First and Second Dissociation Enthalpies in Bi-Component Crystals Consisting of Maleic Acid and L-Phenylalanine.

The energetics of the stepwise dissociation of a A:B2 bi-component crystal, according to A:B2(cr) → A:B(cr) + B(cr) and A:B(cr) → A(cr) + B(cr), was investigated using MA:Phe2 and MA:Phe (MA = maleic acid; Phe = L-phenylalanine) as model systems. The enthalpy changes associated with these sequential processes and with the overall dissociation reaction A:B2(cr) → A(cr) + 2B(cr) were determined by solution calorimetry. It was found that they are all positive, indicating that there is a lattice enthalpy gain when MA:Phe2 is formed, either from the individual precursors or by adding Phe to MA:Phe. Single-crystal X-ray diffraction (SCXRD) analysis showed that MA:Phe2 is best described as a protic salt containing a maleate anion (MA-) and two non-equivalent L-phenylalanine units, both linked to MA- by NH···O hydrogen bonds (H-bond): one of these units is protonated (HPhe+) and the other zwitterionic (Phe±). Only MA- and HPhe+ molecules are present in the MA:Phe lattice. In this case, however, NH···O and OH···O H-bonds are formed between each MA- unit and two HPhe+ molecules. Despite these structural differences, the enthalpy cost for the removal of the zwitterionic Phe± unit from the MA:Phe2 lattice to yield MA:Phe is only 0.9 ± 0.4 kJ mol-1 higher than that for the dissociation of MA:Phe, which requires a proton transfer from HPhe+ to MA- and the rearrangement of L-phenylalanine to the zwitterionic, Phe±, form. Finally, a comparison of the dissociation energetics and structures of MA:Phe and of the previously reported glycine maleate (MA:Gly) analogue indicated that parameters, such as the packing coefficient, density, hydrogen bonds formed, or fusion temperature, are not necessarily good descriptors of dissociation enthalpy or lattice enthalpy trends when bi-component crystals with different molecular composition are being compared, even if the stoichiometry is the same.

Exploratory Study on Lercanidipine Hydrochloride Polymorphism: pH-Dependent Solubility Behavior and Simulation of its Impact on Pharmacokinetics.

This work describes an exploratory experimental and in silico study of the influence of polymorphism, particle size, and physiology on the pharmacokinetics of lercanidipine hydrochloride (LHC). Equilibrium and kinetic solubility studies were performed on LHC forms I and II, as a function of pH and buffer composition. GastroPlus® was used to evaluate the potential effect of solubility differences due to polymorphism, particle size, and physiological conditions, on the drug pharmacokinetics. The results indicated that solubilities of LHC polymorphs are strongly dependent on the composition and pH of the buffer media. The concentration ratio (CI/CII) is particularly large for chloride buffer (CI/CII = 3.3-3.9) and exhibits a slightly decreasing tendency with the pH increase for all other buffers. Based on solubility alone, a higher bioavailability of form I might be expected. However, exploratory PBPK simulations suggested that (i) under usual fasted (pH 1.3) and fed (pH 4.9) gastric conditions, the two polymorphs have similar bioavailability, regardless of the particle size; (ii) at high gastric pH in the fasted state (e.g., pH 3.0), the bioavailability of form II can be considerably lower than that of form I, unless the particle size is < 20 µm. This study demonstrates the importance of investigating the effect of the buffer nature when evaluating the solubility of ionizable polymorphic substances. It also showcases the benefits of using PBPK simulations, to assess the risk and pharmacokinetic relevance of different solubility and particle size between crystal forms, for diverse physiological conditions.

Mechanistic Insights into a Sustainable Mechanochemical Synthesis of Ettringite.

