Free Energy Of Activation Research Articles

Feature Analysis in High-Dimensional Data: Structure-Activity Relationships of Lewis Acid-Transition-Metal Complex-Catalyzed H2 Activation.

Lewis acid-transition metal (LA-TM) catalysts have been proven to have an advantage in catalyzing hydrogen activation. Herein, a high-dimensional structure-activity relationship study is performed for LA-TM-catalyzed hydrogen activation by density functional theory calculations. The DPB-Ni complex is taken as the representative catalyst, and the explored Lewis acid sites and transition-metal centers include B, Al, Ga and Ni, Pd, Pt, respectively. Totally, four general hydrogen activation mechanisms are systematically studied among the nine catalytic systems. The Ga-Ni system undergoes the lowest free energy of activation (11.0 kcal/mol), which is considered to be the optimal combination of the Lewis acid site and transition-metal center. Furthermore, more than 100 parameters are used to analyze the structure-activity relationship, including the physical structure, the bond order, the atom charge, and many other properties. Key parameters of important structures are dug out to show a high correlation with the activity of the LA-TM systems, including the M-H2 distance, the H-H bond length, the second-order perturbation stabilization energy of M-H2, the bond order of the LA-TM, and so on. The multivariable analysis indicates that the feature related to the basic elemental properties and the global feature codetermine the activity of the catalyst. In the LA-TM system, the combination of IpLA/IpTM (Ip, the first ionization energy, the feature related to basic elemental properties) and the chemical hardness (the global feature) can better explain the activity of the catalyst. The IpLA/IpTM reflects the difficulty of breaking the LA-TM bond, affecting the reaction site of activating hydrogen. The hardness reflects the stability and reactivity of LA-TM-RC complexes. The above two features with the addition of the LA-TM bond length (the local feature) can better reflect the activity of the LA-TM system-catalyzed H2 activation. The feature combinations and the method of multidimensional data analysis should be informative guidance for the rational design of efficient LA-TM catalysts for H2 activation.

Influence of alkyl chain length and temperature on the densities, viscosities and speeds of sound of binary liquid mixtures of {2-methyl-1-butanol + 2-ethoxyethanol, or 2-propoxyethanol, or 2-butoxyethanol} at T = (298.15, 303.15 and 308.15) K

Density (ρ), viscosity (η) and speed of sound (u) of pure components and binary mixtures of 2-methyl-1-butanol with alkoxyethanols viz., 2-ethoxyethanol, 2-propoxyethanol and 2-butoxyethanol have been measured over the entire range of composition at different temperatures T = (298.15, 303.15 and 308.15) K. Using the experimental data various thermophysical properties such as excess molar volume (VmE), excess molar isentropic compressibility (Ks,mE), excess speed of sound (uE), deviation in viscosity (Δη) and excess Gibb’s free energy of activation for viscous flow (ΔG∗E) have been calculated. The excess or deviation properties used to define the molecular interactions between like and unlike components. Further, the values of excess and deviation properties have been fitted to the Redlich-Kister polynomial equation. The measured viscosities have been correlated with various semi-empirical relations such as Grunberg-Nissan, Tamura-Kurata, Hind et al., Katti-Chaudhri, McAllister (three body interaction) model, Heric-Brewer and McAllister (four body interaction) model. Apart from this, Prigogine–Flory–Patterson (PFP) theory is also used for the estimation of VmE values for studied binary mixtures.

Insights into C-C Bond Cleavage Mechanisms in Dichloroacetonitrile Formation during Chlorination of Long-Chain Primary Amines, Amino Acids, and Dipeptides.

Dichloroacetonitrile (DCAN) as one of the potentially prioritized regulated DBPs has drawn great attention; however, understanding its formation, especially the C-C bond cleavage mechanisms, is limited. In this study, DCAN formation mechanisms from long-chain primary amines, amino acids, and dipeptides during chlorination were investigated by a combined computational and experimental approach. The results indicate that nitriles initially generate for all of the above precursors, then they undergo β-C-hydroxylation or/and α-C-chlorination processes, and finally, DCAN is produced through the Cα-Cβ bond cleavage. For the first time, the underlying mechanism of the C-C bond cleavage was unraveled to be electron transfer from the O- anion into its attached C atom in the chlorinated nitriles, leading to the strongly polarized Cα-Cβ bond heterocleavage and DCAN- formation. Moreover, DCAN molar yields of precursors studied in the present work were found to be determined by their groups at the γ-site of the amino group, where the carbonyl group including -CO2-, -COR, and -CONHR, the aromatic group, and the -OH group can all dramatically facilitate DCAN formation by skipping over or promoting the time-consuming β-C-hydroxylation process and featuring relatively lower activation free energies in the C-C bond cleavage. Importantly, 4-amino-2-hydroxybutyric acid was revealed to possess the highest DCAN yield among all the known aliphatic long-chain precursors to date during chlorination. Additionally, enonitriles, (chloro-)isocyanates, and nitriles can be generated during DCAN formation and should be of concern due to their high toxicities.

