Porous anionic, cationic, and neutral metal-carboxylate frameworks constructed from flexible tetrapodal ligands: syntheses, structures, ion-exchanges, and magnetic properties

A series of coordination polymers with anionic, cationic, and neutral metal-carboxylate frameworks have been synthesized by using a flexible tetrapodal ligand tetrakis[4-(carboxyphenyl)oxamethyl] methane acid (H(4)X). The reactions between divalent transition-metal ions and H(4)X ligands gave [M(3)X(2)]·[NH(2)(CH(3))(2)](2)·8DMA (M = Co (1), Mn (2), Cd(3)) which have anionic metal-carboxylate frameworks with NH(2)(CH(3))(2)(+) cations filled in channels. The reactions of trivalent metal ions Y(III), Dy(III), and In(III) with H(4)X ligands afforded cationic metal-carboxylate frameworks [M(3)X(2)·(NO(3))·(DMA)(2)·(H(2)O)]·5DMA·2H(2)O (M = Y(4), Dy(5)) and [In(2)X·(OH)(2)]·3DMA·6H(2)O (6) with the NO(3)(-) and OH(-) serving as counterions, respectively. Moreover, a neutral metal-carboxylate framework [Pb(2)X·(DMA)(2)]·2DMA (7) can also be isolated from reaction of Pb(II) and H(4)X ligands. The charged metal-carboxylate frameworks 1-5 have selectivity for specific counterions in the reaction system, and compounds 1 and 2 display ion-exchange behavior. Moreover, magnetic property measurements on compounds 1, 2, and 5 indicate that there exists weak antiferromagnetic interactions between magnetic centers in the three compounds.     

New extended magnetic systems based on oxalate and iron(III) ions

A series of oxalate-bridged iron(III) complexes have been synthesized by the reaction of FeCl 3 with oxalic acid (H 2ox) and XCl, where X is a substituted univalent ammonium or an alkaline cation. We have obtained basically two different types of compounds by varying the nature and the shape of the counterion, with the dimensionality of the resulting product being strongly influenced by the counterion. Three-dimensional (3D) networks of oxo- and oxalato-bridged iron(III) ions of the general formula {X 2[Fe 2O(ox) 2Cl 2]. pH 2O} n have been obtained for X = Li (+) ( 1), Na (+) ( 2), and K (+) ( 3) with p = 4 and X = MeNH 3 (+) ( 4), Me 2NH 2 (+) ( 5), and EtNH 3 (+) ( 6) with p = 2. Similar 3D hydroxo- and oxalato-bridged iron(III) networks of the formula {X[Fe 2(OH)(ox) 2Cl 2].2H 2O} n resulted for X = EtNH 3 (+) ( 7a) and PrNH 3 (+) ( 8). Compound 7a undergoes a solid-to-solid transformation, leading to a new species of the formula {(H 3O)(EtNH 3)[Fe 2O(ox) 2Cl 2].H 2O} n ( 7b). Chainlike compounds of the formula {X 2[Fe 2(ox) 2Cl 4]. pH 2O} n [X = Me 2NH 2 (+)( 9, p = 1), Me 3NH (+) ( 10, p = 2), and Me 4N (+) ( 11, p = 0)] have been obtained for the bulkier alkylammonium cations. Magnetic susceptibility measurements in the temperature range 1.9-295 K show the occurrence of weak ferromagnetic ordering due to spin canting in the 3D networks 1- 8, with the value of the critical temperature ( T c) varying with the cation in the range 26 K ( 2) to 70 K ( 8) without significant structural modifications. The last three one-dimensional compounds exhibit the typical behavior of antiferromagnetically coupled chains of interacting spin sextets [ J = -8.3 ( 9), -6.9 ( 10), and -8.4 ( 11) cm (-1) with H = - J summation operator i S i S i+1 ].     

