Complexation of tetrandrine with calcium ion probed by various spectroscopic methods and molecular modeling

The formation of the complex between tetrandrine and the calcium ion, in solution, was studied using FTIR, UV-Vis, ¹H NMR, ¹³C NMR and electrospray mass spectroscopy spectroscopic methods and molecular modeling. The calcium salts used were: Ca(ClO₄)₂·4H₂O and Ca(Picrate)₂ in the solvents: acetonitrile (CH₃CN), deuterated acetonitrile (CD₃CN) and tetrahydrofurane (THF). The determined complex stability constant was: 20277±67 dm³ mol⁻¹ and corresponding free energy ΔG₀=−5.820±0.002 kcal mol⁻¹. The molecular simulation of the complex formation with the MM3 Augmented force field integrated in CAChe provided useful data about its energy. Combining the experimental results and molecular modeling we propose a model for the structure of tetrandrine-Ca complex with an eight coordinated geometry.     

Dimeric water embedded within a hydrophobic cavity of tetra-(p-tert-butyl)thiacalix[4]arene

X-ray crystallographic analysis and density functional B3LYP/6-31G(d) calculation confirm that dimeric water is embedded within a hydrophobic cavity of tetra-(p-tert-butyl)thiacalix[4]arene and stabilized by hydrogen bondings of aromatic π?H₂O(1) and methyl?H₂O(2) in the dimeric water inclusion complex with binding energies of 1.4 and 0.9 kcal mol⁻¹ respectively, and by hydrogen bonding formed between H₂O(2) and four phenolic OH groups from an adjacent tetra-(p-tert-butyl)thiacalix[4]arene, with binding energy of 3.8-4.2 kcal mol⁻¹.     

An experimental and theoretical reinvestigation of the Michael addition of primary and secondary amines to dimethyl acetylenedicarboxylate

Theoretical calculations of the Michael addition of diethylamine, pyrrolidine, and benzylamine to DMAD at the DFT (B3LYP/6-31+G^∗) level indicate that the reaction follows a stepwise mechanism via a zwitterionic intermediate. The reactions have low activation barriers, 13–15 kcal mol⁻¹ and are exothermic, ΔH° = −29 to −44 kcal mol⁻¹. The detailed investigation of the reaction of benzylamine with DMAD reveals participation of the reactant-, transition structure-, and the product-complexes and that the 1,3-prototropic shift occurs through the benzylamine molecule. It also predicts formation of dimethyl 2-(N-benzylimino)butane-1,4-dicarboxylate as one of the products, which has been duly isolated and characterized experimentally.     

Tetramidocavitand: strong anion receptor by well-organized four -(CO)N-H?X interactions

Resorcin[4]arene-based tetramidocavitands containing four secondary amide groups on their upper rim showed strong (R = methyl or ethyl) binding properties. The caviplex formation through hydrogen bonds of -(CO)N-H?X⁻ was supported by ¹H NMR and crystal structure analyses. In a mixture of C₂D₂Cl₄/DMSO/D₂O = 5:15:2 at 25 °C, the thermodynamic parameters for caviplex @1, ΔG (kcal mol⁻¹), ΔH (kcal mol⁻¹), and ΔS (cal K⁻¹ mol⁻¹), are −3.7, −8.6, and −16.7, respectively.     

Towards the rational design of palladium-N-heterocyclic carbene catalysts by a combined experimental and computational approach

A combined experimental and computational approach towards the development of Pd-NHC catalysts is described. A range of benzimidazolylidinium ligands incorporating electron-rich and electron-poor substituents were prepared and evaluated in the Suzuki reaction. The most electron-rich ligand showed the highest catalytic activity. Based on this information, the first alkyl-alkyl Negishi cross-coupling reaction protocol was developed. Evaluation of N,N′-diaryl-(4,5-dihydro)imidazolylilidinium ligands showed a strong dependence on the steric topography around the metal centre. A computational study of the most active ligand in the Negishi reaction, its Pd(0) and PdCl₂-complexes and related structures were modelled at the B3LYP/DZVP and HF/3-21G levels of theory. The potential energy hypersurfaces flattened with increase in ligand size. Binding energies were computed for carbene/Pd(0) adducts (in the range ∼31-40 kcal mol⁻¹), roughly double that for PH₃ (∼16 kcal mol⁻¹). Weak intramolecular interactions were found using AIM analyses.     