Mechanochemistry offers an environmentally benign and facile synthesis method for a variety of cement paste constituents. In addition, these methods can be used to selectively tune the properties of cement components. The mineral ettringite is an important component of cementitious materials and has additional technological potential due to its ion exchange properties. Synthesis of ettringite via mechanochemistry is an environmentally friendly alternative to conventional wet-chemical synthesis established in industry. This contribution explores the mechanism of a two-step mechanochemical synthesis of ettringite, which was previously found to greatly improve the reaction conversion as compared with one-pot synthesis. The crystallinity of Al(OH)3 was found to decrease during the first stage of this mechanochemical synthesis. This was correlated to a significant decrease in the particle size of Al(OH)3 in this stage. No other significant changes were found for the other components, suggesting that mechanochemical activation of Al(OH)3 is responsible for the enhanced formation of ettringite by the two-step approach. The environmentally friendly approach developed for ettringite synthesis offers a versatile synthetic strategy, which can be applied to synthesise further cementitious materials.

Polymorphism in Simvastatin: Twinning, Disorder, and Enantiotropic Phase Transitions.

Simvastatin is one of the most widely used active pharmaceutical ingredients for the treatment of hyperlipidemias. Because the compound is employed as a solid in drug formulations, particular attention should be given to the characterization of different polymorphs, their stability domains, and the nature of the phase transitions that relate them. In this work, the phase transitions delimiting the stability domains of three previously reported simvastatin forms were investigated from structural, energetics, and dynamical points of view based on single crystal X-ray diffraction (SCXRD), hot stage microscopy (HSM), and differential scanning calorimetry (DSC) experiments (conventional scans and heat capacity measurements), complemented with molecular dynamics (MD) simulations. Previous assignments of the crystal forms were confirmed by SCXRD: forms I and II were found to be orthorhombic ( P212121, Z'/ Z = 1/4) and form III was monoclinic ( P21, Z'/ Z = 2/4). The obtained results further indicated that (i) the transitions between different forms are observed at 235.9 ± 0.1 K (form III → form II) and at 275.2 ± 0.2 K (form II → form I) in DSC runs carried out at 10 K min-1 and close to these values when other types of techniques are used (e.g., HSM). (ii) They are enantiotropic (i.e., there is a transition temperature relating the two phases before fusion at which the stability order is reversed), fast, reversible, with very little hysteresis between heating and cooling modes, and occur under single crystal to single crystal conditions. (iii) A nucleation and growth mechanism seems to be followed since HSM experiments on single crystals evidenced the propagation of an interface, accompanied by a change of birefringence and crystal contraction or expansion (more subtle in the case of form III → form II), when the phase transitions are triggered. (iv) Consistent with the reversible and small hysteresis nature of the phase transitions, the SCXRD results indicated that the molecular packing is very similar in all forms and the main structural differences are associated with conformational changes of the "ester tail". (v) The MD simulations further suggested that the tail is essentially "frozen" in two conformations below the III → II transition temperature, becomes progressively less hindered throughout the stability domain of form II, and acquires a large conformational freedom above the II → I transition. Finally, the fact that these transitions were found to be fast and reversible suggests that polymorphism is unlikely to be a problem for pharmaceutical formulations employing crystalline simvastatin because, if present, the III and II forms will readily convert to form I at ambient temperature.

Extraction Optimization and Structural and Thermal Characterization of the Antimicrobial Abietane 7α-Acetoxy-6ß-hydroxyroyleanone.

