Dissecting intermolecular interactions in the condensed phase of ibuprofen and related compounds: the specific role and quantification of hydrogen bonding and dispersion forces

Quantum Chemistry and Pharmacy: Diagnostic Check of the Thermochemistry of Ibuprofen

The thermodynamic data on ibuprofen available in the literature shows that the disarray of experimental results is unacceptable for this very important drug. The data on ibuprofens available in the literature were collected, combined with our complementary experimental results and evaluated. The enthalpies of combustion and formation of the crystalline RS-(±)- and S-(+)-ibuprofens were measured using high-precision combustion calorimetry. The temperature dependence of the vapour pressure of S-(+)-ibuprofen was measured using the transpiration method and the enthalpy of vaporization was derived from this measurement. The enthalpies of fusion of both compounds were measured using DSC. The G4 calculations have been carried out to determine the enthalpy of formation in the gaseous state of the most stable conformer. Thermochemical properties of the compounds studied were evaluated and tested for consistency with the "centerpiece approach". A set of reliable and consistent values of thermodynamic properties of ibuprofens at 298.15 K is recommended for thermochemical calculations of the pharmaceutical processes. The diagnostic protocol was developed to distinguish between the "sick" or "healthy" thermodynamic data. This diagnostic is also applicable to other drugs with a different structure than ibuprofen.

Health is the greatest wealth: Quest for a diagnostic check for thermochemistry of pure drug compounds

The development of pharmaceutical formulations and the optimisation of drug synthesis are not possible without knowledge of thermodynamics. At the same time, the quantity and quality of the available data is not at a level that meets modern requirements. A convenient diagnostic approach is desirable to assess the quality of available experimental thermodynamic data of drugs. A comprehensive set of available data on phase transitions of profens family drugs was analysed using new complementary measurements and structure-property correlations. The consistent sets of solid-gas, liquid-gas and solid-liquid phase transitions were evaluated for twelve active pharmaceutical ingredients based on alkanoic acid derivatives and recommended for the calculations of the pharmaceutical processes. A "centerpiece approach" proposed in this work helped to perform the "health check" of the thermochemical data. The evaluated data on the sublimation enthalpies were used to derive the crystal lattice energies of the profens and to correlate the water solubilities with the sublimation vapour pressures and molecular parameters. A "paper-and-pen" approach proposed in this work can be extended to the diagnosis of "sick" or "healthy" thermodynamic data for drugs with a different structure than those studied in this work.

Variation of Activation Volume as an Indicator of the Difference in Clusterization Phenomenon Induced by H-Bonding and F-Π Stacking Interactions in Enantiomers and a Racemate of Flurbiprofen

In this paper, X-ray diffraction (XRD), differential scanning calorimetry (DSC), broadband dielectric (BDS), and Fourier transform infrared (FTIR) spectroscopy supported by molecular dynamics (MD) simulations and quantum chemical computations were applied to investigate the structural and thermal properties, molecular dynamics, and H-bonding pattern of R-, S-, and RS-flurbiprofen (FLP). Experimental data indicated various spatial molecular arrangements in crystalline forms of examined systems, which seemed to disappear in the liquid state. Surprisingly, deeper analysis of high-pressure dielectric data revealed unexpected variation in the activation volume of pure enantiomers and a racemate. MD simulations showed that it is an effect of the clusterization phenomenon and a higher population of small associates in the former samples. Moreover, theoretical consideration exposed the particular role of unspecific F-Π interactions as a driving force underlying local molecular arrangements of molecules in the liquid and the crystal lattice of R-, S-, and RS-FLP.

New insights into the degradation mechanism of ibuprofen in the UV/H2O2 process: role of natural dissolved matter in hydrogen transfer reactions

Ibuprofen (IBU), a widely used antipyretic and analgesic, has been frequently detected in various natural water systems. Advanced oxidation processes (AOPs) are effective ways to remove pollutants from water. The degradation of IBU under UV/H2O2 conditions in the presence of various kinds of natural dissolved matter was investigated using density functional theory (DFT). The eco-toxicological properties were predicted based on a quantitative structure-activity relationship (QSAR) model. The calculated results showed that two H-abstraction reactions occurring at the side chain are predominant pathways in the initial reaction. H2O, NH3, CH3OH, C2H5OH, HCOOH and CH3COOH can catalyze the H transfer in the degradation process through decreasing the energy barriers and the catalysis effects follow the order of NH3 > alcohols > acids > H2O. The catalysis effects differ under acid or alkaline conditions. The overall rate coefficient of the reaction of IBU with ˙OH is calculated to be 5.04 × 109 M-1 s-1 at 298 K. IBU has harmful effects on aquatic organisms and human beings and the degradation process cannot significantly reduce its toxicity. Among all products, 2-(4-formylphenyl)propanoic acid, which is more toxic than IBU, is the most toxic with acute and chronic toxicity, developmental toxicity, mutagenicity, genotoxic carcinogenicity and irritation/corrosivity to skin. The findings in this work provide new insights into the degradation of IBU and can help to assess its environmental risks.

