On the Lewis Basicity of Phosphoramides: A Critical Examination of Their Donor Number through Comparison of Enthalpies of Adduct Formation with SbCl5 and BF3

The Lewis basicity of a series of phosphoryl compounds was examined using DFT and ab initio methods, including solvation effects. The enthalpies of adduct formation with two archetypal Lewis acids, antimony pentachloride and boron trifluoride, used to define the donor number DN and the BF₃ affinity (BF₃A) respectively, were examined. The BF₃ adducts allow the use of the high-accuracy G4 approach, whereas for SbCl₅ adducts, three different DFT formalisms, including empirical dispersion corrections, were used because the G4 formalism is not available for third-row elements. For a comparison with experimental data, solvation effects were taken into account by using the polarizable continuum model. The experimental BF₃ affinities were well reproduced by G4 calculations when including PCM solvation. Conversely, comparisons of our calculated values and experimental results reported in the literature show that SbCl₅ enthalpies for phosphoramides are in error. In particular the DN for HMPA should be revised.     

The Strongest Acid: Protonation of Carbon Dioxide

The strongest carborane acid, H(CHB₁₁F₁₁), protonates CO₂ while traditional mixed Lewis/Brønsted superacids do not. The product is deduced from IR spectroscopy and calculation to be the proton disolvate, H(CO₂)₂⁺. The carborane acid H(CHB₁₁F₁₁) is therefore the strongest known acid. The failure of traditional mixed superacids to protonate weak bases such as CO₂ can be traced to a competition between the proton and the Lewis acid for the added base. The high protic acidity promised by large absolute values of the Hammett acidity function (H₀) is not realized in practice because the basicity of an added base is suppressed by Lewis acid/base adduct formation.     

Solvation of alkyllithium compounds. Steric effects on heats of interaction of bases with hexameric versus tetrameric alkyllithiums

Heats of interaction of Lewis bases with hexameric and tetrameric alkyllithiums in hydrocarbon solution at 25° have been determined by high dilution solution calorimetry at low base to lithium atom ratios. The Lewis bases utilized include tetrahydrothiophene, tetrahydrofuran, triethylphosphine, triethylamine, and diethyl ether. The organolithiums investigated were $\text{n}$-butyllithium, ethyllithium, isopropyllithium, trimethylsilylmethyllithium, and $\text{t}$-butyllithium. The basicity order based on initial enthalpies of interaction is independent of the alkyllithium compound. Larger enthalpies of interaction were observed for the tetrameric $\text{versus}$ hexameric alkyllithiums with the exception of tetrameric $\text{t}$-butyllithium which does not interact significantly with these bases. The sensitivities of the enthalpies to the steric requirements of the base were probed by comparison of the enthalpies for tetrahydrofuran, 2-methyltetrahydrofuran, and 2,5-dimethyltetrahydrofuran. Base coordination to hexameric $\text{n}$-butyllithium is more sensitive to the steric requirements of the tetrahydrofuran bases than is coordination to tetrameric trimethylsilylmethyllithium or isopropyllithium. These results are interpreted in terms of coordination of tetrahydrofuran bases to the intact hexameric aggregate for $\text{n}$-butyllithium; however, it is concluded that the corresponding interaction with hexameric trimethylsilylmethyllithium leads directly to base-solvated tetramers.     

The Affinity of Some Lewis Bases for Hexafluoroisopropanol as a Reference Lewis Acid: An ITC/DFT Study

To figure out the possible role of 1,1,1,3,3,3-hexafluoropropan-2-ol (HFIP) as well as to provide reference thermochemical data in solution, the formation of Lewis acid-base complexes between HFIP (Lewis acid) and a series of 8 different Lewis bases (3 sulfoxides, 3 Nsp² pyridine derivatives, 1 aromatic amine, 1 cyclic aliphatic ether) was examined by isothermal titration calorimetry (ITC) experiments and static density functional theory augmented with Dispersion (DFT−D) calculations. Measured ITC association enthalpy values (ΔH_a) lie between −9.3 and −14 kcal mol⁻¹. Computations including a PCM implicit solvation model produced similar exothermicity of association of all studied systems compared to the ITC data with ΔH_a values ranging from −8.5 to −12.7 kcal mol⁻¹. An additional set of calculations combining implicit and explicit solvation by chlorobenzene of the reactants, pointed out the relatively low interference of the solvent with the HFIP-base complexation: its main effect is to slightly enhance the Gibbs energy of the HFIP-Lewis base association. It is speculated that the interactions of bulk HFIP with Lewis bases therefore may significantly intervene in catalytic processes not only via the dynamic microstructuring of the medium but also more explicitly by affecting bonds’ polarization at the Lewis bases.     