Bonding in tropolone, 2-aminotropone, and aminotroponimine: no evidence of resonance-assisted hydrogen-bond effects

The properties of the intramolecular hydrogen bond (IMHB) in tropolone, aminotropone, and aminotroponimine have been compared with those in the corresponding saturated analogues at the B3LYP/6-311+G(3df,2p)//B3LYP/6-311+G(d,p) level of theory. In general, all those compounds in which the seven-membered ring is unsaturated exhibit a stronger IMHB than their saturated counterparts. Nevertheless, this enhanced strength is not primarily due to resonance-assisted hydrogen-bond effects, but to the much higher intrinsic basicity and acidity of the hydrogen-bond acceptor and donor groups, respectively, in the unsaturated compounds. These acidity and basicity enhancements have a double origin: 1) the unsaturated nature of the moiety to which the hydrogen-bond donor and acceptor are attached and 2) the cyclic nature of the compounds under scrutiny. As has been found for hydroxymethylene and aminomethylene cyclobutanones, and cyclobutenones and their nitrogen-containing analogues, the IMHB strength follows the [donor, acceptor] trend: [OH, C=NH]>[OH, C=O]>[NH(2), C=NH]>[NH(2), C=O] and fulfills a Steiner-Limbach correlation similar to that followed by intermolecular hydrogen bonds.     

Accurate prediction of cation-π interaction energy using substituent effects

Substituent effects on cation-π interactions have been quantified using a variety of Φ-X···M(+) complexes where Φ, X, and M(+) are the π-system, substituent, and cation, respectively. The cation-π interaction energy, E(M(+)), showed a strong linear correlation with the molecular electrostatic potential (MESP) based measure of the substituent effect, ΔV(min) (the difference between the MESP minimum (V(min)) on the π-region of a substituted system and the corresponding unsubstituted system). This linear relationship is E(M(+)) = C(M(+))(ΔV(min)) + E(M(+))' where C(M(+)) is the reaction constant and E(M(+))' is the cation-π interaction energy of the unsubstituted complex. This relationship is similar to the Hammett equation and its first term yields the substituent contribution of the cation-π interaction energy. Further, a linear correlation between C(M(+))() and E(M(+))()' has been established, which facilitates the prediction of C(M(+)) for unknown cations. Thus, a prediction of E(M(+)) for any Φ-X···M(+) complex is achieved by knowing the values of E(M(+))' and ΔV(min). The generality of the equation is tested for a variety of cations (Li(+), Na(+), K(+), Mg(+), BeCl(+), MgCl(+), CaCl(+), TiCl(3)(+), CrCl(2)(+), NiCl(+), Cu(+), ZnCl(+), NH(4)(+), CH(3)NH(3)(+), N(CH(3))(4)(+), C(NH(2))(3)(+)), substituents (N(CH(3))(2), NH(2), OCH(3), CH(3), OH, H, SCH(3), SH, CCH, F, Cl, COOH, CHO, CF(3), CN, NO(2)), and a large number of π-systems. The tested systems also include multiple substituted π-systems, viz. ethylene, acetylene, hexa-1,3,5-triene, benzene, naphthalene, indole, pyrrole, phenylalanine, tryptophan, tyrosine, azulene, pyrene, [6]-cyclacene, and corannulene and found that E(M)(+) follows the additivity of substituent effects. Further, the substituent effects on cationic sandwich complexes of the type C(6)H(6)···M(+)···C(6)H(5)X have been assessed and found that E(M(+)) can be predicted with 97.7% accuracy using the values of E(M(+))' and ΔV(min). All the Φ-X···M(+) systems showed good agreement between the calculated and predicted E(M(+))() values, suggesting that the ΔV(min) approach to substituent effect is accurate and useful for predicting the interactive behavior of substituted π-systems with cations.     

Fast estimation of hydrogen-bonding donor and acceptor propensities: a GMIPp study

The suitability of the GMIPp energy functional as a fast, efficient method for estimating the hydrogen-bond donor and acceptor propensities of a wide variety of organic compounds is examined. Comparison of the GMIPp values is made with two experimental hydrogen-bond scales: i) the hydrogen-bond basicity scale for N-heteroaromatics in carbon tetrachloride, and ii) the hydrogen-bond acidities for NH/OH donors and hydrogen-bond basicities of N/O acceptors determined in 1,1,1-trichloroethane. Attention is paid to i) the reliability of semiempirical versus ab initio treatments of the quantum mechanical molecule, ii) the role of solvation, and iii) the effect of including the polarization energy component in the calculation of the GMIPp functional. The statistical analysis of the results reveals that the GMIP functional, which combines electrostatic and steric energy components, predicts with reasonable accuracy and computational efficiency the hydrogen-bond strength for a wide variety of compounds.     

