Intrinsic Acidity Research Articles

Modeling intrinsic basicities and acidities

AbstractWe illustrate the potential of ab initio and density functional theory (DFT) methods to model and gain some understanding of the intrinsic reactivity of molecules. We show how the use of high‐level ab initio or DFT calculations permit to rationalize (a) basicity and acidity trends along a series of homologous compounds, namely thiocarbonyl and selenocarbonyl derivatives, α,β‐unsaturated amines, phosphines and arsines, and α,β‐unsaturated alkanes, silanes, germanes and stannanes, (b) the role of intramolecular hydrogen bonds in intrinsic acidities and basicities, (c) the incidence of induced proton transfer process effects on the intrinsic basicity of compounds stabilized by intramolecular hydrogen bonds, (d) hybridization effects on intrinsic basicities, (e) the role of non‐classical structures in intrinsic basicities and (f) the observation of bond cleavage associated with gas‐phase protonation and the influence of these dissociative proton attachment mechanisms on the intrinsic basicity of the system. Copyright © 2002 John Wiley & Sons, Ltd.

Surface properties of silicon nitride powders

The surface charge densities of silicon nitride powders in NaNO 3 solutions of various concentrations were investigated by the potentiometric acid–base titration method. The points of zero charge, pH pzc, of silicon nitride powders were determined by the batch equilibration method. The site-binding model was applied to calculate the intrinsic acidity constants describing association–dissociation of silicon nitride surface groups.

Surface properties and catalytic activity of ferrospinels of nickel, cobalt and copper, prepared by soft chemical methods

Ferrospinels of nickel, cobalt and copper and their sulphated analogues were prepared by the room temperature coprecipitation route to yield samples with high surface areas. The intrinsic acidity among the ferrites was found to decrease in the order: cobalt>nickel>copper. Sulphation caused an increase in the number of weak and medium strong acid sites, whereas the strong acid sites were left unaffected. Electron donor studies revealed that copper ferrite has both the highest proportion of strong sites and the lowest proportion of weak basic sites. All the ferrite samples proved to be good catalysts for the benzoylation of toluene with benzoyl chloride, copper and cobalt ferrites being much more active than nickel ferrite. The catalytic activity for benzoylation was not much influenced by sulphation, but it increased remarkably with calcination temperature of the catalyst. Surface Lewis acid sites, provided by the octahedral cations on the spinel surface, are suggested to be responsible for the catalytic activity for the benzoylation reaction.

Acidity trends in alpha,beta-unsaturated alkanes, silanes, germanes, and stannanes.

The gas-phase acidity of ethyl-, vinyl-, ethynyl-, and phenyl-substituted silanes, germanes, and stannanes has been measured by means of FT-ICR techniques. The effect of unsaturation on the intrinsic acidity of these compounds and the corresponding hydrocarbons was analyzed through the use of G2 ab initio and DFT calculations. In this way, it was possible to get a general picture of the acidity trends within group 14. As expected, the acid strength increases down the group, although the acidity differences between germanium and tin derivatives are already rather small. As has been found before for amines, phosphines, and arsines, the carbon, silicon, germanium, and tin alpha,beta-unsaturated compounds are stronger acids( )than their saturated analogues. The acidifying effect of unsaturation is much larger for carbon than for Si-, Ge-, and Sn-containing compounds. The allyl anion is better stabilized by resonance than its Si, Ge, and Sn analogues, [CH(2)(-)(delta)--CH(+)(delta)(') --CH(2)(-)(delta)](-) vs [CH(2)(-)(delta)()II = CH(-)(delta)()III - XH(2)(-)(delta)()IV](-) (X = Si, Ge, Sn). The enhanced acid strength of unsaturated compounds is essentially due to a greater stabilization of the anion with respect to the neutral, because the electronegativity of the alpha,beta-unsaturated carbon group increases with its degree of unsaturation. The phenyl derivatives are systematically weaker acids than the corresponding ethynyl derivatives by 15-20 kJ mol(-)(1). Experimentally, toluene acidity is very close to that of propyne, because the deprotonation of propyne takes place preferentially at the =CH group rather than at the -CH(3) group.