Noncovalent Interactions in Crowded Benzene Systems: How Much Strain Is Too Much? Attractions Overcome Repulsions!

In molecular design, large alkyl groups are used to introduce bulk and steric crowding of the catalytic center to improve catalytic efficiency and selectivity. The bulky groups are highly polarizable, increasing their ability to participate in stabilizing noncovalent interactions. The rationalization of noncovalent interaction trends is of both fundamental and practical interest as it provides new design concepts for catalysis and synthesis. Highly congested molecules always present challenges to chemists. Crowded benzene systems are an important class of compounds with well-established thermodynamic properties. The latter were used in this work to develop tools to quantify the degree of stabilization or destabilization in benzene systems crowded with bulky isopropyl and tert-butyl substituents. The basic idea was to quantify the delicate balance between repulsive and attractive interactions inherent in crowded benzene systems. The ensemble of experimental thermodynamic data and DFT-D3 calculations enabled the development of quantitative scales of the dispersion contributions and their understanding at the molecular level.

Molecular Liquids versus Ionic Liquids: The Interplay between Inter-Molecular and Intra-Molecular Hydrogen Bonding as Seen by Vaporisation Thermodynamics

In this study, we determined the enthalpies of vaporisation for a suitable set of molecular and ionic liquids using modern techniques for vapour pressure measurements, such as the quartz crystal microbalance, thermogravimetric analysis (TGA), and gas chromatographic methods. This enabled us to measure reasonable vapour pressures, avoiding the problem of the decomposition of the ionic liquids at high temperatures. The enthalpies of vaporisation could be further analysed by applying the well-known "group contribution" methods for molecular liquids and the "centerpiece" method for ionic liquids. This combined approach allowed for the dissection of the enthalpies of vaporisation into different types of molecular interaction, including hydrogen bonding and the dispersion interaction in the liquid phase, without knowing the existing species in both the liquid and gas phases.

Purely Predicting the Pharmaceutical Solubility: What to Expect from PC-SAFT and COSMO-RS?

A pair of popular thermodynamic models for pharmaceutical applications, namely, the perturbed-chain statistical associating fluid theory (PC-SAFT) equation of state and the conductor-like screening model for real solvents (COSMO-RS) are thoroughly benchmarked for their performance in predicting the solubility of active pharmaceutical ingredients (APIs) in pure solvents. The ultimate goal is to provide an illustration of what to expect from these progressive frameworks when applied to the thermodynamic solubility of APIs based on activity coefficients in a purely predictive regime without specific experimental solubility data (the fusion properties of pure APIs were taken from experiments). While this kind of prediction represents the typical modus operandi of the first-principles-aided COSMO-RS, PC-SAFT is a relatively highly parametrized model that relies on experimental data, against which its pure-substance and binary interaction parameters (k_ij) are fitted. Therefore, to make this benchmark as fair as possible, we omitted any binary parameters of PC-SAFT (i.e., k_ij = 0 in all cases) and preferred pure-substance parameter sets for APIs not trained to experimental solubility data. This computational approach, together with a detailed assessment of the obtained solubility predictions against a large experimental data set, revealed that COSMO-RS convincingly outperformed PC-SAFT both qualitatively (i.e., COSMO-RS was better in solvent ranking) and quantitatively, even though the former is independent of both substance- and mixture-specific experimental data. Regarding quantitative comparison, COSMO-RS outperformed PC-SAFT for 9 of the 10 APIs and for 63% of the API-solvent systems, with root-mean-square deviations of the predicted data from the entire experimental data set being 0.82 and 1.44 log units, respectively. The results were further analyzed to expand the picture of the performance of both models with respect to the individual APIs and solvents. Interestingly, in many cases, both models were found to qualitatively incorrectly predict the direction of deviations from ideality. Furthermore, we examined how the solubility predictions from both models are sensitive to different API parametrizations.