Resonance-assisted hydrogen bonds: a critical examination. Structure and stability of the enols of beta-diketones and beta-enaminones

The characteristics of the intramolecular hydrogen bond (IMHB) for a series of 40 different enols of beta-diketones and their nitrogen counterparts have been systematically analyzed at the B3LYP/6-311+G(3df,2p)//B3LYP/6-311+G(d,p) level of theory. In some cases, two tautomers may exist which are interconnected by a hydrogen shift through the IMHB. In tautomer a the HB donor group (YH) is attached to the six-membered ring, while in tautomer b the HB acceptor (X) is the one that is attached to the six-membered ring. We found that changing an O to a N favors the a tautomer when the atom is endo and the contrary when it is exo, while the presence of a double bond favors the a tautomers. As expected, the OH group behaves as a better HB donor than the NH2 group and the C=NH group as a better HB acceptor than the C=O group, although the first effect clearly dominates. Accordingly, the expected IMHB strength follows the [donor, acceptor] trend: [OH, C=NH] > [OH, C=O] > [NH2, C=NH] > [NH2, C=O]. For all those compounds in which the functionality exhibiting the IMHB is unsaturated (I-type), the IMHB is much stronger than in their saturated counterparts (II-type). However, when the systems of the II-type subset, which are saturated, are constrained to have the HB donor and the HB acceptor lying in the same plane and at the same distance as in the corresponding unsaturated analogue, the IMHB is of similar or even larger strength. Hence, we conclude that, at least for this series of unsaturated compounds, the resonance-assisted hydrogen bond effect is not the primary reason behind the strength of their IMHBs, which is simply a consequence of the structure of the sigma-skeleton of the system that keeps the HB donor and the HB acceptor coplanar and closer to each other.     

Influence of bond fixation in benzo-annulated N-salicylideneanilines and their ortho-C(=O)X derivatives (X = CH3, NH2, OCH3) on tautomeric equilibria in solution

1H, 13C, and 15N NMR spectra show that an ortho-C(=O)X group present in the molecules of N-salicylideneanthranilamide (X = NH2), methyl N-salicylideneanthranilate (X = OCH3), N-salicylidene-o-aminoacetophenone (X = CH3), and their benzo analogues have only a minor effect on the tautomeric OH/NH-equilibrium in solution. Only two of three possible tautomers were detected. Lability of the absent form was proved by theoretical calculations. Calculated energies show that the enolimino form (OH) is less stable than the enaminone (NH) form only for dibenzo-annulated N-salicylideneanilines. The population of each species in the tautomeric mixture was found to be inversely proportional to its energy. Application of the geometry-based aromaticity index HOMA shows that the effectiveness of the pi-electron delocalization in different rings in the molecule depends mostly on the position of benzo-annulation. Both the NH...O and N...HO hydrogen bonds present in the NH and OH tautomers, respectively, increase the aromaticity of the quasirings H-O-C=C-C=N and O=C-C=C-N-H and decrease the aromatic character of the fused benzene ring. These results seem to be reliable when N-salicylideneanilines studied are compared with naphthalene and their benzo-annulated derivatives, i.e., phenanthrene, anthracene, and triphenylene. An analysis of the effectiveness of pi-electron delocalization confirms that in all cases studied, the OH form is more stable. Although the HOMA values and calculated energies are not a criterion that allows determination of the dominating tautomer, both of these parameters correctly show the effect of changes in the molecular topology on tautomeric preferences.     

Nanoswitches based on DNA base pairs: why adenine-thymine is less suitable than guanine-cytosine.

Substituted Watson–Crick guanine–cytosine (GC) base pairs were recently shown to yield robust three‐state nanoswitches. Here, we address the question: Can such supramolecular switches also be based on Watson–Crick adenine‐thymine (AT) base pairs? We have theoretically analyzed AT pairs in which purine‐C8 and/or pyrimidine‐C6 positions carry a substituent X=NH^?, NH₂, NH₃⁺ (N series), O^?, OH or OH₂⁺ (O series), using the generalized gradient approximation (GGA) of density functional theory at the BP86/TZ2P level. Thus, we explore the trend in geometrical shape and hydrogen bond strengths in AT pairs along a series of stepwise protonations of the substituents. Introducing a charge on the substituents leads to substantial and characteristic changes in the individual hydrogen bond lengths when compared to the neutral AT pair. However, the trends along the series of negative, neutral, and positive substituents are less systematic and less pronounced than for GC. In certain instances, internal proton transfer from thymine to adenine occurs. Our results suggest that AT is a less suitable candidate than GC in the quest for chemically controlled nanoswitches.     