Thiouracils: Acidity, Basicity, and Interaction with Water

The optimized geometries, harmonic vibrational frequencies, and the energies of the cyclic structures of monohydrated 2-thiouracil, 4-thiouracil, and 2,4-dithiouracil are calculated using density functional theory (B3LYP) combined with the 6-31+G(d,p) basis set. In the three most stable cyclic structures, the water molecule accepts the NH proton and donates a proton to the carbonyl oxygen or thiocarbonyl sulfur atoms. The intermolecular distances between the water molecule and the acceptor atom of thiouracils are about 0.5 Å longer for hydrogen bonds involving a sulfur atom. Less stable cyclic complexes involving the O4 atom and the C5H bond are also formed. The frequency shifts of the ν(OH) stretching vibrations of water and the ν(NH) stretching vibrations of thiouracils are compared with recent data on the 1:1 adducts of uracil and water. The proton affinity of the oxygen and sulfur atoms and the deprotonation enthalpy of the NH bonds of thiouracils are calculated at the same level of theory. Although intrinsic acidities and basicities are larger in thiouracils than in uracils, the binding energies with one water molecule do not differ markedly for uracil and thiouracils. Comparison with previous data obtained for the 1:1 adducts of uracil and thymine with water suggests that the same binding energy is obtained for a much larger proton affinity of the sulfur atom as compared with the oxygen atom. The complexes of the thiouracils with three water molecules are also investigated and the hydrogen bonding cooperativity is discussed. Comparison with uracil indicates an alteration of the first hydration shell caused by the substitution of the oxygen atom by the sulfur one.

Acidity of the Br�nsted acid sites of zeolites

The acidity of Brönsted acid sites of the (H3SiO)nOq(X3−n−qSi)(OH)AlpY3−m−q(OSiH2)r(OSiH3)m zeolitic clusters, with X=(H,OH), Y=(H,OH), (n,m)=(0,1,3), r=(0,1,8), and (p,q)=(0,1), is evaluated in terms of the intrinsic acidity þ of the OH groups. þ is defined as the ratio between the OH distance and the frequency of the OH vibration mode and has been calculated at ab initio Hartree–Fock (HF) and second-order Møller–Plesset (MP2) levels and the standard 6-31+G(d,p) basis set. The acidity þ is rationalized in terms of the topology of the charge density ρ(r) calculated at the position of the critical point of the OH bond ρ(rc) of the acid sites. It is shown that there exists a linear relationship between þ and the charge concentration ρ(rc), with a correlation coefficient of r=−0.98 for HF results, and r=−0.96 for MP2 ones. © 2001 John Wiley & Sons, Inc. Int J Quant Chem 82: 143–150, 2001

Ab initio Models for Six-Center Multiple Proton Exchange and Ion Pair Formation Assisted by Lewis Acids

High level ab initio and density functional calculations, extrapolated to QCISD(T)/6-311+G(3df,2p)//MP2/6-31+G**+ZPE, reveal that cyclic ion pairs can form in the hydrogen bonded complexes of haloboric acids BHnX3-n–HX, X=F, Cl, with Lewis bases HX, H2O, CH3OH, and NH3, even in isolation (e.g., in the gas phase). The intrinsic acidities (deprotonation energies) required for protonation of these bases with formation of gas phase ion pairs are calculated to be <295 kcal/mol for water, <301 kcal/mol for methanol, and <306 kcal/mol for ammonia; such values are common for acidic sites in zeolites. All gas phase ion pairs prefer symmetric bidentate or tridentate structures. In the other cases where hydrogen bonded complexes prevail, symmetric ion pair-like transition structures for multiple hydrogen exchange are computed.