The abietane 7α-acetoxy-6ß-hydroxyroyleanone (AHR), obtained from plant extracts, is an attractive lead for drug development, given its known antimicrobial properties. Two basic requirements to establish any compound as a new drug are the development of a convenient extraction process and the characterization of its structural and thermal properties. In this work seven different methods were tested to optimize the extraction of AHR from Plectranthus grandidentatus. Supercritical fluid extraction (SFE) proved to be the method of choice, delivering an amount of AHR (57.351 µg·mg-1) approximately six times higher than the second best method (maceration in acetone; 9.77 µg·mg-1). Single crystal X-ray diffraction analysis of the ARH molecular and crystal structure carried out at 167 ± 2 K and 296 ± 2 K showed only a single phase, here dubbed form III (orthorhombic space group P21212), at those temperatures. The presence of two other polymorphs above room temperature was, however, evidenced by differential scanning calorimetry (DSC). The three forms are enantiotropically related, with the form III → form II and form II → form I transitions occurring at 333.5 ± 1.6 K and 352.0 ± 1.6 K, respectively. The fact that the transitions are reversible suggests that polymorphism is not likely to be an issue in the development pharmaceutical formulations based on ARH. DSC experiments also showed that the compound decomposes on melting at 500.8 ± 0.8 K. Melting should therefore be avoided if, for example, strategies to improve solubility based on the production of glassy materials or solid dispersions are considered.

Structure-property relationships in protic ionic liquids: a study of solvent-solvent and solvent-solute interactions.

The ionic nature of a functionalized protic ionic liquid cannot be rationalized simply through the differences in aqueous proton dissociation constants between the acid precursor and the conjugate acid of the base precursor. The extent of proton transfer, i.e. the equilibrium ionicity, of a tertiary ammonium acetate protic ionic liquid can be significantly increased by introducing an hydroxyl functional group on the cation, compared to the alkyl or amino-functionalized analogues. This increase in apparent ionic nature correlates well with variations in solvent-solute and solvent-solvent interaction parameters, as well as with physicochemical properties such as viscosity.

Structure-property relationships in protic ionic liquids: a thermochemical study.

How does cation functionality influence the strength of intermolecular interactions in protic ionic liquids (PILs)? Quantifying the energetics of PILs can be an invaluable tool to answer this fundamental question. With this in view, we have determined the standard molar enthalpy of vaporization, , and the standard molar enthalpy of formation, , of three tertiary ammonium acetate PILs with varying cation functionality, and of their corresponding precursor amines, through a combination of Calvet-drop microcalorimetry, solution calorimetry, and ab initio calculations. The obtained results suggest that these PILs vaporize as their neutral acid and base precursors. We also found a strong correlation between of the PILs and of their corresponding amines. This suggests that, within this series of PILs, the influence of cation modification on their cohesive energies follows a group additivity rule. Finally, no correlation between the of PILs and the extent of proton transfer, as estimated from the difference in aqueous pKa between the precursor acid and the conjugate acid of the precursor base, was observed.

Dynamic spin interchange in a tridentate Fe(iii) Schiff-base compound.

The thermosalient effect is still a rare and poorly understood phenomenon, where crystals suddenly jump, bend, twist or explode upon undergoing a thermally activated phase transition. The synthesis and characterisation of the new spin transition Fe(iii) compound [Fe(5-Br-salEen)2][ClO4] (salEen = N-ethyl-N-(2-aminoethyl)salicylaldiminate) is described and its thermosalient behaviour reported. It is the first example of a thermosalient effect with a spin transition and magnetic, calorimetric, diffraction, microscopy and computational studies are used to characterise these effects. Both thermosalient effect and spin transition occur around 320 K upon heating and are accompanied by an anisotropic unit cell change with conservation of crystal symmetry that causes a large enough stress of the crystal lattice to induce crystal explosion. This stress can ultimately be traced back to a diffusionless and distortive structural perturbation resulting in a coupled spin transition-thermosalient effect.

Gas-Phase Affinity Scales for Typical Ionic Liquid Moieties Determined by using Cooks' Kinetic Method.

Gas-phase affinity studies based on cations and anions commonly present in ionic liquid structures, give quantitative information about the magnitude of the interactions holding the two species together when ILs are formed. They also provide clues on how these interactions depend on the nature of the cationic and anionic moieties. In the present work, mass spectrometric experiments, performed using electrospray ionization quadrupole ion-trap and Fourier transform ion cyclotron resonance mass spectrometry, were used to obtain two affinity scales by Cooks' kinetic method: one scale for the various cations for the bis(trifluoromethylsulfonyl)imide anion, [NTf2 ](-) , and another for the different anions for the 1-butyl-3-methylimidazolium cation, [C4 mim](+) . The obtained results are compared with previously reported data and discussed in terms of the structural characteristics of the different cationic and anionic species.

