Hydrogen-bonding Solvents Research Articles

A Study of the Reichardt $$E_{{\text{T}}}^{{\text{N}}} \left( {30} \right)$$ Parameter using Solvent Molecular Properties Derived from Computation Chemistry and Consideration of the Kamlet and Taft α Scale of Solvent Hydrogen Bond Donor Acidities

Reichardt’s normalized $$E_{{\text{T}}}^{{\text{N}}} \left( {30} \right)$$ parameter for solvent polarity has been analyzed in terms of properties of solvent molecules estimated from quantum–mechanical calculations of isolated solvent molecules. The analyses show that $$E_{{\text{T}}}^{{\text{N}}} \left( {30} \right)$$ has a strong dependence on the partial charge on the most positive hydrogen atom in the solvent molecule, reflecting hydrogen bonding at the pendant oxygen atom of the betaine dye used to define the ET(30) scale. There are smaller, and roughly equal, dependences on the dipole moments and quadrupolar amplitudes of the solvent molecules and an inverse dependence on the solvent polarizability. These three dependences reflect the solvent polarity, that is, the ability to stabilize charge through longer-range interactions. The reason for the inverse dependence on the solvent polarizability is unclear, but a similar dependence was found previously in the analysis of the Kamlet, Abboud and Taft π* scale. The resulting equation for $$E_{{\text{T}}}^{{\text{N}}} \left( {30} \right)$$ reproduces the experimental values for around 160 solvents, representing most classes of organic solvents, with a standard deviation of around 0.07 { $$E_{{\text{T}}}^{{\text{N}}} \left( {30} \right)$$ values range from 0 to 1}. The Kamlet and Taft α scale of hydrogen bond donor acidities, which is, in effect, derived from the differences between the $$E_{{\text{T}}}^{{\text{N}}} \left( {30} \right)$$ and π* values for a solvent, is discussed. The results of the present analyses of $$E_{{\text{T}}}^{{\text{N}}} \left( {30} \right)$$ and earlier analyses of π* indicate that, while the α values capture the effect of solvent hydrogen bond donor acidity, it also contains residual dependences on other molecular properties. These residual dependences result from the differences in the dependences of the $$E_{{\text{T}}}^{{\text{N}}} \left({30} \right)$$ and π* on solvent properties.

N-Heterocyclic Carbene Organocatalysis: With or Without Carbenes?

In this work the mechanism of the aldehyde umpolung reactions, catalyzed by azolium cations in the presence of bases, was studied through computational methods. Next to the mechanism established by Breslow in the 1950s that takes effect through the formation of a free carbene, we have suggested that these processes can follow a concerted asynchronous path, in which the azolium cation directly reacts with the substrate, avoiding the formation of the carbene intermediate. We hereby show that substituting the azolium cation, and varying the base or the substrate do not affect the preference for the concerted reaction mechanism. The concerted path was found to exhibit low barriers also for the reactions of thiamine with model substrates, showing that this path might have biological relevance. The dominance of the concerted mechanism can be explained through the specific structure of the key transition state, avoiding the liberation of the highly reactive, and thus unstable carbene lone pair, whereas activating the substrate through hydrogen‐bonding interactions. Polar and hydrogen‐bonding solvents, as well as the presence of the counterions of the azolium salts facilitate the reaction through carbenes, bringing the barriers of the two reaction mechanisms closer, in many cases making the concerted path less favorable. Thus, our data show that by choosing the exact components in a reaction, the mechanism can be switched to occur with or without carbenes.

Solvent Dependence of Cationic-Exciplex Emission: Limitation of Solvent Polarity Functions and the Role of Hydrogen Bonding.