QM/MM Study of a Nucleophilic Substitution Reaction Catalyzed by Uridine Phosphorylase from Vibrio cholerae

Uridine phosphorylases are used for biotechnological synthesis of pyrimidine derivatives and, moreover, their substrates and inhibitors are used in medicine. Therefore, studies of the mechanisms of the chemical reaction catalyzed by the enzyme and its specificity for various substrates are relevant. The research into the enzymatic reaction main stage—nucleophilic substitution of the nitrogenous base in uridine with an orthophosphate or orthovanadate group by hybrid QM/MM methods—was carried out. A comparison of various levels of theory and calculation schemes showed that preliminary optimization of the reactants’s geometry, as well as calculation of the initial trajectory of the minimum energy path, can be achieved by semi-empirical methods. At the same time, for the minimum energy path clarification, transition state geometry optimization, and calculation of the thermochemical parameters, it is preferable to use density functional theory in combination with modern ab initio methods. In comparison with the calculations of the activation barrier carried out in a solvent without an enzyme, differences in the kinetics of the enzymatic reaction due to the orientation and concentration actions of amino acid residues of the enzyme were revealed. This led to lowering the activation barrier by 20 kcal/mol and contributed to the reaction under physiologically acceptable conditions. It was shown that the free activation energy during the nucleophilic attack for uridine with hydrovanadate ion is 2 kcal/mol lower than for the hydrophosphate ion and this is consistent with the literature data.

Estimation of frequency factors for the calculation of kinetic isotope effects from classical and path integral free energy simulations.

We use the modified Bigeleisen-Mayer equation to compute kinetic isotope effect values for non-enzymatic phosphoryl transfer reactions from classical and path integral molecular dynamics umbrella sampling. The modified form of the Bigeleisen-Mayer equation consists of a ratio of imaginary mode vibrational frequencies and a contribution arising from the isotopic substitution's effect on the activation free energy, which can be computed from path integral simulation. In the present study, we describe a practical method for estimating the frequency ratio correction directly from umbrella sampling in a manner that does not require normal mode analysis of many geometry optimized structures. Instead, the method relates the frequency ratio to the change in the mass weighted coordinate representation of the minimum free energy path at the transition state induced by isotopic substitution. The method is applied to the calculation of 16/18O and 32/34S primary kinetic isotope effect values for six non-enzymatic phosphoryl transfer reactions. We demonstrate that the results are consistent with the analysis of geometry optimized transition state ensembles using the traditional Bigeleisen-Mayer equation. The method thus presents a new practical tool to enable facile calculation of kinetic isotope effect values for complex chemical reactions in the condensed phase.

Viscosity, Free Activation Energy, and Glass Transition Temperature of Potassium Boron K2O–B2O3 Melts

The viscosity of potassium–boron melts is measured in the temperature range 918–1699 K using a vibrational viscosimeter. The content of potassium oxide is varied from 0.74 to 28.46 mol %. The parameters of the viscous flow in melts (the configuration–activation energy(εh) and the shifting energy of the bridging oxygen atoms’ bonds (U∞)) using the configuration–activation model are calculated for two temperature intervals 918–1400 K and 1400–1699 K. The glass transition temperature (Tg) is measured by differential scanning calorimetry (DSC) and the concentration dependence of the glass transition temperature on the content of potassium oxide in the melt is shown.