Thermal stability of simvastatin under different atmospheres.

Simvastatin (SV) is a widely used drug for the treatment of hypercholesterolemia in humans. Nevertheless, serious efforts are still being made to develop new SV formulations with, for example, improved tabletability or bioavailability properties. These efforts frequently involve heating the compound well above ambient temperature or even fusion. In this work, the thermal stability of solid SV under different atmospheres was investigated by using isothermal tests in glass ampules, differential scanning calorimetry, and Calvet-drop microcalorimetry experiments. These tests were combined with analytical data from diffuse reflectance infrared Fourier-transform spectroscopy and liquid chromatography coupled with tandem mass spectrometry or Fourier transform ion cyclotron resonance mass spectrometry (LC-FT-ICR-MS). No decomposition was observed when the sample was kept at a temperature ≤373 K under N2 or reduced pressure (13.3 Pa) atmospheres. Thermal degradation was, however, observed for temperatures ≥353 K in the presence of pure or atmospheric oxygen. The nature of the two main oxidative degradation products was determined through MS/MS experiments and accurate mass measurements of the precursor ions using FT-ICR-MS. The obtained results indicated that the decomposition process involves the oxidation of the hexahydronaphthalene fragment of SV.

All-atom force field for molecular dynamics simulations on organotransition metal solids and liquids. Application to M(CO)(n) (M = Cr, Fe, Ni, Mo, Ru, or W) compounds.

A previously developed OPLS-based all-atom force field for organometallic compounds was extended to a series of first-, second-, and third-row transition metals based on the study of M(CO)(n) (M = Cr, Fe, Ni, Mo, Ru, or W) complexes. For materials that are solid at ambient temperature and pressure (M = Cr, Mo, W) the validation of the force field was based on reported structural data and on the standard molar enthalpies of sublimation at 298.15 K, experimentally determined by Calvet-drop microcalorimetry using samples corresponding to a specific and well-characterized crystalline phase: Δ(sub)H(m)° = 72.6 ± 0.3 kJ·mol(­1) for Cr(CO)(6), 73.4 ± 0.3 kJ·mol(­1) for Mo(CO)(6), and 77.8 ± 0.3 kJ·mol(­1) for W(CO)(6). For liquids, where problems of polymorphism or phase mixtures are absent, critically analyzed literature data were used. The force field was able to reproduce the volumetric properties of the test set (density and unit cell volume) with an average deviations smaller than 2% and the experimentally determined enthalpies of sublimation and vaporization with an accuracy better than 2.3 kJ·mol(­1). The Lennard-Jones (12-6) potential function parameters used to calculate the repulsive and dispersion contributions of the metals within the framework of the force field were found to be transferable between chromium, iron, and nickel (first row) and between molybdenum and ruthenium (second row).

Energetics and structure of simvastatin.

The study of structure-energetics relationships for active pharmaceutical ingredients has received considerable attention in recent years, due to its importance for the effective production and safe use of drugs. In this work the widely prescribed cholesterol-lowering drug simvastatin was investigated by combining experimental (combustion calorimetry and differential scanning calorimetry, DSC) and computational chemistry (quantum chemistry and molecular dynamics calculations) results. The studies addressed the crystalline form stable at ambient temperature (form I) and the liquid and gaseous phases. Heat capacity determinations by DSC showed no evidence of polymorphism between 293 K and the fusion temperature. It was also found that the most stable molecular conformation in the gas phase given by the quantum chemistry calculations (B3LYP-D3/cc-pVTZ) is analogous to that observed in the crystal phase. The molecular dynamics simulations correctly captured the main structural properties of the crystalline phase known from published single crystal X-ray diffraction results (unit cell dimensions and volume). They also suggested that, while preferential conformations are exhibited by the molecule in the solid at 298.15 K, these preferences are essentially blurred upon melting. Finally, the experiments and calculations led to enthalpies of formation of simvastatin at 298.15 K, in the crystalline (form I) ΔfH(m)(o) (cr I) = -1238.4 ± 5.6 kJ · mol(-1), liquid ΔfH(m)(o) (l) = -1226.4 ± 5.7 kJ · mol(-1), and gaseous ΔfH(m)(o) (g) = -1063.0 ± 7.1 kJ · mol(-1) states.