Conventional exciplexes are products of excited-state charge generation reactions between neutral reactants (e.g., A* + D → A•-D•+), whereas cationic exciplexes are products of charge shift reactions of cations with neutral donors (e.g., A+* + D → A•D•+). Compared herein is the solvent-dependent fluorescence of a cationic exciplex with extant data for conventional exciplexes. Although linear correlations of conventional exciplex emission maxima with the Lippert-Mataga solvent polarity function are well documented in low to moderate polarity solvents, the correlations are often poor in more polar solvents. A number of such plots in moderate to high polarity solvents show a strong curvature. Intriguingly, for these same cases, plots of emission maxima versus the solvent dielectric constant (ε) are remarkably linear. Interestingly, emission maxima for the cationic exciplex of 1+ in nitrile and alcohol solvents also correlate linearly with ε. The solvent dependency for cationic exciplex emission maxima on ε is ca 1/3 of that for conventional exciplexes, which is ascribed to solvent stabilization of both the excited state and the ground state for cationic exciplexes. Differences in exciplex emissions between nitrile and alcohol solvents for 1+ are attributed to hydrogen bonding, with larger differences in higher hydrogen-bond acidity solvents.

Electrochemically Controlled Polymerization through the Use of Redox-Responsive Ureidopyrimidinones for Supramolecular Polymers.

Supramolecular polymers are constructed along weaker, non-covalent interactions such as hydrogen-bonding. Compared to conventional polymers where monomer units react to form covalent bonds, non-covalent interactions of monomer units grant these supramolecular polymers dynamic behavior. One example of these systems that utilizes non-covalent interactions to construct supramolecular polymers are the well-investigated ureidopyrimidinones, UPy. UPy’s dimerize along their 4 strong directional hydrogen bonds in non-polar organic solvents. UPy’s have been covalently linked at the ends of smaller monomer chains so as to function as chain-linkers in the construction of larger polymer structures under favorable dimerization conditions. One interest in the use of these systems in supramolecular applications is their inherent ability of self-healing. Applying heat or mechanical stress to these systems will prevent polymerization at the H-bonds. However, upon relief of these external stimuli, H-bonding resumes. In this study, the use of electron transfer as an external stimulus to perturb dimerization and affect UPy chain polymerization will be explored. In order to exemplify the aims of this study, previously studied electroactive UPy’s will be investigated as redox-responsive chain terminators with regards to linear polymer chains that are functionalized with UPy units at their terminal ends, allowing for long-chain polymerization indicated by an increase in solution viscosity. In this study, a decrease in solution viscosity should be observed once these long chain bi-functionalized UPy polymers are in the presence of redox-responsive mono-functional UPy. Upon electrochemical oxidation / reduction of the mono-functional electroactive UPy units, an increase in viscosity is expected. The weakened H-bonding of the mono-functional UPy units will prevent chain termination and long-chain polymerization between bi-functional long chain UPy’s is expected to resume.

The effect of solvents on solid-liquid phase equilibrium of 1,3-Di-o-tolylguanidine

The crystal structure of 1,3-Di-o-tolylguanidine (DOTG) were investigated, as well as the thermodynamic properties about the solid-liquid phase equilibrium. The intermolecular interactions examined by Hirshfeld surface analysis demonstrate that the N-H···N and C-H···C interactions are dominant in the solid-state structure of DOTG. The solubility of DOTG in thirteen pure solvents (ethanol, 1-propanol, 2-propanol, 1-butanol, isobutanol, 2-butanol, 1-pentanol, propyl acetate, isopropyl acetate, acetone, butanone, acetonitrile, 1,4-dioxane) was measured by a gravimetric method from 283.15 K to 323.15 K at atmospheric pressure. According to the experimental results, the solubility of DOTG in all solvents increase with the increase of temperature. The determined solubility data were correlated using the Apelblat equation, λh equation, van't Hoff equation and the NRTL model. Additionally, the effects of solvent properties on the solubility of DOTG were discussed in terms of solvent polarity, hydrogen bond and cohesive energy density. Furthermore, the mixing thermodynamic properties of DOTG were studied in selected solvents based on the NRTL model, showing that the process of DOTG dissolving is spontaneous and entropy-driving. • The solubility of DOTG increases with temperature in thirteen systems. • The experimental data was well correlated by the Apelblat, van't Hoff, λh and NRTL models. • Thermodynamic analyses were performed based on functions of mixing. • The structure properties of DOTG crystal were investigated.