Gas-Phase Acidities of Some Neutral Brønsted Superacids: A DFT and ab Initio Study

For the first time G2 or G2(MP2) calculations or both have been performed to calculate the acidity and deprotonation enthalpy of classical strong mineral acids HClO4, CF3SO3H, FSO3H, H2SO4, HBF4, HPO3, and HNO3. Also, the intrinsic acidities and gas-phase deprotonation enthalpies for 39 neutral strong or superstrong Brønsted acids, Brønsted−Lewis conjugate acids, and some compounds modeling the acidic clusters of zeolites were calculated using the DFT B3LYP 6-311+G** approach. DFT B3LYP method at 6-31+G* basis was used for the calculation of the intrinsic Brønsted acidities of the conjugate acids of the carborane anion CB11H12- and its mono-, hexa-, and dodecafluorinated analogues. G2 and G2(MP2) theories describe the acidities of different compounds better than DFT B3LYP//6-311+G**. However, the DFT results could also be used for the estimation of the acidity of compounds which are out of reach of G2 or G2(MP2) theory. The estimated ΔGacid values obtained this way can be used as the substitutes for the unavailable experimental values, especially for those (rather numerous) compounds for which the experimental determination of ΔGacid is very difficult. In the case of practically all considered families of compounds extremely high acidities (low ΔGacid values) could be reached. If the compounds were started from HF as the parent acid, then the estimated ΔGacid as low as 249.0 kcal/mol (for F(OSO2)4H) could be reached by formation of Brønsted−Lewis conjugate acids by consecutive complexation with SO3 molecules. Also very low ΔGacid value (ΔGacid(HSbF6) = 255.5) could be reached by complexation of HF with SbF5. At least as high intrinsic acidities as in case of the strongest Brønsted−Lewis superacids could be reached in the case of progressive introduction of highly electronegative, correctly oriented polarizable dipolar electron-accepting substituents into the acidity site. Indeed, the introduction of five CN groups into cyclopentadiene is expected to lead to the acidity ΔGacid = 250.1 kcal/mol which is lower than the corresponding quantity even for hexafluoroantimonic acid (ΔGacid = 255.5 kcal/mol). However, by far the strongest intrinsic Brønsted acidity (ΔGacid = 209 kcal/mol) for dodecafluorosubstituted carborane acid CB11F12H is predicted to exceed the intrinsic acidity of sulfuric acid by about 90 kcal/mol or by almost 70 powers of ten, whereas semi-empirical PM3 calculations suggest that the conjugate acid of the dodecatrifluoromethylmonocarborane anion CB11(CF3)12- could be the first neutral Brønsted superacid whose acidity (deprotonation energy) is expected to be below the landmark 200 kcal/mol level. An approximate linear relationship is found to hold between the calculated gas-phase acidities of strong and superstrong Brønsted acids and the corresponding Hammett acidity functions of the corresponding neat acids. The simultaneous existence of the widely overlapping areas on the gas-phase acidity scale of neutral and cationic Brønsted acids evidences strongly for the feasibility of the spontaneous proton-transfer equilibria between neutral Brønsted acids and bases.

Intrinsic acidity of aluminum, chromium (III) and iron (III) μ 3-hydroxo functional groups from ab initio electronic structure calculations

Density functional calculations are performed on M 3(OH) 7(H 2O) 6 2+ and M 3O(OH) 6(H 2O) 6 + clusters for MAl, Cr(III), and Fe(III), allowing determination of the relative acidities of the μ 3-hydroxo and aquo functional groups. Contrary to previous predictions and rationalizations, Fe 3OH and Al 3OH groups have nearly the same intrinsic acidity, while Cr 3OH groups are significantly more acidic. The gas-phase acidity of the Fe 3OH site is in good agreement with the value predicted by the molecular mechanics model previously used to estimate the relative acidities of surface sites on iron oxides. [ J. R. Rustad et al. (1996) Geochim. Cosmochim. Acta 60, 1563]. Acidities of aquo functional groups were also computed for Al and Cr. The AlOH 2 site is more acidic than the Al 3OH site, whereas the Cr 3OH site is more acidic than the CrOH 2 site. These findings predict that the surface charging behavior of chromium oxides/oxyhydroxides should be distinguishable from their Fe, Al counterparts. The calculations also provide insight into why the lepidocrocite/boehmite polymorph is not observed for CrOOH.