A general strategy for the experimental study of the thermochemistry of protic ionic liquids: enthalpy of formation and vaporisation of 1-methylimidazolium ethanoate.

A general strategy to determine enthalpies of formation of protic ionic liquids, based solely on enthalpy of solution measurements, was conceived and tested for 1-methylimidazolium ethanoate, leading to Δ(f)H°(m){[Hmim][O(2)CCH(3)], 1} = -(425.7 ± 1.2) kJ mol(-1). This result in conjunction with the enthalpy of formation of gaseous 1-methylimidazole (mim) proposed in this work, Δ(f)H°(m)(mim, g) = 126.5 ± 1.1 kJ mol(-1), and Δ(f)H°(m)(CH(3)COOH, g) taken from the literature, allowed the calculation of the enthalpy of the vaporisation process [Hmim][O(2)CCH(3)](l) → mim(g) + CH(3)COOH(g) as Δ(vap)H°(m){[Hmim][O(2)CCH(3)]} = 119.4 ± 3.0 kJ mol(-1). The agreement between this value and Δ(vap)H°(m){[Hmim][O(2)CCH(3)]} = 117.3 ± 0.5 kJ mol(-1), obtained for the direct vaporisation of [Hmim][O(2)CCH(3)], by Calvet-drop microcalorimetry, gives a good indication that, as previously suggested by Fourier transform ion cyclotron resonance mass spectrometry, Raman spectroscopy, and GC-MS experiments, the vaporisation of [Hmim][O(2)CCH(3)] essentially involves a proton transfer mechanism with formation of the two volatile neutral precursor molecules (mim and CH(3)COOH). Although being a low ionicity protic ionic liquid, [Hmim][O(2)CCH(3)] was chosen to validate the methodology proposed here, since its vaporisation mechanism has been unequivocally demonstrated by different methods and for different pressure ranges.

The structure of aqueous solutions of a hydrophilic ionic liquid: the full concentration range of 1-ethyl-3-methylimidazolium ethylsulfate and water.

Several structural features of aqueous solutions of the ionic liquid 1-ethyl-3-methylimidazolium ethylsulfate were analyzed in the entire concentration range using molecular dynamics simulation results. Different analysis tools developed in-house were applied to describe the size and connectivity of different water and ion aggregates as a function of the solution concentration. Four concentration ranges-x(H(2)O)<0.5, 0.50.95-with four distinct structural regimes--isolated water molecules, chain-like water aggregates, bicontinuous system, and isolated ions or small ion clusters--were identified and discussed, including two different percolation limits: that of water in the ionic liquid network (around x(H(2)O)=0.8) and that of the ionic liquid in water (around x(H(2)O)=0.95).

Vaporisation of a dicationic ionic liquid revisited.

The vaporization of a dicationic ionic liquid at moderate temperatures and under reduced pressures--recently studied by line-of-sight mass spectrometry--was further analyzed using an ion-cyclotron resonance mass spectroscopy technique that allows the monitoring of the different species present in the gas phase through the implementation of controlled ion-molecule reactions. The results support the view that the vapour phase of an aprotic dicationic ionic liquid is composed of neutral ion triplets (one dication attached to two anions). Molecular dynamics simulations were also performed in order to explain the magnitude of the vaporization enthalpies of dicationic ionic liquids vis-à-vis their monocationic counterparts.