The curious case of SDS self-assembly in glycerol: Formation of a lamellar gel

HypothesisHydrogen-bonding capacities of polar nonaqueous media significantly affect self-assembly behaviours of surfactants in these media. IntroductionGlycerol, a nonaqueous hydrogen-bonding solvent, is widely used in industrial formulations due to its desirable physical properties. Surfactants are ubiquitous in such applications; however, surfactant self-assembly in glycerol is not well understood. MethodsThe microscopic structure of the gel phase was studied using a series of imaging techniques: polarised light microscopy (PLM), confocal laser scanning microscopy (CLSM), and environmental scanning electron microscopy (ESEM). The rheological properties of the gel were studied using viscometry and oscillation rheology measurements. Further nano-structural characterisation was carried out using small-angle neutron scattering (SANS). ResultsWe have observed the unexpected formation of a microfibrillar gel in SDS and glycerol mixtures at a critical gelation concentration (CGC) as low as ~2 wt%; such SDS gelation has not been observed in aqueous systems. The microscopic structure of the gel consisted of microfibres some mm in length and with an average diameter of D ~ 0.5 μm. The fibres in the gel phase exhibited shear-induced alignment in the viscometry measurements, and oscillation tests showed that the gel was viscoelastic, with an elastic-dominated behaviour. Fitting to SANS profiles showed lamellar nano-structures in the gel microfibres at room temperature, transforming into cylindrical-micellar solutions above a critical gelation temperature, TCG ~ 45 °C. ConclusionsThese unprecedented observations highlight the markedly different self-assembly behaviours in aqueous and nonaqueous H-bonding solvents, which is not currently well understood. Deciphering such self-assembly behaviour is key to furthering our understanding of self-assembly on a fundamental level.

Solvophobically Driven Complexation of Adamantyl Mannoside with β-Cyclodextrin in Water and Structured Organic Solvents.

The effects of solvent and temperature on the complexation of adamantyl mannoside with β-cyclodextrin and 6-O-monotosyl-6-deoxy-β-cyclodextrin were explored experimentally and by means of molecular dynamics simulations. Efficient binding was observed only in hydrogen-bonded solvents, which indicated solvophobically driven complexation. The stability of the inclusion complex was considerably higher in aqueous media. A pronounced temperature dependence of Δr H○ and Δr S○ , resulting in perfect enthalpy-entropy compensation, was observed in water. The complexation thermodynamics was in line with classical rationale for the hydrophobic effect at lower temperatures and the nonclassical explanation at higher temperatures. This finding linked cyclodextrin complexation thermodynamics with insights regarding the effect of temperature on the hydration water structure. The complexation enthalpies and entropies were weakly dependent on temperature in organic media. The signs of Δr H○ and Δr S○ were in accordance with the nonclassical hydrophobic (solvophobic) effect. The structures of the optimized product corresponded to those deduced spectroscopically, and the calculated and experimentally obtained values of Δr G○ were in very good agreement. This investigation clearly demonstrated that solvophobically driven formation of cyclodextrin complexes could be anticipated in structured solvents in general. However, unlike in water, adamantane and the host cavity behaved solely as structure breakers in the organic media explored so far.

Correlation analysis of solvent effects on solvolysis rates: What can the empirical parameters of solvents actually say?

AbstractThe logarithm of heterolysis rates of 4‐methoxyneophyl tosylate, Et3CBr,tert‐BuCl, andtert‐BuBr in a set of 12 to 28 hydrogen‐bond (HB) and non‐HB donor solvents are correlated to four sets of solvent parameters: (a)ET(30), (b)π*,α,β, (c)SP,SdP,SA,SB, and (d)DI,ES,α1,β1. Disjointed results are found for Et3CBr because of an insufficient diversity of solvents. For the other solvolyses, the determination coefficientsr2are good to excellent, the gas‐phase values are fairly predicted, and the contributions of various intermolecular forces to the global solvent effect agree satisfactorily within the four solvent sets. However, the most complete description of the solvent effect is given by the four‐parameter sets because their parameters refer to a single intermolecular force, whereasET(30) andπ*correspond to a fixed blend of solute/solvent interactions. For the application of theDI,ES,α1, andβ1set to correlatetert‐BuCl solvolysis,r2= .976, lgk(gas) = −18.6 (experimental −19.3), and the contributions of HB donation (α1), electrostatic forces (ES), and dispersion‐induction (DI) to the solvent effect amounts to 59%, 21%, and 14%, respectively. The dependence of rates on the solvent HB basicity is nearly zero. The inclusion of the solvent cohesive energy density parameterinto the correlation equations does not improve these correlations.