Direct C–C bond scission of xylitol to ethylene and propylene glycol precursors using single-atom catalysts (SACs) anchored on MgO

Shorter chain alcohols, as opposed to longer ones, are beneficial as biomass feedstock for chemicals and fuels, including hydrogen production. More so, it has been demonstrated that carbon–carbon rather than carbon–oxygen bond-cleaving activity determines the product selectivity of a metal catalyst for higher oxygenates reforming. In this report, we investigate the direct C2–C3 bond-cleaving activity of xylitol via first-principles, periodic density functional theory calculations to identify the differences in activities between single-crystal catalysts (SCCs) and single-atom catalysts (SACs). A comparison of the kinetic barriers revealed that xylitol's C–C bond scission appears to be a near-impossible task on SCCs. However, SACs demonstrated higher performance. For example, Ir1/MgO and Ir1/MgO_Ovac (having surface oxygen vacancy) yielded ∼72% and 54% decrease, respectively, in Gibb’s free activation energy compared to Ir (111) at the xylitol reforming operating temperature of 473 K. Furthermore, electronic structure calculations revealed an up-shift in the DOS for the surface M1 atoms in all investigated SACs compared to the surface atoms of their respective SCCs, resulting in M1 higher d-band center and stronger adsorbate (s) binding. This study highlights the importance of SACs for boosting the atom efficiency of costly metals while also offering a new strategy for tuning the activity of catalytic reactions.

Viscosity, heat capacity and refractive index studies for binary liquid mixtures containing 1,3-diaminopropane and alkyl acetates: Experimental and theoretical interpretation

The viscosity (η), heat capacity (CP) and refractive index (nD) of 1,3-DAP (1,3-Diaminopropane) (1) + AAc (MAc(methyl acetate), EAc(ethyl acetate), PAc(propyl acetate) and BAc(butyl acetate)) (2) binary mixture were measured at standard ambient temperature i.e. 298.15 K under the atmospheric pressure 0.1 MPa. The data of η, CP and nD were further used to determine deviation in viscosity (Δη), excess molar heat capacity (CPE), excess Gibbs free energy of activation (G∗E), and deviation in refractive index (ΔnD). η were analyzed by Bloomfield and Dewan model (BFD). The η values were further correlated by various correlations like Kendall, Bingham, Kendall-Monroe, Arrhenius, Katti-Chaudhari, Tamura-Kurata, Grunberg-Nissan, Frenkel, Hind-McLaughlin-Ubbelohde, Heric, McAllister, Lulian, Krishnan-Laddha and Heric-Brewer. Further various correlations like Heller, Lorentz-Lorenz, Arago-Biot, Newton, Gladstone-Dale, Erying and Weiner were used to correlate nD data. The derived thermo-physical properties were also interpreted in terms of molecular interactions and correlated with Redlich- Kister polynomial equation (R.K.).

A computational study of 1,3-dipolar cycloadditions of nitrile oxides with dienes

Density functional theory calculations at the M06-2X level of theory were employed to study the reactivity and regioselectivity of 1,3-dipolar cycloaddition reactions of nitrile oxides with cyclodienes. Calculated relative activation free energies reproduce the experimentally observed product ratios. The electronic energies of activation are found to be mainly controlled by distortion energies required to achieve the transition states. Both electronic and steric effects influence regioselectivities. Theoretical predictions were performed on previous experimental data and analyzed with the use of the distortion/interaction model. The obtained results will help in obtaining a better understanding of the factors that affect the relative cycloaddition rate.

Predictive models for the aqueous phase reactivity of inorganic radicals with organic micropollutants

Single-electron transfer (SET) is one of the most common reaction mechanisms for degrading organic micropollutants (OMPs) in advanced oxidation processes. We collected 300 SET reactions (CO3•–, SO4•–, Cl2•–, and Br2•–-mediated) and calculated three key parameters for understanding the SET mechanism: aqueous phase free energies of activation (ΔG‡), free energies of reactions (ΔG), and orbital energy gaps of reactants (EOMPsHOMO-ERadiLUMO). Then, we classified the OMPs according to their structure, developed and evaluated linear energy relationships of the second-order rate constants (k) with ΔG‡, ΔG, or EOMPsHOMO-ERadiLUMO in each class. Considering that a single descriptor cannot capture all the chemical diversity, we combined ΔG‡, ΔG, and EOMPsHOMO-ERadiLUMO as inputs to develop multiple linear regression (MLR) models. Chemical classification is critical to the linear model described above. However, OMPs usually have multiple functional groups, making the classification challenging and uncertain. Therefore, we tried machine learning algorithms to predict k values without chemical classification. We found that decision trees (R2 = 0.88–0.95) and random forest (R2 = 0.90–0.94) algorithms show better performance on the prediction of the k values, whereas boosted tree algorithm cannot make an accurate prediction (R2 = 0.19–0.36). Overall, our study provides a powerful tool to predict the aqueous phase reactivity of OMP to certain radicals without the need for chemical classification.