The thermochemistry of solvation of imidazolium-based ionic liquids in benzene

ABSTRACT

The present work is devoted to the thermochemical study of solvation of ionic liquids (IL) in benzene. The solution enthalpies of 1-ethyl-3-methylimidazolium tricyanomethanide [EMIM][C(CN)₃], 1-butyl-3-methylimidazolium tetrafluoroborate [BMIM][BF₄], 1-hexyl-3-methylimidazolium hexafluorophosphate [HMIM][PF₆], 1-octyl-3-methylimidazolium tetrafluoroborate [OMIM][BF₄], 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [EMIM][NTf₂], 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [BMIM][NTf₂] and 1-butyl-3-methylimidazolium trifluoromethanesulfonate [BMIM][TfO] in benzene were measured. The solvation enthalpies of imidazolium-based IL were calculated. Molar refractions of imidazolium-based IL form literature data on density and refractive indexes of IL were also calculated. The linear correlation between solvation enthalpy and molar refraction of IL was observed. This correlation can be used to calculate the vaporization enthalpy of imidazolium-based IL from solution calorimetry data.     

Effect of Organic Donor on the Enthalpy of Its Mixing with Gallium Chloride

Enthalpies of solution of gallium chloride in organic solvents are controlled by their basicity. The enthalpies of mixing with such a weak electron acceptor as tetracyanoethylene are influenced by both specific and nonspecific solvation.     

Activation of the B−F Bond by Diphenylcarbene: A Reversible 1,2‐Fluorine Migration between Boron and Carbon

Experiments in low‐temperature matrices reveal that triplet diphenylcarbene inserts into the very strong B−F bond of BF₃ in a two‐step reaction. The first step is the formation of a strongly bound Lewis acid–base complex between the singlet state of diphenylcarbene and BF₃. This step involves an inversion of the spin state of the carbene from triplet to singlet. The second step requires visible‐light photochemical activation to induce a 1,2‐F migration from boron to the adjacent carbon atom under formation of the formal insertion product of the carbene center into BF₃. The 1,2‐F migration is reversible under short‐wavelength UV irradiation, thus leading back to the Lewis acid–base adduct.     

Lewis acidity of methyltrioxorhenium(VII) (MTO) based on the relative binding strengths of N-donors

This article presents a sigma acceptor strength scale for methyltrioxorhenium(VII) (MTO), one of the most versatile and useful high oxidation state organometallics ever described. The spectrophotometric titration of MTO with a series of N-donor bases in CCl(4) gives formation constants (K(f)) and enthalpies for the adduct formation reactions. An excellent linearity of log K(f) with respect to the Hammett sigma constants of the substituents on the ligands was observed. The resulting rho constant is proposed to be a good indication of the Lewis acidity of MTO. The enthalpies of adduct formation of N-donors with MTO also fit the ECW model to predict the values of E(A) and C(A) parameters for MTO. The parameters can be used to predict an acidity scale for MTO. These parameters also allow the chemists to predict and correlate quantitatively the enthalpies of MTO.Lewis base interactions. Significant chemical insights result from the fit of the data to the ECW model.     

Thermodynamic investigations on derivatives of pyrimidine nucleic acid bases: Joint use of calorimetric, volumetric and structural data for the description of properties of pyrimidine nucleic acid bases and their derivatives

The experimental enthalpies of solution Δ_solH_m^∞, van’t Hoff enthalpies of sublimation Δ_s^gH_m⁰ of solid compounds, partial molar volumes V₂⁰, and partial molar heat capacities C_p,2⁰ of aqueous solutions of pyrimidine nucleic acid bases and their derivatives, determined previously and collected here, are discussed in terms of calculated structural parameters. Relations have been established between the calorimetric and volumetric properties. Correlations have been developed to relate both the enthalpies of solvation and the partial molar heat capacities to the polar and apolar parts of the accessible molecular surface areas.     