Hydrogen bond capability tunable different “relay-race” mechanisms of the excited-state proton transfer process for 4′-methoxy-3-hydroxyflavone

Abstract The different “relay-race” mechanisms of proton transfer process are investigated for 4′-methoxy-3-hydroxyflavone (MHF) in methanol (MeOH) and dimethylformamide (DMF) characterized by different hydrogen bond (HB) capabilities. Based on the density functional theory and time-dependent density functional theory, we demonstrate that the asynchronous excited-state intermolecular double proton transfer via two intermolecular HBs happens for MHF in MeOH. Differently, in DMF solvent, it breaks the intramolecular HB and forms a single intermolecular HB. And the intermolecular single proton transfer occurs in DMF upon photoexcitation. The excited-state intermolecular HBs both in MHF-MeOH and MHF-DMF complex are proved to be significantly strengthened, which facilitate the proton transfer process. Importantly, it reveals that the higher electrostatic potential and the stronger proton capture ability of DMF than MeOH induce a faster proton transfer for MHF-DMF in experiment. The results highlight the significance of the solvent HB capability in effecting the proton transfer mechanism, and hence have potential to design and develop excellent fluorescent probes.

Choline Chloride as a Nano-Crowder Protects HP-36 from Urea-Induced Denaturation: Insights from Solvent Dynamics and Protein-Solvent Interactions.

Urea at sufficiently high concentration unfolds the secondary structure of proteins leading to denaturation. In contrast, choline chloride (ChCl) and urea, in 1 : 2 molar ratio, form a deep eutectic mixture, a liquid at room temperature, protecting proteins from denaturation. In order to get a microscopic picture of this phenomenon, we perform extensive all-atom molecular dynamics simulations on a model protein, HP-36. Based on our calculation of Kirkwood-Buff integrals, we analyze the relative accumulation of urea and ChCl around the protein. Additional insights are drawn from the translational and rotational dynamics of solvent molecules and hydrogen bond auto-correlation functions. In the presence of urea, water shows slow subdiffusive dynamics around the protein owing to a strong interaction of water with the backbone atoms. Urea also shows subdiffusive motion. The addition of ChCl further slows down the dynamics of urea, restricting its accumulation around the protein backbone. Adding to this, choline cations in the first solvation shell of the protein show the strongest subdiffusive behavior. In other words, ChCl acts as a nano-crowder by excluding urea from the protein backbone and thereby slowing down the dynamics of water around the protein. This prevents the protein from denaturation and makes it structurally rigid, which is supported by the smaller radius of gyration and root mean square deviation values of HP-36.

Hydrogen-bond facilitated intramolecular proton transfer in excited state and fluorescence quenching mechanism of flavonoid compounds in aqueous solution

The density functional theory (DFT) and time-dependent density functional theory (TDDFT) were performed to investigate the ground state and excited state hydrogen-bonding dynamics of flavonoid in hydrogen donating aqueous solution. We demonstrated that the intermolecular hydrogen bond C=O•••H-O between flavonoid and water molecules is significantly strengthened in the electronically excited-state upon photoexcitation of the hydrogen-bonded complex. The radiationless deactivation via intramolecular proton transfer in excited state (ESIPT) can be facilitated by the hydrogen bond strengthening in the excited state. At the same time, quantum yields of the excited-state deactivation via fluorescence are correspondingly decreased. The total fluorescence of flavonoid in polar protic solvents can be drastically quenched by hydrogen bonding. As a consequence, we propose a fluorescence modulation mechanism by hydrogen bonding to explain fluorescence emissions of flavonoid in hydrogen- bonding solvents and no hydrogen-bonding solvents. Given that water molecules have anomalous properties that are different from other small organic molecules that form hydrogen bonds with flavonoids, this theoretical study has reference significance for the development of in vivo probes. Hopefully, the newly proposed mechanism can inspire experimentalists to develop and synthetic non-toxic high signal-to-noise ratios without the need to wash fluorescent probes for individual targets in vivo.

Boron-Nitrogen Double Tweezers Comprising Arylboronic Esters and Diamines: Self-Assembly in Solution and Adaptability as Hosts for Aromatic Guests in the Solid State.

