Nearest-Neighbor and Non-Nearest-Neighbor Interactions between Substituents in the Benzene Ring. Experimental and Theoretical Study of Functionally Substituted Benzamides

Standard molar enthalpies of formation of 2- and 4-hydroxy­benz­amides were measured by combustion calorimetry. Vapor pressures of benz­amide and 2-hydroxy­benz­amide were derived by the transpiration method. Standard molar enthalpies of sublimation or vaporization of these compounds at 298 K were obtained from vapor pressure temperature dependence. Thermochemical data on benz­amides with hydroxyl, methyl, methoxy, amino, and amide substituents were collected, evaluated, and tested for internal consistency. The high-level G4 quantum-chemical method was used for mutual validation of the experimental and theoretical gas-phase enthalpies of formation. Sets of nearest-neighbor and non-nearest-neighbor interactions between substituents in the benzene ring have been evaluated. A simple incremental procedure has been suggested for a quick appraisal of the vaporization and gas-phase formation enthalpies of the substituted benz­amides

Thermochemical Properties of Xanthine and Hypoxanthine Revisited

The standard molar enthalpies of formation of xanthine and hypoxanthine were measured by using high-precision combustion calorimetry. The standard molar enthalpies of sublimation of these compounds at 298.15 K were derived by the quartz-crystal microbalance technique. Limited thermodynamic data available in the literature are compared with our new experimental data. In addition, we use the G4 method to calculate the molar enthalpies of formation of xanthine and hypoxanthine in the gas phase. There is good agreement between the evaluated experimental data and the quantum-chemical calculations

Biomass-Derived Platform Chemicals: Thermodynamic Studies on the Extraction of 5‑Hydroxymethylfurfural from Ionic Liquids

Activity coefficients at infinite dilution, γ_<i>i</i>^∞, of 13 solutes such as alkanes, alkenes, alkylbenzenes, alcohols, esters, and ethers in six 1,3-dialkylimidazolium- or tetraalkylphosphonium-based ionic liquids have been determined by gas chromatography using the ionic liquids as the stationary phase. Furthermore, the solubility of 5-hydroxymethylfurfural (HMF) in these solutes and the solubility of the solutes in 1-butyl-3-methylimidazolium methanesulfonate ([C₄mim]­[CH₃SO₃]) was assessed. The combination of these data allowed for the interpretation of prevailing interactions on molecular level and resulted in the hypothesis that an ideal extracting agent must feature hydrogen bond acceptor properties to obtain high extraction efficiencies of hydrogen bond donor molecules such as HMF from this ionic liquid. Extraction data obtained using the thus proposed extracting agents demonstrated that this hypothesis was correct and can in future be transposed to other separation problems. In the case of a multifunctional molecule such as HMF, extraction efficiencies are however in general low and can only be little improved by removing other potential interaction sites from the ionic liquid’s cation: hence, in the <i>N</i>-ethyl-<i>N</i>-methylpyrrolidinium analogue [C₂C₁pyr]­[CH₃SO₃], π–π-interactions between cation and HMF cannot form, which increases the extraction efficiency somewhat

Liquid Organic Hydrogen Carriers: An Upcoming Alternative to Conventional Technologies. Thermochemical Studies.

A system based on the catalytic hydrogenation/dehydrogenation reactions of <i>N</i>-ethylcarbazole is one of the most promising as the new class of the liquid organic hydrogen carrier (LOHC) compounds. Enthalpy of formation of the liquid dodecahydro-<i>N</i>-ethylcarbazole (fully hydrogenated <i>N</i>-ethylcarbazole) was measured using combustion calorimetry. Vaporization enthalpy for this compound was derived from vapor pressure–temperature dependence measured by transpiration. The enthalpy of formation of the gaseous dodecahydro-<i>N</i>-ethylcarbazole was derived and validated with the high-level quantum chemical calculation. Vapor pressures of the liquid <i>N</i>-ethylcarbazole (0.0008 bar) and dodecahydro-<i>N</i>-ethylcarbazole (0.01 bar) at a practical and relevant temperature (400 K) were assessed from the new experimental data. It has turned out that these vapor pressures were low enough to fulfill the basic requirement for an LOHC

Transfer Hydrogenation as a Redox Process in Nucleotides

Using a combined theoretical and experimental strategy, the heats of hydrogenation of the nucleotide bases uracil, thymine, cytosine, adenine, and guanine have been determined. The most easily hydrogenated base is uracil, followed by thymine and cytosine. Comparison of these hydrogenation enthalpies with those of ketones and aldehydes derived from sugar models indicates the possibility of near-thermoneutral hydrogen transfer between uracil and the sugar phosphate backbone in oligonucleotides

Ionic Liquids: Differential Scanning Calorimetry as a New Indirect Method for Determination of Vaporization Enthalpies