Phase diagrams for the YBa2Cu3O7 family — ANNO 1996

Phase diagrams for cuprates of alkaline earth and rare earth elements are presented, covering binary to quintenary oxides and including selected solid solution series with other elements. Elementary crystal chemical data are included for identification of the occurring phases. Chemical stability is discussed with respect to the high-temperature reactions with Lewis acids like CO₂, protons, etc. Particularly the occurrence of oxide carbonates is consistently pointed out as one of the possible reasons for contradictory results in phase diagrams which comprise oxides with high basicity.     

Stand up for Electrostatics: The Disiloxane Case

The basicity of the simplest silicone, disiloxane (H₃Si−O−SiH₃), is strongly affected by the Si−O−Si angle (α). We use high-level ab initio MP2/aug′-cc-pVTZ calculations and the molecular electrostatic potential (MEP) to analyze the relationship between the increase in basicity and the reduction of α. Our results clearly point out that this increase can be explained through the MEP, as the interactions between oxygen from disiloxane and the acceptors are mostly electrostatic. Furthermore, the effect of α on the tetrel bond between disiloxane and several Lewis bases can again be rationalized using the MEP. Finally, we explore the cooperativity throughout α for ternary complexes where disiloxane simultaneously interacts with a Lewis acid and a Lewis base. Both non-covalent interactions remain cooperative for all α values, although the largest cooperativity effects are not always those maximizing the binding energy in the binary complexes. Overall, the MEP remains a powerful predictor for noncovalent interactions.     

Updated Abraham model correlations for enthalpies of solvation of organic solutes dissolved in benzene and acetonitrile

A search of the published chemical and engineering literature found enthalpy of solution data for an additional 104 and 49 organic compounds dissolved in benzene and acetonitrile, respectively. Standard thermodynamic relationships were used to convert the experimental enthalpy of solution data, ΔH_solv, to enthalpies of solvation, ΔH_solv. Updated Abraham model correlations were derived for describing gas-to-benzene and gas-to-acetonitrile enthalpies of solvation by combining the 104 and 49 additional values to existing benzene and acetonitrile ΔH_solv databases. The updated Abraham model correlations for benzene and acetonitrile described the observed ΔH_solv values to within overall standard deviations of less than 3.4 kJ mol^?1.     

Borohydrides from Organic Hydrides: Reactions of Hantzsch’s Esters with B(C6F5)3

We report herein that the reaction between a series of Hantzsch’s ester analogues 1 a – d with the Lewis acidic species B(C₆F₅)₃ results in facile transfer of hydride to boron. The main products of this reaction are pyridinium borohydride salts 2 a – d , which are obtained in high to moderate yields. The N‐substituted substrates (N‐Me, N‐Ph) reacted in high yield 90–98 % and the connectivity of the products were confirmed by an X‐ray crystallographic analysis of the N‐Me borohydride salt 2 a . Unsubstituted Hanztsch’s ester 1 a reacted less effectively generating only 60 % of the corresponding borohydride salt, with the balance of the material sequestered as the ester‐bound Lewis acid–base adduct 3 a . Formation of the Lewis acid–base adduct could be minimized by increasing the steric bulk about the ester groups as in 1 d . The connectivity of the carbonyl‐bound adduct was confirmed by an X‐ray crystallographic analysis of 3 e the product of the reaction of methyl ketone 1 e with B(C₆F₅)₃. We also explored the generation of these pyridinium salts by employing frustrated Lewis pair methodology. However, the reaction of mixtures of the corresponding pyridine and B(C₆F₅)₃ with hydrogen gas only resulted in formation of trace amounts of the pyridinium borohydride, along with the Lewis acid–base adduct of the starting material and B(C₆F₅)₃. The 1,2‐dihydropyridine adduct was the final product of this reaction. This was ascribed to the low basicity of the pyridine nitrogen and the complicating formation of an ester bound Lewis acid–base adduct.     

