

The thermodynamic and structural study of a series of polyphenylbenzenes, from benzene, n(Ph) = 0, to hexaphenylbenzene, n(Ph) = 6, is presented. The available literature data for this group of compounds was extended by the determination of the relevant thermodynamic properties for 1,2,4-triphenylbenzene, 1,2,4,5-tetraphenylbenzene, and hexaphenylbenzene, as well as structural determination by X-ray crystallography for some of the studied compounds. Gas phase energetics in this class of compounds was analyzed from the derived standard molar enthalpies of formation in the gaseous phase. The torsional profiles relative to the phenyl-phenyl hindered rotations in some selected polyphenylbenzenes, as well as the gas phase structures and energetics, were derived from quantum chemical calculations. In the ideal gas phase, a significant enthalpic destabilization was observed in hexaphenylbenzene relative to the other polyphenylbenzenes, due to steric crowding between the six phenyl substituents. A relatively low enthalpy of sublimation was observed for hexaphenylbenzene, in agreement with the decreased surface area able to establish intermolecular interactions. The apparently anomalous low entropy of sublimation observed for hexaphenylbenzene is explained by its high molecular symmetry and the six highly hindered phenyl internal rotations. For the series of polyphenylbenzenes considered, it was shown that the differentiation in the entropy of sublimation can be chiefly ascribed to the torsional freedom of the phenyl substituents in the gas phase and the entropy terms related with molecular symmetry.

High-accuracy vapor pressure data of the extended [C(n)C1im][Ntf2] ionic liquid series: trend changes and structural shifts.

For the first time, two distinct trends are clearly evidenced for the enthalpies and entropies of vaporization along the [Cnmim][Ntf2] ILs series. The trend shifts observed for Δ(l)(g)H(m)(o) and Δ(l)(g)S(m)(o), which occur at [C6mim][Ntf2], are related to structural modifications. The thermodynamic results reported in the present article constitute the first quantitative experimental evidence of the structural percolation phenomenon and make a significant contribution to better understanding of the relationship among cohesive energies, volatilities, and liquid structures of ionic liquids. A new Knudsen effusion apparatus, combined with a quartz crystal microbalance, was used for the high-accuracy volatility study of the 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide series ([Cnmim][Ntf2], where n = 2, 3, 4, 5, 6, 7, 8, 10, 12). Vapor pressures in the (450­500) K temperature range were measured, and the molar standard enthalpies, entropies, and Gibbs energies of vaporization were derived. The thermodynamic parameters of vaporization were reported, along with molecular dynamic simulations of the liquid phase structure, allowing the establishment of a link between the thermodynamic properties and the percolation phenomenon in ILs.

Thermodynamic insights on the structure and energetics of s-triphenyltriazine.

For s-triphenyltriazine, at T = 298.15 K, were measured the standard (p(0) = 10(5) Pa) molar enthalpy of combustion, by static bomb combustion calorimetry, and the standard molar enthalpy, entropy, and Gibbs energy of sublimation by Knudsen/Quartz crystal effusion. A comparison between the entropies of sublimation of s-triphenyltriazine and the isosteric 1,3,5-triphenylbenzene gave a good indication that the higher symmetry of the former contributes significantly to the decrease of its volatility. A computational study at the MP2/cc-pVDZ and B3LYP/6-311++g(d,p) levels of theory was carried out in order to obtain the gas phase geometry, enthalpy, and barriers to internal rotation about the phenyl-triazine bonds. Making use of homodesmotic reaction schemes, a marked stabilization was observed in the molecule of s-triphenyltriazine relative to analogous systems. This result is supported both experimentally and computationally and, combined with a detailed analysis of the literature data concerning the energetics and structure of related compounds, pointed to a significant enthalpic stabilization associated with the exchange of an intramolecular Ar-H···H-Ar close contact by an Ar-H···N(Ar) one. An inspection of the ring-ring torsional profiles in azabenzenes and biphenyls, obtained computationally at the SCS-MP2/cc-pVDZ level, showed that the ring-ring torsions are the dimensions of the potential energy surface (PES) that chiefly determine the energetic differentiation in this class of compounds.

The role of aromatic interactions in the structure and energetics of benzyl ketones.

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 π-π interaction in DBK was found. The gas phase intramolecular ππ 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 ππ interaction in DBK and a weak C-Hπ 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.

Neutral, ion gas-phase energetics and structural properties of hydroxybenzophenones.

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.

1,3-Diphenyl-propan-2-one (2,4-dinitro-phen-yl)hydrazone.

In the title compound, C(21)H(18)N(4)O(4), there is an intra-molecular N-H⋯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)° with the two phenyl rings. In the crystal structure, mol-ecules are linked into a three-dimensional network by weak C-H⋯N, C-H⋯O and C-H⋯π inter-actions.

(E)-1-Phenyl-butan-2-one (2,4-dinitro-phen-yl)hydrazone.

In the title compound, C(16)H(16)N(4)O(4), the dihedral angle between the aromatic rings is 79.04 (8)° and an intra-molecular N-H⋯O hydrogen bond occurs. In the crystal, weak C-H.·O and C-H..π inter-actions link the mol-ecules, forming sheets.