Experimental Study and Modeling of the Radium Sorption onto Goethite

ABSTRACTIron oxyhydroxides play a significant role in radium behavior in uranium mill tailings. Although they are present at trace level (less than 1% in mass), iron oxyhydroxides are responsible of the retention of significant amounts of radium in these residues. A part of these oxyhydroxides is present as amorphous or ill-crystallized forms, the other part corresponds to crystalline iron oxyhydroxides like goethite.In this paper, we present data concerning radium sorption on a well-characterized goethite (α-FeOOH). Batch experiments have been performed and the influence of different parameters such as contact time, pH and electrolyte concentration of the solution has been examined. The sorption equilibrium was obtained after a contact time of 1 hour. In sodium perchlorate medium, the amount of sorbed radium reaches a maximum for pH higher than 8 and no significant influence of sodium concentration was observed. However, the calcium concentration greatly influences the radium sorption: the greater the calcium concentration, the less radium is sorbed. The kinetics of the radium desorption are slower than those of sorption: more than 30% of radium remains sorbed on goethite after 1 week of contact time.Using the characteristics of the goethite surface (total number of sites, intrinsic acidity constants, intrinsic constants of calcium), modelling of the experimental data with inner sphere complexes gave satisfactory results. Two surface species involving radium can be formed (SORa+ and SORaOH). These sorption constants determined allow prediction of the radium sorption onto goethite in the chemical conditions of uranium mill tailings.

A Comparative Study of Surface Acid–Base Characteristics of Natural Illites from Different Origins

The acid–base characteristics of naturally occurring illites, collected from different locations, were investigated by potentiometric titrations. The experimental data were interpreted using the constant capacitance surface complexation model. Considerable release of Al and Si from illite samples and subsequent complexation or precipitation of hydroxyl aluminosilicates generated during the acidimetric forward titration and the alkalimetric back titration, respectively, were observed. Therefore, the acidimetric supernatant, rather than the neutral one, was regarded as the system blank for each illite suspension to yield the surface site concentrations. In order to describe the acid–base chemistry of aqueous illite surfaces, two surface proton-reaction models, introducing the corresponding reactions between the dissolved aluminum species and silicic acid, as well as a surface Al–Si complex on homogeneous illite surface sites, were proposed as follows:The Kf2 constant in Model II was obtained by simulating the complex formation between the dissolved aluminum species and silicic acid that occurred in acidimetric supernatant when the hydroxide was added. Additionally, the following cation exchange reaction was also considered for a special case, where a large amount of K+ is released during the corresponding acidimetric titration, in which a high concentration of protons are consumed.Optimization results indicated that both models could provide a good description of the titration behavior for all aqueous illite systems in this study. The intrinsic acidity constants for the different illites were similar in Model I, showing some generalities in their acid–base properties. Model I may be considered as a simplification of Model II, evident in the similarities between the corresponding constants. In addition, the formation constant for surface Al–Si species (complexes or precipitates) is relatively stable in this study.

Protonation and Deprotonation Enthalpies of Guanine and Adenine and Implications for the Structure and Energy of Their Complexes with Water: Comparison with Uracil, Thymine, and Cytosine

The optimized geometries, harmonic vibrational frequencies, and energies of the cyclic structures of monohydrated guanine and adenine are computed using density functional theory (B3LYP) combined with the 6-31+G(d,p) basis set. The proton affinity of the O and N atoms and the deprotonation enthalpy of the different NH bonds of guanine and adenine are computed at the same level of theory. The results are compared with recent data on uracil, thymine, and cytosine. The intrinsic acidities and basicities of the five nucleobases are discussed. Complex formation with water results in a moderate change of the pyramidal character of the amino group. For closed complexes where water interacts with the O atom of the nucleobase, the intermolecular distances and the hydrogen bond energies are correlated to the proton affinities and deprotonation enthalpies of the sites involved in complex formation.