Energetics of aqueous solutions of the ionic liquid 1-ethyl-3-methylimidazolium ethylsulfate.

The energetics of aqueous solutions of the ionic liquid (IL) 1-ethyl-3-methylimidazolium ethylsulfate in the concentration range m < 165 mol·kg(-1) was analyzed on the basis of the enthalpies of solution of the IL in water (Δ(sol)H(m)) and the enthalpies of dilution (Δ(dil)H(m)) of solutions with different IL concentrations. The data were both obtained experimentally, by calorimetry, and theoretically, by using Molecular Dynamics (MD) simulations. Particular attention was given to the low-concentration range (m < 5 mol·kg(-1)), which had not been covered in previous experimental studies of this system. The dependence of Δ(sol)H(m) from the molality of the IL observed within this m < 5 mol·kg(-1) range could be fitted to a fourth-order polynomial with an average relative deviation of ∼0.13%. This polynomial function shows a minimum of Δ(sol)H(m) at m ≈ 0.6 mol·kg(-1) (or a molar fraction x(IL) ≈ 0.01) that could be approximately captured by the MD simulations performed in this work but not through extrapolations based on previously reported experimental or simulation data. The decomposition of our MD simulation Δ(sol)H(m) results in contributions from different types of interaction (IL-IL, H(2)O-H(2)O, and IL-H(2)O), indicated that the minimum essentially results from two opposite effects: the differences between the IL-IL and H(2)O-H(2)O interactions in the solution and in the pure liquids are both positive and increase with the dilution of the IL, while the contribution of the IL-H(2)O interactions (that is only present in the solution) is negative and decreases with the IL dilution. It was also found that the observed trends in Δ(sol)H(m) are dominated by electrostatic rather than dispersion interactions.

The nature of protic ionic liquids in the gas phase revisited: Fourier transform ion cyclotron resonance mass spectrometry study of 1,1,3,3-tetramethylguanidinium chloride.

The sublimation/vaporization of the protic ionic liquid 1,1,3,3-tetramethylguanidinium chloride, [Htmg]Cl, was studied by Fourier transform ion cyclotron resonance mass spectrometry in the temperature range 298-488 K and under a reduced pressure of 3.2 x 10(-6) to 1.5 x 10(-5) Pa. The results show that no charged species are present in the vapor over the condensed phase. Furthermore, ion-molecule reaction studies found no evidence of neutral ion pairs in the gas phase. This indicates that the sublimation/vaporization of [Htmg]Cl conforms to the general mechanism postulated for the distillation of protic ionic liquids, which involves a proton transfer leading to the formation of the neutral acid and base precursors, in this case hydrogen chloride and 1,1,3,3-tetramethylguanidine.

Energetics and structure of nicotinic acid (niacin).