The thermodynamic stability of 1 : 1 and 2 : 1 boron-nitrogen (B←N) adducts formed between aromatic boronic esters with mono- and diamines was studied in solution by NMR and UV-vis spectroscopy with association energies (ΔG°) ranging from -11 to -28 kJ mol-1 . The effect of different substituents in the boronic ester, the nature of the diamine linker, and the effect of the solvent was explored. Stable 2 : 1 B←N adducts with diamines such as 1,3-diaminopropane were produced in solutions of hydrogen-bonding acceptor solvents (acetonitrile and ethyl acetate), which can be isolated in the solid state as crystalline solvates, whereas the use of noncoordinating solvents such as 1,2-dichloroethane afforded mainly 1 : 1 B←N adducts. In suitable combinations, aromatic bis-pyridyl diamines produced stable 2 : 1 B←N adducts that were isolated either as solvent-free solids, solvates, or cocrystals. In these crystalline forms, double-tweezer hosts were observed with an exceptional syn/anti conformational guest-adaptability driven by simultaneous donor-acceptor and C-H⋅⋅⋅π interactions in the tweezer cavities, resembling preorganized covalent tweezer hosts. Interestingly, cocrystals with electron-rich guests such as tetrathiafulvalene and pyrene showed non-centrosymmetric crystal lattices with infinite π-stacked donor-acceptor columns.

Three-dimensional docking of alcohols to ketones: an experimental benchmark based on acetophenone solvation energy balances.

The two hydrogen bond solvation sites exhibited by the carbonyl group in acetophenone are influenced by alkylation of the methyl group in both the acetophenone and in the prototype solvent methanol, largely due to London dispersion forces. Phenyl docking and alkyl docking preferences can be realized at will by appropriate substitution. In particular, cyclopropylation helps to stabilize the opposite phenyl docking site. In all cases, the energy gap is small enough to allow for a simultaneous detection even under low temperature conditions. This density functional prediction is checked experimentally by jet FTIR spectroscopy and largely confirmed. A spurious out-of-plane solvation preference predicted for cyclopropylphenylketone with tert-butyl alcohol by B3LYP-D3 calculations is not confirmed experimentally. It is unlikely that this discrepancy is due to zero-point energy effects. Instead, the second most stable alkyl-side solvation motif predicted with a more in-plane coordination is found in the jet expansion. Overall, the ability of carbonyl solvation balances to benchmark subtle electronic structure effects for non-covalent interactions without major nuclear motion corrections is supported.

Insight into Speciation and Electrochemistry of Uranyl Ions in Deep Eutectic Solvents.

Understanding the speciation of metal ions in heterogeneous hydrogen-bonded deep eutectic solvents (DES) has immense importance for their wide range of applications in green technology, environmental remediation, and nuclear industry. Unfortunately, the fundamental nature of the interaction between DES and actinide ions is almost completely unknown. In the present work, we outline the speciation, solvation mechanism, and redox chemistry of uranyl ion (UO22+) in DES consisting of choline chloride (ChCl) and urea as the hydrogen-bond donor. Electrochemical and spectroscopic techniques along with molecular dynamics (MD) simulations have provided a microscopic insight into the solvation and speciation of the UO22+ ion in DES and also on associated changes in physical composition of the DES. The hydrogen-bonded structure of DES plays an important role in the redox behavior of the UO22+ ion because of its strong complexation with DES components. X-ray absorption spectroscopy and MD simulations showed strong covalent interactions of uranyl ions with the constituents of DES, which led to rearrangement of the hydrogen-bonding network in it without formation of any clusters or aggregations. This, in turn, stabilizes the most unstable pentavalent uranium (UO2+) in the DES. MD analysis also highlights the fact that the number of H-bonds is reduced in the presence of uranyl nitrate irrespective of the presence of water with respect to pristine reline, which suggests high stability of the formed complexed species. The effect of added water up to 20 v/v % on speciation is insignificant for DES, but the presence of water influences the redox chemistry of UO22+ ions considerably. The fundamental findings of the present work would have far reaching consequences on understanding DES, particularly for application in the field of nuclear fuel reprocessing.