Hydrophobic interaction chromatography of proteins: Thermodynamic analysis of conformational changes

For BSA and β-lactoglobulin adsorption to hydrophobic interaction chromatography (HIC) stationary phases leads to conformational changes. In order to study the enthalpy (ΔH_ads), entropy (ΔS_ads), free energy (ΔG_ads) and heat capacity (Δc_p,ads) changes associated with adsorption we evaluated chromatographic data by the non-linear van’t Hoff model. Additionally, we performed isothermal titration calorimetry (ITC) experiments. van’t Hoff analysis revealed that a temperature raise from 278 to 308 K increasingly favoured adsorption seen by a decrease of ΔG_ads from −12.9 to −20.5 kJ/mol for BSA and from −6.6 to −13.2 kJ/mol for β-lactoglobulin. Δc_p,ads values were positive at 1.2 m (NH₄)₂SO₄ and negative at 0.7 m (NH₄)₂SO₄. Positive Δc_p,ads values imply hydration of apolar groups and protein unfolding. These results further corroborate conformational changes upon adsorption and their dependence on mobile phase (NH₄)₂SO₄ concentration. ITC measurements showed that ΔH_ads is dependent on surface coverage already at very low loadings. Discrepancies between ΔH_ads determined by van’t Hoff analysis and ITC were observed. We explain this with protein conformational changes upon adsorption which are not accounted for by van’t Hoff analysis.     

Dynamic H NMR study of 4-methylphenoxyimidoyl azides: conformational or configurational isomerisation?

Dynamic ¹H NMR (500 MHz) investigation of 4-methylphenoxyimidoyl azides (4-CH₃-C₆H₄-O-CN-Y)-N₃, Y=4-CH₃-C₆H₄-SO₂-, 4-Br-C₆H₄-SO₂-, C₆H₅SO₂-, CH₃-SO₂-, -CN in acetone-d₆ at temperature range of 195-280 K is reported. The observed free energy barrier (almost 12 kcal mol⁻¹) is attributed to conformational isomerisation about the N-S bond for Y=4-CH₃-C₆H₄-SO₂-, 4-Br-C₆H₄-SO₂-, C₆H₅SO₂-, CH₃-SO₂- and (almost 14 kcal mol⁻¹) to configurational isomerisation (E/Z) about CN bond for Y=-CN.     

Protonated MF3 (M = N-Bi): Structure, stability, and thermochemistry of the H-MF3 and HF-MF2 isomers

The structure, stability, and thermochemistry of the H(MF₃)⁺ isomers (M = N-Bi) have been investigated by MP2 and coupled cluster calculations. All the HF-MF₂⁺ revealed weakly bound ion-dipole complexes between MF₂⁺ and HF. For M = N, As, Sb, and Bi they are more stable than the H-MF₃⁺ covalent structures (free energy differences) by 6.3, 14.3, 32.1, and 73.5 kcal mol⁻¹, respectively. H-PF₃⁺ is instead more stable than HF-PF₂⁺ by 21.8 kcal mol⁻¹. The proton affinities (PAs) of MF₃ at the M atom range from 91.9 kcal mol⁻¹ (M = Bi) to 156.5 kcal mol⁻¹ (M = P), and follow the irregular periodic trend BiF₃ < SbF₃ < AsF₃ < NF₃ < PF₃. The PAs at the F atom range instead from 131.9 kcal mol⁻¹ (M = P) to 164.9 kcal mol⁻¹ (M = Bi), and increase in the more regular order PF₃ ≈ NF₃ < AsF₃ < SbF₃ < BiF₃. This trend parallels the fluoride-ion affinities of the MF₂⁺ cations. For protonated NF₃ and PF₃, the calculations are in good agreement with the available experimental results. As for protonated AsF₃, they support the formation of HF-AsF₂⁺ rather than the previously proposed H-AsF₃⁺. The calculations indicate also that the still elusive H(SbF₃)⁺ and H(BiF₃)⁺ should be viable species in the gas phase, exothermically obtainable by various protonating agents.     

Heat capacity and thermodynamic functions of LuPO4 in the range 0-320 K

The heat capacity of LuPO₄ was measured in the temperature range 6.51-318.03 K. Smoothed experimental values of the heat capacity were used to calculate the entropy, enthalpy and Gibbs free energy from 0 to 320 K. Under standard conditions these thermodynamic values are: (298.15 K) = 100.0 ± 0.1 J K⁻¹ mol⁻¹, S⁰(298.15 K) = 99.74 ± 0.32 J K⁻¹ mol⁻¹, H⁰(298.15 K) − H⁰(0) = 16.43 ± 0.02 kJ mol⁻¹, −[G⁰(298.15 K) − H⁰(0)]/T = 44.62 ± 0.33 J K⁻¹ mol⁻¹. The standard Gibbs free energy of formation of LuPO₄ from elements Δ_fG⁰(298.15 K) = −1835.4 ± 4.2 kJ mol⁻¹ was calculated based on obtained and literature data.     