The standard molar enthalpies of formation and sublimation of crystalline (monoclinic, space group P2(1)/c) nicotinic acid (NA), at 298.15 K, were determined as Delta(f)H(m)(o)(NA, cr) = -344.7 +/- 1.2 kJ x mol(-1) and Delta(sub)H(m)(o)(NA) = 112.1 +/- 0.5 kJ x mol(-1) by using combustion calorimetry, drop-sublimation Calvet microcalorimetry, and the Knudsen effusion method. The experimental determinations were all based on a sample of NIST Standard Reference Material 2151, which was characterized in terms of chemical purity, phase purity, and morphology. From the above results, Delta(f)H(m)(o)(NA, g) = -232.6 +/- 1.3 kJ x mol(-1) could be derived. On the basis of this value and on published experimental data, the enthalpy of the isodesmic reaction nicotinic acid(g) + benzene(g) --> benzoic acid(g) + pyridine(g) was calculated as -3.6 +/- 2.7 kJ x mol(-1) and compared with the corresponding predictions by the B3LYP/cc-pVTZ (-3.6 kJ x mol(-1)), B3LYP/aug-cc-pVTZ (-3.7 kJ x mol(-1)), B3LYP/6-311++G(d,p) (-4.2 kJ x mol(-1)), G3MP2 (-4.3 kJ x mol(-1)), and CBS-QB3 (-4.0 kJ x mol(-1)) quantum chemistry models. The excellent agreement between the experimental and theoretical results supports the reliability of the Delta(f)H(m)(o) (NA, cr), Delta(sub)H(m)(o)(NA), and Delta(f)H(m)(o)(NA, g) recommended in this work. These data can therefore be used as benchmarks for discussing the energetics of nicotinic acid in the gaseous and crystalline states and, in particular, to evaluate differences imparted to solid forms by the production and processing methods. Such differences are perhaps at the root of the significant inconsistencies found between the published enthalpies of sublimation of this important active pharmaceutical ingredient and thermochemical standard. The molecular packing in the crystalline phase studied in this work was also discussed and its influence on the molecular structure of nicotinic acid was analyzed by comparing bond distances and angles published for the solid state with those predicted by the B3LYP/cc-pVTZ method. No advantage in terms of accuracy of the structural predictions was found by the use of the larger aug-cc-pVTZ or 6-311++G(d,p) basis sets.

Energetics and structure of hydroxynicotinic acids. Crystal structures of 2-, 4-, 6-hydroxynicotinic and 5-chloro-6-hydroxynicotinic acids.

The relationship between energetics and structure in 2-, 4-, 5-, and 6-hydroxynicotinic and 5-chloro-6-hydroxynicotinic acids (2HNA, 4HNA, 5HNA, 6HNA, and 5Cl6HNA, respectively) was investigated in the solid and gaseous phases by means of a variety of experimental and computational chemistry techniques. The molecular and crystal structures of the 2HNA, 4HNA, 6HNA, and 5Cl6HNA solid forms used in this study were determined by single crystal X-ray diffraction at 293 +/- 2 K. The 2HNA, 4HNA, and 5Cl6HNA samples were monoclinic (space groups: P2(1)/n for 2HNA and P2(1)/c for 4HNA and 5Cl6HNA), and that of 6HNA was found to be triclinic (space group: P1). The 2HNA sample investigated corresponds to a new polymorphic form of this compound. The 2HNA, 4HNA, 6HNA, and 5Cl6HNA molecules crystallize as oxo tautomers exhibiting N-H and Cring=O bonds. This is also supported by the observation of bands typical of N-H and Cring=O stretching frequencies in the corresponding FT-IR spectra. The absence of these bands in the spectrum of 5HNA indicates that a hydroxy tautomer with an unprotonated N heteroatom and a Cring-OH bond is likely to be present in this case. Results of theoretical calculations carried out at the G3MP2 and CBS-QB3 levels of theory suggest that in the ideal gas phase, at 298.15 K, 2HNA favors the oxo form, 4HNA prefers the hydroxy form, and no strong dominance of one of the two tautomers exists in the case of 6HNA and 5Cl6HNA. The standard molar enthalpies of formation of 2HNA, 4HNA, 5HNA, 6HNA, and 5Cl6HNA in the crystalline state, at 298.15 K, Delta(f)H(m)(o)(cr), were determined by micro combustion calorimetry. The corresponding enthalpies of sublimation, Delta(sub)H(m)(o), were also derived from vapor pressure versus temperature measurements by the Knudsen effusion method. The obtained Delta(f)H(m)(o)(cr) and Delta(sub)H(m)(o) values led to the enthalpies of formation of 2HNA, 4HNA, 5HNA, 6HNA, and 5Cl6HNA in the gaseous phase. These were discussed together with the corresponding predictions by the B3LYP/cc-pVTZ, B3LYP/aug-cc-pVTZ, G3MP2, and CBS-QB3 methods on the basis of isodesmic or atomization reactions. The experimental "stability" order (more stable meaning a more negative Delta(f)H(m)(o)(g) value) found was 5Cl6HNA > 2HNA > 6HNA > 4HNA > 5HNA, and it was accurately captured by the CBS-QB3 and G3MP2 methods, which give 5Cl6HNA > 2HNA approximately 6HNA > 4HNA > 5HNA, irrespective of the use of isodesmic or atomization reactions. In contrast, only when well-balanced isodesmic reactions were considered did the DFT results agree with the experimental ones. The picture that emerged from the structural and energetic studies carried out in this work was also discussed in light of that typical of hydroxypyridines, which are generally regarded as the archetype systems for the study of the hydroxy <--> oxo tautomerization in N-heterocyclic compounds.