1H, 15N] heteronuclear single quantum coherence NMR study of the mechanism of aquation of platinum(IV) ammine complexes

The aquation and hydrolysis of a series of platinum(IV) complexes of the general form cis, trans, cis-[PtCl 2(X) 2( (15)NH 3) 2] (X = Cl (-), O 2CCH 3 (-), OH (-)) have been followed by [ (1)H, (15)N] Heteronuclear Single Quantum Coherence NMR spectroscopy. Negligible aquation (<5%) is observed for the complexes where X = O 2CCH 3 (-) or OH (-) over 3-4 weeks. Aquation of cis-[PtCl 4( (15)NH 3) 2] ( 1) is observed, and the rate of aquation increases with increasing pH and upon the addition of 0.01 mol equiv of the platinum(II) complex cis-[PtCl 2( (15)NH 3) 2] (cisplatin). The first aquated species formed from cis-[PtCl 4(NH 3) 2] has one of the axial chloro groups (relative to the equatorial NH 3 ligands) replaced by an aqua/hydroxo ligand. The second observed substitution occurs in an equatorial position. Peaks that are consistent with five of the eight possible aquation species were observed in the NMR spectra.     

Metal-catalyzed anaerobic disproportionation of hydroxylamine. Role of diazene and nitroxyl intermediates in the formation of N2, N2O, NO+, and NH3

The catalytic disproportionation of NH(2)OH has been studied in anaerobic aqueous solution, pH 6-9.3, at 25.0 degrees C, with Na(3)[Fe(CN)(5)NH(3)].3H(2)O as a precursor of the catalyst, [Fe(II)(CN)(5)H(2)O](3)(-). The oxidation products are N(2), N(2)O, and NO(+) (bound in the nitroprusside ion, NP), and NH(3) is the reduction product. The yields of N(2)/N(2)O increase with pH and with the concentration of NH(2)OH. Fast regime conditions involve a chain process initiated by the NH(2) radical, generated upon coordination of NH(2)OH to [Fe(II)(CN)(5)H(2)O](3)(-). NH(3) and nitroxyl, HNO, are formed in this fast process, and HNO leads to the production of N(2), N(2)O, and NP. An intermediate absorbing at 440 nm is always observed, whose formation and decay depend on the medium conditions. It was identified by UV-vis, RR, and (15)NMR spectroscopies as the diazene-bound [Fe(II)(CN)(5)N(2)H(2)](3)(-) ion and is formed in a competitive process with the radical path, still under the fast regime. At high pH's or NH(2)OH concentrations, an inhibited regime is reached, with slow production of only N(2) and NH(3). The stable red diazene-bridged [(NC)(5)FeHN=NHFe(CN)(5)](6)(-) ion is formed at an advanced degree of NH(2)OH consumption.     

Apparent stoichiometry of water in proton hydration and proton dehydration reactions in CH3CN/H2O solutions

Gradual solvation of protons by water is observed in liquids by mixing strong mineral acids with various amounts of water in acetonitrile solutions, a process which promotes rapid dissociation of the acids in these solutions. The stoichiometry of the reaction XH(+) + n(H(2)O) = X + (H(2)O)(n)H(+) was studied for strong mineral acids (negatively charged X, X = ClO(4)ˉ, Clˉ, Brˉ, Iˉ, CF(3)SO(3)ˉ) and for strong cationic acids (uncharged X, X = R*NH(2), H(2)O). We have found by direct quantitative analysis preference of n = 2 over n = 1 for both groups of proton transfer reactions at relatively low water concentrations in acetonitrile. At high water concentrations, we have found that larger water solvates must also be involved in the solvation of the proton while the spectral features already observed for n = 2, H(+)(H(2)O)(2), remain almost unchanged at large n values up to at least 10 M of water.     

Basic ancillary ligands promote O-O bond formation in iridium-catalyzed water oxidation: a DFT study