Differential scanning calorimetry (DSC) has been used to measure enthalpies of synthesis reactions of the 1-alkyl-3-methylimidazolium bromide [C_<i>n</i>mim]­[Br] ionic liquids from 1-methylimidazole and <i>n</i>-alkyl bromides (with <i>n</i> = 4, 5, 6, 7, and 8). The optimal experimental conditions have been elaborated. Enthalpies of formation of these ionic liquids in the liquid state have been determined using the DSC results according to the Hess Law. The ideal-gas enthalpies of formation of [C_<i>n</i>mim]­[Br] were calculated using the methods of quantum chemistry. They were used together with the DSC results to derive indirectly the enthalpies of vaporization of the ionic liquids under study. In order to validate the indirect determination, the experimental vaporization enthalpy of [C₄mim]­[Br] was measured by using a quartz crystal microbalance (QCM). The combination of reaction enthalpy measurements by DSC with modern high-level first-principles calculations opens valuable indirect thermochemical options to obtain values of vaporization enthalpies of ionic liquids

Benchmark Thermodynamic Properties of Methyl- and Methoxybenzamides: Comprehensive Experimental and Theoretical Study

The enthalpies of formation of 2-, 3-, and 4-CH₃-benzamide, as well as for 2-CH₃O-benzamide, were measured by using combustion calorimetry. Vapor pressures of the isomeric CH₃- and CH₃O-benzamides were measured by using the transpiration method. The enthalpies of sublimation/vaporization of these compounds at 298 K were obtained from temperature dependencies of vapor pressures. The enthalpies of solution of the isomeric CH₃- and CH₃O-benzamides were measured with solution calorimetry. The enthalpies of sublimation of m- and p-substituted benzamides were independently derived with help of a solution calorimetry-based procedure. The enthalpies of fusion of the CH₃-benzamides were derived from differential scanning calorimetry measurements. Thermochemical data on CH₃- and CH₃O-benzamides were collected, evaluated, and tested for internal consistency. A simple incremental procedure was suggested for a quick appraisal of vaporization enthalpies of substituted benzamides. The high-level G4 quantum-chemical method was used for mutual validation of the experimental and theoretical gas-phase enthalpies of formation. A remarkable ability of the G4-based atomization procedure to calculate reliable enthalpies of formation was established for the set of aliphatic and aromatic amides. An outlook for the proper validation of the G4-AT procedure was discussed

Structure–Property Relationships in Ionic Liquids: A Study of the Influence of N(1) Ether and C(2) Methyl Substituents on the Vaporization Enthalpies of Imidazolium-Based Ionic Liquids

In this work, the QCM and TGA methods were used concurrently to study the two alkoxy-substituted ionic liquid (IL) series: 1-[oligo­(ethylene glycol)]-3-methylimidazolium bis­(triflamide) ([P_<i>x</i>mim]­[NTf₂]) and 1-[oligo­(ethylene glycol)]-2,3-dimethylimidazolium bis­(triflamide) ([P_<i>x</i>mmim]­[NTf₂]). For comparison, enthalpies of vaporization measured at elevated temperatures were adjusted to the reference temperature 298 K and tested for consistency. It was found that the vaporization enthalpies of the alkoxy-substituted ILs are significantly lower than those of the analogous ILs with the alkyl-substituted cation. This is in contrast to molecular solvents, for which alkoxy groups are typically observed to increase vaporization enthalpy relative to those of the hydrocarbon analogues. Two useful group contributions for the quick estimation of vaporization enthalpies of various alkoxy-substituted IL cations (e.g., imidazolium, ammonium, pyridinium) are recommended based on the findings of this work

Asymmetric Hydrogenation of Nonfunctionalized Olefins in Propylene CarbonateKinetic or Thermodynamic Control?

Iridium-catalyzed hydrogenations of nonfunctionalized olefins in propylene carbonate as the solvent allow efficient catalysis with much higher enantioselectivities in comparison with dichloromethane which is usually employed for these reactions. Experimental and computational studies of the hydrogenation of 1-methylene-1,2,3,4-tetrahydronaphthalene have been performed to understand the limitation for this reaction

Building Blocks for Ionic Liquids: Vapor Pressures and Vaporization Enthalpies of N‑Functionalized Imidazoles with Branched and Cycloalkyl Substituents

The imidazole structure offers a versatile means of developing molecules with controlled/tunable physicochemical properties that have significant utility in many applications and can be further derivatized to form ionic liquids. In the literature, the vast majority of studies on structure–property relationships in these types of molecules are devoted to linear (e.g., <i>n</i>-alkyl) substituents. However, imidazoles with branched or cycloalkyl groups are equally accessible through convenient synthetic methods – yet there are essentially no reports on the physical properties of such compounds in the literature. Here, the absolute vapor pressures of branched and cycloalkyl derivatives of imidazole have been determined as a function of temperature by the transpiration method. The standard molar enthalpies of vaporization were derived from the temperature dependences of vapor pressures. The measured data sets were successfully checked for internal consistency by comparison with vaporization enthalpies of the parent species, and a group contribution method is put forth by which the vaporization enthalpies of imidazoles, and imidazolium-based ILs, with alkyl groups in any configuration can be rapidly predicted