取代芳烃水中溶解度的测定与估算

本文应用线性溶剂化能相关法回归分析了多种有机有机物的溶解度与范德华体积、Ｌｅｗｉｓ酸性及Ｌｅｗｉｓ碱性之间的相关性，结果表明，溶解度与范德华体积、Ｌｅｗｉｓ酸性及碱性之间呈良好的相关性，用这种方法估算的溶解度值与测定吻合得较好。     

Cationic Symmetrically and Unsymmetrically Substituted Diboranes and Bis(diboranes) with Direct Boron-Boron Bond: Synthesis by Substitution,Stability and Properties

Starting with diboranes with two electron-rich bridging bicyclic guanidinate substituents, we report in this work the rational synthesis of new dicationic symmetrically- and unsymmetrically-substituted diboranes in S_N1-type substitution reactions in which triflato or bromo substituents are replaced by neutral Lewis bases. The scope of such substitution reactions and their rate are analyzed with different pyridine derivatives of variable Lewis basicity. The first substitution step, leading to a monocationic diborane with one anionic substituent (triflate or bromide) and one neutral Lewis base, proceeds much faster than the second substitution step leading to a dicationic diborane with two neutral Lewis bases. The different time scales for the substitution steps could be used to conveniently synthesize in one-pot reactions several dicationic, unsymmetrically-substituted diboranes with two different neutral Lewis bases.     

The boron trifluoride nitromethane adduct

The separation of the boron isotopes using boron trifluoride·organic-donor, Lewis acid·base adducts is an essential first step in preparing ¹⁰B enriched and depleted crystalline solids so vital to nuclear studies and reactor applications such as enriched MgB₂, boron carbide, ZrB₂, HfB₂, aluminum boron alloys, and depleted silicon circuits for radiation hardening and neutron diffraction crystal structure studies. The appearance of this new adduct with such superior properties demands attention in the continuing search for more effective and efficient means of separation. An evaluation of the boron trifluoride nitromethane adduct, its thermodynamic and physical properties related to large-scale isotopic separation is presented. Its remarkably high separation factor was confirmed to be higher than the expected theoretical value. However, the reportedly high acid/donor ratio was proven to be an order of magnitude lower. On-going research is determining the crystal structure of deuterated and ¹¹B enriched ¹¹BF₃·CD₃NO₂ by X-ray and neutron diffraction.     

Enthalpies of solution of n-alcohols in formamide and ethylene glycol

The enthalpies of solution of 1-pentanol, 1-hexanol and 1-heptanol and water were determined in formamide and in ethylene glycol, at 25‡C, by macrosolution calorimetry. The observed enthalpies of solution for the n-alcohols are positive in the two solvents, and of similar magnitude. The enthalpy of solution of water is positive in formamide, and negative in ethylene glycol. From the enthalpies of solution, the enthalpies of solvation and the enthalpies of transfer for organic solvent↿ water were calculated. Using our values and literature data for alkanols, it was possible to see that both the enthalpies of solution and the enthalpies of solvation presented a constant CH₂ increment for the entire series, in contrast with their behavior in water. The methylene increments for these properties in different solvents are compared with parameters considered to reflect the cohesive energy of the solvent.     

Solvent effects on hydrogen bond formation

Enthalpies of solution have been used to calculate transfer enthalpies for phenol, pyridine, and DMSO between the solvent cyclohexane and the solvents CCl₄, benzene, and CHCl₃. By use of model compounds, enthalpies due to interactions with phenol, pyridine, and DMSO have been determined. These enthalpies are used to calculate the effect of solvation relative to cyclohexane on hydrogen bonded complexes in CCl₄ and benzene solvents. Correlations with enthalpies due to interactions and frequency shifts for the hydroxyl stretch in these solvents have also been made.     

Enthalpies of 1,4-Dioxane, 1,2-Dimethoxyethane, and Ethylacetate Solvation in the Water–1-Propanol and Water–Glycerol Binary Mixtures

The enthalpies of 1,4-dioxane, 1,2-dimethoxyethane, and ethylacetate solution in binarymixtures of water with 1-propanol and glycerol were measured at 25°C using a precise isoperibol calorimeter. The enthalpies of the solute solvation were calculated and compared with the experimental data for other solutes. The results obtained were interpreted in terms of universal and specific solute solvation using parameters of a solvent polarity. It was found that the extreme shape of the curve _solv H° vs. X for ethylacetate in the mixtures of water with 1-propanol results from peculiarities of carbotylate-group solvation and appears to be not connected with the influence of alcohol microaggregates in the mixed solvent.