Surface acidity constants of α-Al2O3 between 25 and 70°c

The effect of temperature on the adsorption of H+ onto corundum was investigated experimentally by conducting potentiometric titrations at 25, 30, 40, 50, 60 and 70°C. These titrations were used to determine apparent acidity constants, given by the product of an intrinsic adsorption constant and a coulombic term (Kiapp = Kiinte(−ΔZFΨ/RT)) at the above temperatures.First and second intrinsic acidity constants were determined using a constant capacitance model (CCM). These constants and the point of zero charge change linearly with inverse temperature. These data were used to determine thermodynamic constants for the proton adsorption reactions.In the coulombic term, only the capacitance and the surface charge change with temperature and both were determined with the titration data at the various temperatures. Results show that the change in capacitance can be predicted with changes in the dielectric constant of water with increasing temperature. At a given pH, changes in the surface charge with temperature can, in turn, be predicted with a linear regression.With the above model, apparent acidity constants of corundum (including the chemical and electrostatic interactions) can be predicted for any temperatures between 25 and 70°C and possibly higher. These apparent constants change over several orders of magnitude in this temperature range (mainly due to a change in the coulombic term) and small temperature changes could have a strong influence on the stability of surface complexes.

Ionic Hydrogen Bond Effects on the Acidities, Basicities, Solvation, Solvent Bridging, and Self-Assembly of Carboxylic Groups

Ionic assemblies of acetic acid and water form unlimited hydrogen bond networks. The stabilities of the networks correlate with the intrinsic acidities of the components, leading to strong CH3COO-···HOOCCH3 bonds and weak CH3COO-···H2O bonds. These relations apply from strong bonds in small aggregates to weak bonds in large assemblies, and affect the energies of acid dissociation and self-assembly. Partial solvation of CH3COO- by four H2O molecules facilitates acid dissociation and decreases the CH3COO-−H+ bond dissociation energy by 332 kJ/mol (80 kcal/mol). The stabilites of the hydrogen bond networks increase with CH3COOH content, and aggregation decreases further the acid dissociation energy by forming strong CH3COO-···HOOCCH3 bonds about the ions and by stabilizing the released protons in (CH3COOH)m(H2O)nH+ assemblies. The combination of strong CH3COO-···HOOCCH3 bonds and weak CH3COO-···H2O bonds makes self-assembly with solvent displacement particularly favorable for carboxylic acids, explaining the...

NH3 Sorption Isotherms in ALPO-11 and Its Si, Co, and Mn Analogues

Equilibrium sorption capacities of different probe molecules such as water, n-hexane, cyclohexane, and ammonia were used as a tool for the characterization of ALPO-11 and protonic forms of SAPO-11, CoAPO-11, and MnAPO-11. Sorption uptake at P/P0 = 0.8 and 298 K, 2 h for water, n-hexane, and cyclohexane was found to be affected by incorporating Si, Co, and Mn in ALPO-11. Generation of Bronsted acidity is expected to increase the hydrophilicity; Si, Co, and Mn analogues have exhibited such trends depending upon the extent of substitution and characteristics of the substituted cation. The amount of ammonia retained irreversibly (on account of chemisorption) during sorption measurements was found to be correlated with the intrinsic acidity possessed by ALPO-11 and its analogues. Isotherms of ammonia sorption in ALPO-11, SAPO-11, CoAPO-11, and MnAPO-11 in the temperature range 333−483 K up to 500 Torr have been measured. The NH3 sorption capacity was also found to decrease in the order ALPO-11 > SAPO-11 > CoAP...

Substituent Effects on the Strength of the Intramolecular Hydrogen Bond of Thiomalonaldehyde

The effect of CH3, NH2, OH, and F substituents on the intramolecular hydrogen bond (IHB) of thiomalonaldehyde (TMA) was analyzed through the use of B3LYP density functional theory calculations. The geometries of the C1-, C2-, and C3-susbtituted enol and enethiol tautomers were optimized at the B3LYP/6-31G(d) level while their final energies were evaluated using a 6-311+G(3df,2p) basis set expansion. In general C1-substitution strengthens the IHB of the enolic tautomer, while C3-substitution strengthens the IHB of the enethiolic form. These changes are related with an enhancement of the intrinsic acidity of the OH and the SH groups, respectively. Important cooperative effects are also present when the substituent can form an additional IHB with either the oxygen atom or the sulfur atom of TMA. However, the trends observed in the relative stabilities of the enol and the enethiol tautomers do not follow the changes observed in the strength of the IHB. C1-substitution specifically stabilizes the enethiol form...