2-(2-Hydroxyaryl)cinnamic amides: a new class of axially chiral molecules

The syntheses of several differently substituted amides formally derived from a chiral amine, either E-2-(2-hydroxyphenyl)cinnamic acid or both E- and Z-2-(2-hydroxynaphthyl)cinnamic acid, are reported. These molecules display a restricted rotation about the C₂-C_aryl bond. The barriers to rotation about the C₂-C_aryl bond were measured by the dynamic ¹H NMR and were found to vary between 11.8 and 24.5 kcal mol⁻¹, depending on the substitution. In particular, E-2-(2-hydroxynapthyl)cinnamic amides, displayed a high barrier to rotation (ΔG_c^‡=24.4 kcal mol⁻¹) and could be isolated in both diastereomerically pure forms at room temperature. The X-ray structure of one E-2-(2-hydroxynapthyl)cinnamic amide, was resolved, enabling for the determination of the absolute configuration of the chiral axis (aR).     

Ethylene addition to OsO3(CH2) - A theoretical study

Quantum chemical calculations at the B3LYP/TZVP level of theory have been carried out for the initial steps of the addition reaction of ethylene to OsO₃(CH₂). The calculations predict that there are two reaction channels with low activation barriers. The kinetically and thermodynamically most favored reaction is the [3+2]_O, C addition which has a barrier of only 2.3 kcal mol⁻¹. The [3+2]_O, O addition has a slightly higher barrier of 6.5 kcal mol⁻¹. Four other reactions of OsO₃(CH₂) with C₂H₄ have significantly larger activation barriers. The addition of ethylene to one oxo group with concomitant migration of one hydrogen atom from ethylene to the methylene ligand yields thermodynamically stable products but the activation energies for the reactions are 16.7 and 20.9 kcal mol⁻¹. Even higher barriers are calculated for the [2+2] addition to the OsO bond (32.6 kcal mol⁻¹) and for the addition to the oxygen atom yielding an oxiran complex (41.2 kcal mol⁻¹). The activation barriers for the rearrangement to the bisoxoosmaoxirane isomer (36.3 kcal mol⁻¹) and for the addition reactions of the latter with C₂H₄ are also quite high. The most favorable reactions of the cyclic isomer are the slightly exothermic [2+2] addition across the OsO bond which has an activation barrier of 46.6 kcal mol⁻¹ and the [3+2]_O, O addition which is an endothermic process with an activation barrier of 44.3 kcal mol⁻¹.     

High basicity of tris-(tetramethylguanidinyl)-phosphine imide in the gas phase and acetonitrile—a DFT study

It is shown by reliable density functional theory (DFT) calculations that compounds 4 and 6 are very powerful neutral organic superbases as evidenced by the calculated proton affinities in the gas phase 305.4 kcal mol⁻¹ (44.8) and 287.8 kcal mol⁻¹ (37.8), respectively, and the corresponding calculated pK_a values in acetonitrile given within parentheses.     

Heat-capacity measurements and thermodynamic functions of crystalline adenine; revised thermodynamic properties of aqueous adenine

The standard molar heat capacity C^°_p,m of adenine(cr) has been measured using adiabatic calorimetry over the range 6<(T/K)<310 and the results used to derive thermodynamic functions for adenine(cr) at smoothed temperatures. At T=298.15 K, C^°_p,m=(142.67±0.29) J · K⁻¹ · mol⁻¹ and the third law entropy S^°_m=(145.62±0.29) J · K⁻¹ · mol⁻¹. The standard molar Gibbs free energy of formation Δ_fG^°_m at T=298.15 K for crystalline adenine was calculated, using the standard molar enthalpy of formation for the compound and entropies of the elements from the literature, and found to be Δ_fG^°_m=(301.4±1.0) kJ · mol⁻¹. The results were combined with solution calorimetry and solubility measurements from the literature to yield revised values for the standard molar thermodynamic properties of aqueous adenine at T=298.15 K: Δ_fG^°_m=(313.4±1.0) kJ · mol⁻¹, Δ_fH^°_m=(129.5±1.4) kJ · mol⁻¹, and S_m^°=(217.68±0.44) J · K⁻¹ · mol⁻¹.     