Thermodynamically Stable Cationic Dimers in Carboxyl-Functionalized Ionic Liquids: The Paradoxical Case of "Anti-Electrostatic" Hydrogen Bonding

We show that carboxyl-functionalized ionic liquids (ILs) form doubly hydrogen-bonded cationic dimers (c⁺=c⁺) despite the repulsive forces between ions of like charge and competing hydrogen bonds between cation and anion (c⁺–a⁻). This structural motif as known for formic acid, the archetype of double hydrogen bridges, is present in the solid state of the IL 1−(carboxymethyl)pyridinium bis(trifluoromethylsulfonyl)imide [HOOC−CH₂−py][NTf₂]. By means of quantum chemical calculations, we explored different hydrogen-bonded isomers of neutral (HOOC–(CH₂)_n–py⁺)₂(NTf₂⁻)₂, single-charged (HOOC–(CH₂)_n–py⁺)₂(NTf₂⁻), and double-charged (HOOC– (CH₂)_n−py⁺)₂ complexes for demonstrating the paradoxical case of “anti-electrostatic” hydrogen bonding (AEHB) between ions of like charge. For the pure doubly hydrogen-bonded cationic dimers (HOOC– (CH₂)_n−py⁺)₂, we report robust kinetic stability for n = 1–4. At n = 5, hydrogen bonding and dispersion fully compensate for the repulsive Coulomb forces between the cations, allowing for the quantification of the two equivalent hydrogen bonds and dispersion interaction in the order of 58.5 and 11 kJmol⁻¹, respectively. For n = 6–8, we calculated negative free energies for temperatures below 47, 80, and 114 K, respectively. Quantum cluster equilibrium (QCE) theory predicts the equilibria between cationic monomers and dimers by considering the intermolecular interaction between the species, leading to thermodynamic stability at even higher temperatures. We rationalize the H-bond characteristics of the cationic dimers by the natural bond orbital (NBO) approach, emphasizing the strong correlation between NBO-based and spectroscopic descriptors, such as NMR chemical shifts and vibrational frequencies.

The explicit role of electron exchange in the hydrogen bonded molecular complexes

We applied a set of advanced bonding descriptors to establish the hidden electron density features and binding energy characteristics of intermolecular DH∙∙∙A hydrogen bonds (OH∙∙∙O, NH∙∙∙O and SH∙∙∙O) in 150 isolated and solvated molecular complexes. The exchange-correlation and Pauli potentials as well as corresponding local one-electron forces allowed us to explicitly ascertain how electron exchange defines the bonding picture in the proximity of the H-bond critical point. The electron density features of DH∙∙∙A interaction are governed by alterations in the electron localization in the H-bond region displaying itself in the exchange hole. At that, they do not depend on the variations in the exchange hole mobility. The electrostatic interaction mainly defines the energy of H-bonds of different types, whereas the strengthening/weakening of H-bonds in complexes with varying substituents depends on the barrier height of the exchange potential near the bond critical point. Energy variations between H-bonds in isolated and solvated systems are also caused the electron exchange peculiarities as follows from the corresponding potential and the interacting quantum atom analyses complemented by electron delocalization index calculations. Our approach is based on the bonding descriptors associated with the characteristics of the observable electron density and can be recommended for in-depth studies of non-covalent bonding.

Determination of the dispersion forces in the gas phase structures of ionic liquids using exclusively thermodynamic methods

Ionic liquids are described by a delicate balance of Coulomb interaction, hydrogen bonding and dispersion forces. Dissecting the different types of interactions from thermodynamic properties is still a challenge. Here, we show that comparison of vaporization enthalpies of tetra-alkyl-ammonium ionic liquids with bis(trifluoromethylsulfonyl)imide [NTf₂]^- anions and the related molecular liquids, trialkylamines, allows for determining dispersion interactions in the gas phase ion-pairs. For this purpose, we measured vapor pressures and vaporization enthalpies of these ionic and molecular liquids by using a quartz-crystal microbalance. For supporting these data, we determined the vaporization enthalpies additionally from experimental activity coefficients at infinite dilution. Characteristic alkyl chain length dependences of the vaporization enthalpies have been established and were used for quantifying the dispersion forces in the gas phase species. The dissected dispersion contributions suggest that the alkyl chains do not show star-like topologies but embrace the anion maximizing the dispersion interactions. For the longest alkyl chains with eight carbon atoms, the dispersion interaction is as strong as two and a half hydrogen bonds. The proportion of dispersion in the gas phase species depending on the number of methylene groups in the ammonium cations is strongly supported by quantum chemical calculations.

Inter- vs. Intramolecular Hydrogen Bond Patterns and Proton Dynamics in Nitrophthalic Acid Associates

Noncovalent interactions are among the main tools of molecular engineering. Rational molecular design requires knowledge about a result of interplay between given structural moieties within a given phase state. We herein report a study of intra- and intermolecular interactions of 3-nitrophthalic and 4-nitrophthalic acids in the gas, liquid, and solid phases. A combination of the Infrared, Raman, Nuclear Magnetic Resonance, and Incoherent Inelastic Neutron Scattering spectroscopies and the Car-Parrinello Molecular Dynamics and Density Functional Theory calculations was used. This integrated approach made it possible to assess the balance of repulsive and attractive intramolecular interactions between adjacent carboxyl groups as well as to study the dependence of this balance on steric confinement and the effect of this balance on intermolecular interactions of the carboxyl groups.