The cationic iridium complex [Ir(OH(2))(2)(phpy)(2)](+) (phpy = o-phenylpyridine) is among the most efficient mononuclear catalysts for water oxidation. The postulated active species is the oxo complex [Ir(O)(X)(phpy)(2)](n), with X = OH(2) (n = +1), OH(-) (n = 0) or O(2-) (n = -1), depending on the pH. The reactivity of these species has been studied computationally at the DFT(B3LYP) level. The three [Ir(O)(X)(phpy)(2)](n) complexes have an electrophilic Ir(v)-oxo moiety, which yields an O-O bond by undergoing a nucleophilic attack of water in the critical step of the mechanism. In this step, water transfers one proton to either the Ir(V)-oxo moiety or the ancillary X ligand. Five different reaction pathways associated with this acid/base mechanism have been characterized. The calculations show that the proton is preferably accepted by the X ligand, which plays a key role in the reaction. The higher the basicity of X, the lower the energy barrier associated with O-O bond formation. The anionic species, [Ir(O)(2)(phpy)(2)](-), which has the less electrophilic Ir(V)-oxo moiety but the most basic X ligand, promotes O-O bond formation through the lowest energy barrier, 14.5 kcal mol(-1). The other two active species, [Ir(O)(OH)(phpy)(2)] and [Ir(O)(OH(2))(phpy)(2)](+), which have more electrophilic Ir(V)-oxo moieties but less basic X ligands, involve higher energy barriers, 20.2 kcal mol(-1) and 25.9 kcal mol(-1), respectively. These results are in good agreement with experiments showing important pH effects in similar catalytic systems. The theoretical insight given by the present study can be useful in the design of more efficient water oxidation catalysts. The catalytic activity may increase by using ligand scaffolds bearing internal bases.     

Generalized anomeric effect on N-alkyl-amino cation affinities: a G2(+)M investigation

The gas-phase N-alkyl-amino-cation affinities (NAACA) of archetypal anionic main-group element hydrides across the Periodic Table have been investigated by means of a modified G2(+) scheme. The reactions studied include R(2)NB → R(2)N(+) + B(-) (R = H, Me; B = XH(n), n = 0-3; X = F, Cl, Br, O, S, Se, N, P, As, C, Si, Ge). Our calculations indicate that the reasonable linear correlations between NAACA and proton affinities (PA) only exist within the Period 2 anions, including H(3)C(-), H(2)N(-), HO(-), and F(-), or the anions within Periods 3-4 in the Periodic Table, which is significantly different from the alkyl cation affinities, where there is a reasonable correlation between the computed alkyl cation affinity and PA values of the set of anionic main-group element hydrides. The interesting differences can be ascribed to the generalized anomeric effect induced by n(N) → σ*(X-H) negative hyperconjugation found in R(2)NXH(n), with central atom X belonging to Groups 14-16 (X = O, S, Se, N, P, As, C, Si, Ge).     

Methyl and phenyl substitution effects on the proton affinities of hydrides of first and second row elements and substituent effects on the proton affinities of ring carbons in benzene: a DFT study

Theoretical calculations using B3LYP density functional theory (DFT) with the 6-311++G(d,p) basis set have been performed to determine proton affinities (PAs) of a series of H-X compounds and the corresponding methyl- (H(3)C-X) and phenyl- (Ph-X) substituted derivatives with a variety of proton acceptor atoms, such as C, O, N, F, Si, P, S, Cl, etc. Our results illustrate an interesting substituent effect on PAs. The PAs of ring carbon atoms for a series of monosubstituted benzene molecules (Y-C(6)H(5); Y = F, Cl, CH(3), OCH(3), NH(2), PH(2), OH, SH, SiH(3), CN, CF(3), and NO(2)) have also been estimated. Correlations between proton affinities of H-X, H(3)C-X, and Ph-X and substituent effects on the PAs of the ring carbon atoms for a series of monosubstituted benzene molecules have been studied. It has been observed that substituent effects on the PAs of the ring carbon atoms follow a good Hammett-type correlation.     

B3LYP and MP2 calculations of the enthalpies of hydrogen-bonded complexes of methanol with neutral bases and anions: comparison with experimental data

To perfect a method for building a theoretical hydrogen-bond basicity scale, the enthalpy of hydrogen bonding between methanol and thirteen neutral and anionic bases (MeOH, MeNH2, Me2NH, Et2NH, Me3N, Et3N, Br-, CN-, SH-, Cl-, HCOO-, MeO-, F-) was calculated by DFT and ab initio methods. The theoretical results were compared to selected experimental ones. It appears that B3LYP/6-31+G(d,p) calculations are satisfactory for optimizing the geometry of complexes and giving a general order of basicity. However, they are deficient for reproducing the large effect of alkyl groups on the hydrogen-bond basicity of amines. This deficiency is explained by intermolecular perturbation theory calculations, which show that the alkylation of nitrogen dramatically increases the dispersion energy component not taken into account by the B3LYP functional. Of the methods considered, only MP2/aug-cc-pVTZ calculations are capable of reproducing the binding enthalpy within the experimental error for the first-row acceptor atoms N, O, and F, and of accounting for dispersion effects created by alkylation at the hydrogen-bond acceptor site.     