Theoretical study of the thermochemistry and the kinetics of the SFxCl (x = 0-5) series

A systematic thermodynamic and kinetic study of the entire SF_xCl (x = 0-5) series has been carried out. High-level quantum chemical composite methods have been employed to derive enthalpy of formation values from calculated atomization and isodesmic energies. The resulting values for the SCl, SFCl, SF₂Cl(C₁), SF₃Cl(C_s), SF₄Cl(C_s) and SF₅Cl molecules are 28.0, −36.0, −64.2, −134.3, −158.2 and −237.1 kcal mol⁻¹. A comparison with previous experimental and theoretical values is presented. Statistical adiabatic channel model/classical trajectory, SACM/CT, calculations of selected complex-forming and recombination reactions of F and Cl atoms with radicals of the series have been performed between 200 and 500 K. The reported rate coefficients span over the normal range of about 6 × 10⁻¹² and 5 × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹ expected for this type of barrierless reactions.     

Anchored thiol smectite clay—kinetic and thermodynamic studies of divalent copper and cobalt adsorption

A natural smectite clay sample from Serra de Maicuru, Pará State, Brazil, had aluminum and zirconium polyoxycations inserted within the interlayer space. The precursor and pillarized smectites were organofunctionalized with the silyating agent 3-mercaptopropyltrimethoxysilane. The basal spacing of 1.47 nm for natural clay increased to 2.58 and 2.63 nm, for pillared aluminum, S_Al/SH, and zirconium, S_Zr/SH, and increases in the surface area from 44 to 583 and 585 m² g⁻¹, respectively. These chemically immobilized clay samples adsorb divalent copper and cobalt cations from aqueous solutions of pH 5.0 at 298±1 K. The Langmuir, Redlich-Peterson and Toth adsorption isotherm models have been applied to fit the experimental data with a nonlinear approach. From the cation/basic center interactions for each smectite at the solid-liquid interface, by using van’t Hoff methodology, the equilibrium constant and exothermic thermal effects were calculated. By considering the net interactive number of moles for each cation and the equilibrium constant, the enthalpy, Δ_intH⁰ (−9.2±0.2 to −10.2±0.2 kJ mol⁻¹) and negative Gibbs free energy, Δ_intG⁰ (−23.9±0.1 to −28.7±0.1 kJ mol⁻¹) were calculated. These values enabled the positive entropy, Δ_intS⁰ (51.3±0.3 to 55.0±0.3 JK⁻¹ mol⁻¹) determination. The cation-sulfur interactive process is spontaneous in nature, reflecting the favorable enthalpic and entropic results. The kinetics of adsorption demonstrated that the fit is in agreement with a second-order model reaction with rate constant k₂, varying from 4.8×10⁻² to 15.0×10⁻² and 3.9×10⁻² to 12.2×10⁻² mmol⁻¹ min⁻¹ for copper and cobalt, respectively.     

The effect of spiroadamantane substitution on the conformational preferences of N-Me pyrrolidine and N-Me piperidine: a description based on dynamic NMR spectroscopy and ab initio correlated calculations

Dynamic NMR spectroscopy and ab initio correlated calculations revealed that the attachment of a spiroadamantane entity at the C-2 position of N-methylpyrrolidine or N-methylpiperidine induces a severe steric crowding around nitrogen, which changes the conformational space of the heterocycle resulting in: (a) the complete destabilization of the N-Me(eq) conformer in spiranic structures; in contrast the N-Me(eq) conformer corresponds to the global minimum in N-methylpyrrolidine or N-methylpiperidine. The spiroadamantane structure raises the energy of the equatorial conformer because of the severe van der Waals repulsion between the N-Me(eq) group and adamantane C-H bonds. (b) The interconversion between the only populated enantiomeric N-Me(ax) conformers ax→[eq]→ax′; the interconversion eq→ax between N-Me(eq) and N-Me(ax) conformers, which are both populated, is observed in N-methylpyrrolidine or N-methylpiperidine. (c) The raising of ring and nitrogen inversion barriers ax→ts by ∼4-6 kcal mol⁻¹. The dynamic NMR study provides evidence that the most important process required for the enantiomerization between the axial N-Me conformers in spiropiperidine 4 and spiropyrrolidine 5 are different, i.e., a nitrogen inversion in 5 (9.10 kcal mol⁻¹) and a ring inversion in 4 (15.2 kcal mol⁻¹). While an enantiomerization interconverts N-Me axial conformers in spiropiperidine 5 and spiropyrrolidine 4, substitution of the pyrrolidine ring of 5 with a C-Me group effects a diastereomerization between two N-Me axial conformers and reduces effectively the nitrogen inversion barrier according to the protonation experiments and the calculations. In general, all the calculations levels used, i.e., the MM3, B3LYP/6-31+G^∗∗ and MP2/6-311++G^∗∗//B3LYP/6-31+G^∗∗, predict correctly the different stability of the local minima; however only MP2/6-311++G^∗∗//B3LYP/6-31+G^∗∗ was found to be reliable for the calculation of the nitrogen inversion barriers.     