Energetics of the O-H bond and of intramolecular hydrogen bonding in HOC6H4C(O)Y (Y = H, CH3, CH2CH=CH2, C[triple bond]CH, CH2F, NH2, NHCH3, NO2, OH, OCH3, OCN, CN, F, Cl, SH, and SCH3) compounds.

The energetics of the phenolic O-H bond in a series of 2- and 4-HOC 6H 4C(O)Y (Y = H, CH3, CH 2CH=CH2, C[triple bond]CH, CH2F, NH2, NHCH 3, NO2, OH, OCH3, OCN, CN, F, Cl, SH, and SCH3) compounds and of the intramolecular O...H hydrogen bond in 2-HOC 6H 4C(O)Y, was investigated by using a combination of experimental and theoretical methods. The standard molar enthalpies of formation of 2-hydroxybenzaldehyde (2HBA), 4-hydroxybenzaldehyde (4HBA), 2'-hydroxyacetophenone (2HAP), 2-hydroxybenzamide (2HBM), and 4-hydroxybenzamide (4HBM), at 298.15 K, were determined by micro- or macrocombustion calorimetry. The corresponding enthalpies of vaporization or sublimation were also measured by Calvet drop-calorimetry and Knudsen effusion measurements. The combination of the obtained experimental data led to Delta f H m (o)(2HBA, g) = -238.3 +/- 2.5 kJ.mol (-1), DeltafHm(o)(4HBA, g) = -220.3 +/- 2.0 kJ.mol(-1), Delta f H m (o)(2HAP, g) = -291.8 +/- 2.1 kJ.mol(-1), DeltafHm(o)(2HBM, g) = -304.8 +/- 1.5 kJ.mol (-1), and DeltafHm(o) (4HBM, g) = -278.4 +/- 2.4 kJ.mol (-1). These values, were used to assess the predictions of the B3LYP/6-31G(d,p), B3LYP/6-311+G(d,p), B3LYP/aug-cc-pVDZ, B3P86/6-31G(d,p), B3P86/6-311+G(d,p), B3P86/aug-cc-pVDZ, and CBS-QB3 methods, for the enthalpies of a series of isodesmic gas phase reactions. In general, the CBS-QB3 method was able to reproduce the experimental enthalpies of reaction within their uncertainties. The B3LYP/6-311+G(d,p) method, with a slightly poorer accuracy than the CBS-QB3 approach, achieved the best performance of the tested DFT models. It was further used to analyze the trends of the intramolecular O...H hydrogen bond in 2-HOC 6H 4C(O)Y evaluated by the ortho-para method and to compare the energetics of the phenolic O-H bond in 2- and 4-HOC 6H 4C(O)Y compounds. It was concluded that the O-H bond "strength" is systematically larger for 2-hydroxybenzoyl than for the corresponding 4-hydroxybenzoyl isomers mainly due to the presence of the intramolecular O...H hydrogen bond in the 2-isomers. The observed differences are, however, significantly dependent on the nature of the substituent Y, in particular, when an intramolecular H-bond can be present in the radical obtained upon cleavage of the O-H bond.