Characterization of hydrophilic interaction liquid chromatography retention by a linear free energy relationship. Comparison to reversed- and normal-phase retentions

The Abraham solvation parameter model, a linear free energy relationship (LFER) approach, has been used to characterize a polymeric zwitterionic (sulfobetaine) column in HILIC mode. When acetonitrile (MeCN) is used in the preparation of mobile phases the main solute characteristics affecting the chromatographic behavior of analytes are the molecular size and the hydrogen-bonding (both acidity and basicity) interactions. The former property is more favorable in the acetonitrile-rich mobile phase, reducing thus the retention, but the latter reveals a higher affinity for the water layer adsorbed on the stationary phase, enhancing retention. However, if the aprotic acetonitrile is replaced by methanol, a hydrogen-bond acidic solvent, solute hydrogen-bond basicity does not contribute any more to retention, quite the opposite. Thus, a slightly different selectivity is observed in methanol/water than in acetonitrile/water. Normal-phase mode and HILIC-MeCN share the same main factors affecting retention. For reversed-phase and immobilized artificial membrane (IAM) chromatography, the solute molecular size increase retention because of the lower amount of energy required in the formation of a cavity in the solvated stationary phase. On the contrary, the analyte hydrogen-bond basicity favors interactions with the hydroorganic mobile phase and reduces retention. The determined parameters justify the reversed selectivity commonly observed in HILIC in reference to reversed-phase. In most instances, the least retained solutes in reversed-phase are the most retained in HILIC.

Tetranuclear Au2Cu2 Clusters with Butterfly- and Planar-Shaped Metal Cores: Strong Rigidochromism Induced by Jahn-Teller Distortion in Two-Coordinated Gold(I) Centers.

Two tetranuclear Au2Cu2 cluster complexes [Au2Cu2(μ-(PPh2)2py)2(μ-OH)](PF6)3, 2, and [Au2Cu2Cl2(μ-(PPh2)2 py)2](OTf)2, 4, have been prepared by the reactions of precursor complexes [Au2(μ-(PPh2)2py)2](OTf)2, 1, and [Cu2(μ-(PPh2)2py)2(μ-SMe2)(OTf)2], 3, with [Cu(NCCH3)4]PF6 and AuCl(SMe2), respectively. The crystal structures of complexes 2 and 4 were determined by X-ray crystallography, indicating a butterfly-shaped Au2Cu2 metal core for 2 and a planar-shaped Au2Cu2 metal core for 4. In complex 2, the Cu atoms occupy the edge-sharing bond, while in complex 4, alternating Au and Cu atoms occupy the tetragon vertices. The optical properties of the complexes were investigated by experimental and computational methods. Although complex 2 displayed a luminescence vapochromic behavior in the presence several volatile organic compounds, complex 4 indicated only an distinguishable change in its emission color when it was exposed to vapor of hydrogen-bond donor solvents. The calculations showed that 2 undergoes an unsymmetrical distortion in its two-coordinated gold(I) centers upon excitation to the first triplet excited state. This distortion induces a large Stokes shift and a strong rigidochromism behavior that is unprecedented for two-coordinated gold(I) complexes.

Role of Explicit Solvation in the Simulation of Resonance Raman Spectra within Short-Time Dynamics Approximation.

In short-time dynamics approximation, relative resonance Raman (RR) intensity of a vibrational mode primarily depends on the magnitude of square of the excited-state gradient along the corresponding normal coordinate, ground-state normal mode eigenvector, and harmonic vibrational wavenumbers. In this study, through simulation of RR spectra of guanosine-5'-monophosphate (GMP) in two ππ* singlet excited states, we analyze how the explicitly hydrogen-bonded local solvation structure of the chromophore dictates intensities of the RR active modes in an unprecedented manner. We show that the accuracy of the structural model of solvated chromophore plays a decisive role in determining an optimal theoretical method for prediction of the Franck-Condon region of the ππ* excited states. 9-Methylguanine (9-meG) in complex with six water molecules (9-meG·6H2O) is found out to be the most accurate one for describing GMP in two different bright electronic states. We find that explicit hydrogen-bonded water molecules strongly influence computed RR intensities of GMP by modulating both the ground-state normal mode vectors and the excited-state energy gradients. We find that simultaneous inclusion of six explicit waters to describe the solute-solvent interaction near all hydration sites is essential for reliable prediction of the features of RR spectra in Lb and Bb electronic states of GMP.