Highly air-stable anionic mononuclear and neutral binuclear palladium(II) complexes for C-C and C-N bond-forming reactions

The short-bite aminobis(phosphonite), PhN{P(-OC10H6(mu-S)C10H6O-)}2 (2), containing a mesocyclic thioether backbone is synthesized by either treating PhN(PCl2)2 with 2 equiv of thiobis(2,2'-naphthol) or reacting chlorophosphite (-OC10H6(mu-S)C10H6O-)PCl (1) with aniline in the presence of a base. Treatment of 2 with an equimolar amount of Pd(COD)Cl2 in the presence of H2O affords a P-N-P-bridged and P,S-metalated binuclear complex, [PhN(P(-OC10H6(mu-S)C10H6O-)-kappaP)2Pd2Cl2{P(-OC10H6(mu-S)C10H6O-)(O)-kappaP,kappaS}2] (3), whereas the same reaction with 2 equiv of Pd(COD)Cl2 in the presence of H2O and Et3N produces the mononuclear anionic complex [{(-OC10H6(mu-S)C10H6O-)P(O)-kappaP,kappaS}PdCl2](Et3NH) (5). By contrast, reaction of 2 with 2 equiv of Pd(COD)Cl2 and H2O in the absence of Et3N gives the hydrogen phosphonate coordinated complex [{(-OC10H6(mu-S)C10H6O-)P(OH)}PdCl2] (4) which converts to the anionic complex in solution or in the presence of a base. Compound 2 on treatment with Pt(COD)X2 (X = Cl or I) afforded P-coordinated four-membered chelate complexes [PhN(P(-OC10H6(mu-S)C10H6O-)-kappaP)2PtX2] (6 X = Cl, 7 X = I). The crystal structures of compounds 2, 3, 5, and 7 are reported. Compound 3 is the first example of a crystallographically characterized binuclear palladium complex containing a bidentate bridging ligand and its hydrolyzed fragments forming metallacycles containing a palladium-phosphorus sigma bond. All palladium complexes proved to be very good catalysts for the Suzuki-Miyaura and Mizoroki-Heck cross-coupling and amination reactions with excellent turnover numbers (TON up to 1.46 x 105 in the case of the Suzuki-Miyaura reaction).     

Substituent effects on the bond dissociation enthalpies of aromatic amines

Bond dissociation enthalpy differences, Z-X DeltaBDE = BDE(4-YC(6)H(4)Z-X) - BDE(C(6)H(5)Z-X), for Z = CH(2) and O are largely independent of X and are determined mainly by the stabilization/destabilization effect of Y on the 4-YC(6)H(4)Z(*) radicals. The effects of Y are small (< or =2 kcal/mol for all Y) for Z = CH(2), but they are large for Z = O, where good correlations with sigma(p)(+)(Y) yield rho(+) = 6.5 kcal/mol. For Z = NH, two sets of electrochemically measured N-H DeltaBDEs correlate with sigma(p)(+)(Y), yielding rho(+) = 3.9 and 3.0 kcal/mol. However, in contrast to the situation with phenols, these data indicate that the strengthening effect on N-H BDEs of electron-withdrawing (EW) Y's is greater than the weakening effect of electron-donating (ED) Y's. Attempts to measure N-H DeltaBDEs in anilines using two nonelectrochemical techniques were unsuccessful; therefore, we turned to density functional theory. Calculations on 15 4-YC(6)H(4)NH(2) gave N-H DeltaBDEs correlating with sigma(p)(+) (rho(+) = 4.6 kcal/mol) and indicated that EW and ED Y's had comparable strengthening and weakening effects, respectively, on the N-H bonds. To validate theory by connecting it to experiment, the N-H DeltaBDEs of four 4,4'-disubstituted diphenylamines and five 3,7-disubstituted phenothiazines were both calculated and measured by the radical equilibration EPR technique. For all compounds, theory and experiment agreed to better than 1 kcal/mol. Dissection of N-H DeltaBDEs in 4-substituted anilines and O-H DeltaBDEs in 4-substituted phenols into interaction enthalpies between Y and NH(2)/OH (molecule stabilization/destabilization enthalpy, MSE) and NH*/O* (radical stabilization/destabilization enthalpy, RSE) reveals that for both groups of compounds, ED Y's destabilize the molecule and stabilize the radical, while the opposite holds true for EW Y's. However, in the phenols the effects of substituents on the radical are roughly 3 times as great as those in the molecule, whereas in the anilines the two effects are of comparable magnitudes. These differences arise from the stronger ED character of NH(2) vs OH and the weaker EW character of NH* vs O*. The relatively large contributions to N-H BDEs in anilines arising from interactions in the molecules suggested that N-X DeltaBDEs in 4-YC(6)H(4)NH-X would depend on X, in contrast to the lack of effect of X on O-X and CH(2)-X DeltaBDEs in 4-YC(6)H(4)O-X and 4-YC(6)H(4)CH(2)-X. This suggestion was confirmed for X = CH(3), H, OH, and F, for which the calculated NH-X DeltaBDEs yielded rho(+) = 5.0, 4.6, 4.0, and 3.0 kcal/mol, respectively.     