Conformational fluxionality in a palladium(II) complex of flexible click chelator 4-phenyl-1-(2-picolyl)-1,2,3-triazole: A dynamic NMR and DFT study

An experimental and theoretical DFT study was carried out on the solution behavior in [D₇]DMF for bis-chelate complex [Pd(L)₂](BF₄)₂·2CH₃CN (L = 4-phenyl-1-(2-picolyl)-1,2,3-triazole). In structure of [Pd(L)₂]²⁺, the central square-planar palladium(II) cation is trans-chelated by two L substrates, each through the pyridine and the triazole N2 nitrogen atoms, forming two six-membered metallacycles. These can adopt boat-like conformations anti-trans-[Pd(L)₂]²⁺ and syn-trans-[Pd(L)₂]²⁺ in which the picolyl methylene carbons are anti or syn, respectively, relative to the palladium coordination plane. In solution, the boat-to-boat inversion at both metallacycles takes place. The conformers are in a dynamic equilibrium, which was monitored by variable-temperature (VT) ¹H NMR spectroscopy in the temperature range of 223-353 K. The equilibrium lies on the side of the anti-trans-[Pd(L)₂]²⁺ conformer and the corresponding reaction enthalpy and entropy is estimated to be 0.6 ± 0.5 kcal mol⁻¹ and 0.8 ± 1 cal mol⁻¹ K⁻¹, respectively. From the full-line-shape analysis of resonances in the VT ¹H NMR spectra, the activation enthalpy and activation entropy was determined to be 13.0 ± 0.4 kcal mol⁻¹ and 2.7 ± 1.6 cal mol⁻¹ K⁻¹, respectively. The activation entropy close to zero suggests a nondissociative mechanism for the isomerisation. DFT investigation revealed that the isomerisation proceeds through a one step mechanism with a barrier of 11.40 kcal mol⁻¹. The structures of the syn and anti conformers as well as that of the transition state were characterized. Energy decomposition analysis was carried out in order to explore the origins of the stability difference between the syn and anti isomers.     

Thermodynamic evidence for phase transition in MoO2−δ

The standard Gibbs free energy of formation of MoO_2−δ, Δ_fG^°(MoO_2−δ), has been measured over a wide temperature range (925 to 1925) K using an advanced version of bi-electrolyte solid-state electrochemical cell incorporating a buffer electrode:

Pt∣Mo + MoO_2−δ∥(Y₂O₃)ThO₂∥(CaO)ZrO₂∥O₂(0.1 MPa)∣Pt     

Kinetic studies on Brook-type isomerization of acylpolysilanes to silenes

First-order rate constants of Brook-type isomerization of acylpolysilanes (Me₃Si)₃SiCOR (R = iso-Pr, tert-Bu, Ad, 2,6-xylyl, and Mes) leading to silenes (Me₃Si)₂SiC(OSiMe₃)R at various temperatures were determined. Their Eyring plots gave kinetic parameters of ΔH^‡ = 26.6-29.4 kcal mol⁻¹ and ΔS^‡ = −11.5 to −14.6 cal mol⁻¹ K⁻¹. The isomerization was accelerated by introducing an electron-donating alkyl substituent on the carbonyl carbon. These results are in accordance with a concerted mechanism involving a four-centered transition state.     

Solid state and theoretical evaluation of β-fluoroethyl esters indicate a fluorine-ester gauche effect

Single crystal X-ray diffraction studies and a theoretical analysis indicate a preferred conformation for O-β-fluoroethyl esters, where the CF and CO(CO) bonds are gauche rather than anti to each other. The OCCF dihedral angles for three compounds and five independent structures indicate a range of only 63.4-69.6°. Evaluation of a rotational energy profile around this bond in a model system (β-fluoroethyl acetate) predicted a similar dihedral angle and the gauche conformation to be the minimum on the rotational energy profile. High level ab initio calculations measured the gauche conformer to be 0.95 kcal mol⁻¹ lower in energy than the anti conformer and application of a solvation model further increased this differential to 1.6 kcal mol⁻¹, consistent with a previous solution state (NMR) evaluation of this system.