Microhydration Structures of Protonated Oxazole

The initial microhydration structures of the protonated pharmaceutical building block oxazole (Ox), H+Ox-Wn≤4, are determined by infrared photodissociation (IRPD) spectroscopy combined with quantum chemical dispersion-corrected density functional theory calculations (B3LYP-D3/aug-cc-pVTZ). Protonation of Ox, achieved by chemical ionization in a H2-containing plasma, occurs at the most basic N atom. The analysis of systematic shifts of the NH and OH stretch vibrations as a function of the cluster size provides a clear picture for the preferred cluster growth in H+Ox-Wn. For n = 1-3, the IRPD spectra are dominated by a single isomer, and microhydration of H+Ox with hydrophilic protic W ligands occurs by attachment of a hydrogen-bonded (H-bonded) Wn solvent cluster to the acidic NH group via an NH···O H-bond. Such H-bonded networks are stabilized by strong cooperativity effects. This is in contrast to previously studied hydrophobic ligands, which prefer interior ion solvation. The strength of the NH···O ionic H-bond increases with the degree of hydration because of the increasing proton affinity (PA) of the Wn cluster. At n = 4, proton-transferred structures of the type Ox-H+Wn become energetically competitive with H+Ox-Wn structures, because differences in solvation energies can compensate for the differences in the PAs, and barrierless proton transfer from H+Ox to the Wn solvent subcluster becomes feasible. Indeed, the IRPD spectrum of the n = 4 cluster is more complex suggesting the presence of more than one isomer, although it lacks unequivocal evidence for the predicted intracluster proton transfer.

Activation of Saturated Fluorocarbons to Synthesize Spirobiindanes, Monofluoroalkenes, and Indane Derivatives.

SummaryFluorinated organic compounds are produced in abundance by the pharmaceutical and agrochemical industry, making such compounds attractive as building blocks for further functionalization. Unfortunately, activation of C(sp3)-F bond in saturated fluorocarbons, especially for aliphatic gem-difluoroalkanes, remains challenging. Here we describe the selective activation of inert C(sp3)-F bonds catalyzed by B(C6F5)3. In hexafluoro-2-propanol (HFIP), chemically robust aliphatic gem-difluorides are converted in high yields to the corresponding substituted 2,2′,3,3′-tetrahydro-1,1′-spirobiindenes via a B(C6F5)3-catalyzed intramolecular cascade Friedel-Crafts cyclization, not requiring a silicon-based trapping reagent. However, in the absence of a hydrogen-bonding donor solvent such as HFIP, the aliphatic gem-difluorides preferentially engage in a defluorination/elimination process that provides monofluorinated alkenes in good yields. Furthermore, a series of substituted 1-alkyl-2,3-dihydro-1H-indenes was obtained in high yield from the B(C6F5)3-catalyzed defluorinative cyclization of aliphatic secondary monofluorides in HFIP. The protocol could inspire development of a new class of main-group Lewis acid-catalyzed C(sp3)-F bond activation in general unactivated fluorocarbons.

A Method of Calculating the Kamlet-Abboud-Taft Solvatochromic Parameters Using COSMO-RS.

There is demand for safer and bio-based solvents, brought on by legislation and sustainability objectives. The prediction of physical properties is highly desirable to help design new molecules. Here we present an in silico approach to obtain calculated Kamlet–Abboud–Taft solvatochromic parameters using virtual experiments. The tautomerisation equilibrium of methyl acetoacetate and dimedone was calculated in different solvents with COSMO-RS theory and converted into estimates of solvent dipolarity and hydrogen bond accepting ability, respectively. Hydrogen bond donating ability was calculated as a function of the electron deficient surface area on protic solvents. These polarity descriptors correlate with rate constants and equilibria, and so ability of calculated Kamlet–Abboud–Taft solvatochromic parameters to recreate experimental free energy relationships was tested with sixteen case studies taken from the literature. The accuracy of the calculated parameters was also satisfactory for solvent selection, as demonstrated with a 1,4-addition reaction and a multicomponent heterocycle synthesis.