Arene-anion based arginine-binding motif on a galactose scaffold: structure-activity relationships of interactions with arginine-rich galectins

Two series of C3-benzamido and O2-anion-substituted galactopyranosides were synthesized and studied as binders to arginine-rich proteins galectin-1, -3, -7, -8N (N-terminal domain), and -9N (N-terminal domain). The first series had a 4-methylbenzamide at C3 and the anionic O2-substituent was varied. The second series varied the 4-substituent of the C3-benzamide, whereas the anionic O2 substituent was kept as a sulfate. The influence of the O2-anion substituent correlated negatively with the oxygen charge density in case of galectin-1, -3, and -9N. In the second series, the electron-donating capacity of the 4-substituent of the C3-benzamides correlated positively with the magnitude of the affinity enhancement by the 2O-sulfate.     

The infrared spectrum of solid L-alanine: influence of pH-induced structural changes

The influence of the pH on the infrared spectrum of L-alanine has been analyzed by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. The amino acid was precipitated from aqueous solutions and dried at 36.5 degrees C, in order to stabilize cationic L-alanine or alaninium [CH3CH(NH3(+))COOH] at pH 1, the zwitterionic form [CH3CH(NH3(+))COO(-)] at pH 6, and anionic L-alanine or alaninate [CH3CH(NH2)COO(-)] at pH 13. New insight on the specific inter and intramolecular interactions in the different forms of L-alanine was reached by a novel methodological approach: an infrared technique not used before to analyze solid amino acid samples (DRIFTS), in combination with a detailed analysis based on spectral deconvolution. The frequency ranges of interest include the carbonyl/carboxyl stretching and amine deformation modes and the OH/NH stretching modes. It was shown that intermolecular hydrogen bonds between the NH3(+) and COO(-) groups are predominant in the zwitterionic form, whereas in cationic L-alanine, H bonds between the COOH groups are responsible for the formation of dimers. In anionic L-alanine, only strong electrostatic interactions between the COO(-) groups and Na(+) ions are proposed, evidencing the relevant role of the counterion.     

Transmission of electronic effects through the {closo-1-CB9} and {closo-1-CB11} cages: apparent dissociation constants for series of [closo-1-CB9H8-1-COOH-10-X] and [closo-1-CB11H10-1-COOH-12-X] acids

The apparent ionization constants pK(a)' for series of carboxylic acids [closo-1-CB(9)H(8)-1-COOH-10-X](-) (1) and [closo-1-CB(11)H(10)-1-COOH-12-X](-) (2), where X = H, I, n-C(6)H(13), (+)NMe(3), (+)N(2), (+)SMe(2), OC(5)H(11), were measured in EtOH/H(2)O (1/1, v/v) at 24 °C. Correlation analysis of the pK(a)' values using Hammett substituent constants σ(p)(X) gave the reaction constant ρ = 0.87 ± 0.04 for series 1 and ρ = 1.00 ± 0.09 for series 2. These values are higher than for derivatives of PhCH═CHCOOH (ρ = 0.70 ± 0.09 in 55% EtOH) and correspond to 56% and 65% efficiencies in transmission of electronic effects by [closo-1-CB(9)H(10)](-) (E) and [closo-1-CB(11)H(12)](-) (F), respectively, as compared to benzene (A). Experimental results were supported with DFT calculations of relative acidity for series of acids derived from A, E, and F in aqueous medium.