Spanning and Expanding the Basicity Scale with Simple Ammonia Derivatives.

The currently known intrinsic basicity (DeltaG degrees ) range spans about 154 kcal/mol, from the basicity of the oxygen molecule (94.7 kcal/mol) to that of 7-isopropyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene (248.4 kcal/mol). In this paper, we show that the basicities of two commercially available derivatives of ammonia, viz. F(3)N and Li(3)N, differ by 182 kcal/mol, which is greater than the above-mentioned range. We also show that the basicity range from F(3)N to Li(3)N can be spanned with simple ammonia derivatives containing single-atom substituents of electronegativity higher or lower than that of nitrogen and that basicities even higher than that of Li(3)N (a superbase) are indeed possible. The large basicity differences exhibited by these compounds are analyzed in the light of the energies of their charged and neutral forms. It should be noted that the most basic compounds reported fill the current gap between the intrinsic acidity and basicity scales, which allows such basic substances to be inserted in the known acidity range for neutral compounds in order to generate the corresponding conjugate charged forms by reacting in the gas phase an appropriate acid with a neutral base.

Intrinsic acidity and redox properties of the adenine radical cation

The intrinsic chemical properties of the gaseous adenine radical cation were examined by using dual cell Fourier transform ion cyclotron resonance mass spectrometry. The adiabatic recombination energy of the radical cation (ionization energy of neutral adenine) was found by bracketing experiments to be 8.55 ± 0.1 eV (at 298 K; earlier literature values range from 8.3 to 8.9 eV). Based on this value, the heat of formation (Δ H f 298) of the adenine radical cation is estimated to be 246 ± 3 kcal/mol. The acidity (Δ H acid 298) of the adenine radical cation was bracketed to be 221 ± 2 kcal/mol. These thermochemical values suggest that the adenine radical cation reacts with neutral guanine by electron abstraction or proton transfer, with neutral cytosine by proton transfer, and via neither pathway with neutral thymine, molecular water or a sugar moiety of DNA (modeled by tetrahydrofuran). Experimental examination of the gas-phase reactivity of the adenine radical cation revealed a slow deuterium atom abstraction from perdeuterated tetrahydrofuran. Hence, in the absence of a nearby guanine or cytosine, the adenine radical cation may be able to abstract a hydrogen atom from a sugar moiety of DNA.

Point of zero charge and intrinsic equilibrium constants of activated carbon cloth

Surface properties of cellulose-based activated carbon cloth were investigated. The point of zero charge was determined by batch equilibrium method. Surface charge densities were obtained from potentiometric titrations. The site-binding model was applied to calculate the intrinsic acidity constants.

Dramatic Medium Effects on Reactivity. The Ionization Sites of Pyrrole and Indole Carboxylic Acids

The intrinsic (gas phase) acidities of pyrrole- and indole-2- and 3-carboxylic acids as well as those of methyl indole-3-carboxylate and 1-methyl indole-3-carboxylic acid have been measured by means of Fourier Transform Ion Cyclotron Resonance (FT-ICR) mass spectrometry. Ab initio molecular orbital and DFT calculations were performed at several levels of theory on all the relevant species. This study was extended to the same species in aqueous solution by means of the continuum model implemented in the SCRFPAC program. The calculated acidities, both in the gas phase and in solution, are in very good agreement with the experimental data and conclusively show that (i) pyrrole- and indole-3-carboxylic acids behave as NH acids in the gas phase, (ii) in the gas phase, pyrrole- and indole-2-carboxylic acids are deprotonated at the COOH group, although competing ionizations possibly take place, and (iii) these four compounds behave in aqueous solution as OH acids.