Excess properties, computational chemistry and spectroscopic analyses of 1,2-propanediamine + n-propanol/isopropanol binary mixtures

To understand excess properties and intermolecular interactions of 1,2-propanediamine and n-propanol/isopropanol binary mixtures, their density and viscosity were systemically measured at T = (298.15–318.15) K under atmospheric pressure (1005 hectopascals in Tianjin, China), and their excess molar volume (VmE), viscosity deviation (Δη) and excess viscous activation free energy (ΔG*E) were also calculated. From the results of VmE, it can be inferred that two n-propanol/isopropanol molecules and one 1,2-propanediamine molecule have the strongest binding capacity and form the most stable intermolecular hydrogen bonds. The experimental density and kinematic viscosity were correlated and predicted by several semi-empirical models, among which the Jouyban-Acree model had a better fit for the density and the McAllister four-body equation had a better fit for the kinematic viscosity, and the calculated results of VmE, Δη and ΔG*E were analyzed using the R-K equation. Meanwhile, the apparent molar volume, partial molar volume, thermal expansion coefficient and viscous activation parameters of 1,2-propanediamine and n-propanol/isopropanol solutions were calculated and analyzed. After that, the existence of intermolecular hydrogen bonds (IHBs) between 1,2-propanediamine and n-propanol/isopropanol binary mixtures was demonstrated on the basis of computational chemistry. And then, the IHBs between 1,2-propanediamine and n-propanol/isopropanol binary mixtures with the form of –OH···NH2– were further confirmed based on Raman, ultraviolet (UV), fluorescence (FLS) and nuclear magnetic resonance (1H NMR) spectral technologies.

Viscometric, rheological, and thermodynamic studies of octafunctional epoxy resins OGDHDPBA and OGDHDPDS

In this study, we looked at the viscometric, rheological, and thermodynamic properties of two octafunctional epoxy resins: octaglycidyl dihydrazine dipropoxy para-bisphenol A (OGDHDPBA) and octaglycidyl dihydrazine 4,4′-dipropoxy diphenylsulfone (OGDHDPDS) (OGDHDPDS). The calculated degree of polymerization (DPn¯) of the synthesized OGDHDPBA and OGDHDPDS resins was in the order of 2 and 3, respectively. The viscometric characterization results revealed that the OGDHDPDS resin is more viscous than the OGDHDPBA resin.The rheological characterization results revealed that the storage (G') and loss (G'') moduli of the OGDHDPDS resin are higher than those of the OGDHDPBA resin. While thermodynamic characterization results showed that thermodynamic parameters such as glass transition temperature (Tg), activation energy (Ea), activation enthalpy variations (ΔHa), activation entropy variations (ΔSa), and activation free energy variations (ΔGa) of the studied resins (OGDHDPBA and OGDHDPDS) had the values of 105.13 and 132.86 °C; −111.49, −74.41 Kj.mol−1 (G'), −53.95 and −38.82 Kj.mol−1 (G''); –32.17, −50.29 Kj.mol−1 (G'), −42.15 and −71.16 Kj. mol−1 (G''); −257.65, −293.81 j.mol−1.K−1 (G'), −282.01, −344.94 j.mol-1.K−1 (G''); −5.08, 11.25 Kj.mol−1 (G'), −12.50 and −25.33 Kj.mol−1 (G'').

Effect of the bipodal Ti species on activity and selectivity of propylene epoxidation with H2O2 over TS-1: A theoretical study

In order to probe into the effect of bipodal Ti species on the catalytic performance, the propylene epoxidation with H2O2 over TS-1 catalyst with bipodal Ti site has been studied using an embedded quantum mechanical/molecular mechanics (QM/MM) approach, and the results shown that the bipodal Ti site has strong Brønsted and Lewis acidity, which not only greatly facilitates the epoxidation reaction but also lower selectivity toward the epoxy products. The excellent epoxidation activity is attributed to the Lewis acidity, in which, H2O2 easily activated to generate active intermediates (i.e., Ti-η1-OOH and Ti-η2-OOH) that is facile to transfer O atom to oxidize the propylene. The poor selectivity toward PO is also due to the Lewis acidity, which leads to the stronger adsorption ability of PO and excellent ring-opening ability of Ti-OH nucleophile to PO. The introduction of methoxy (CH3O) group is beneficial for the enhancement of the selectivity of PO, although it enhances the activation free energy for epoxidation to generate PO. The identifications about the relationship of the intrinsic selectivity and activity with bipodal Ti site would provide a clue for designing highly-efficient TS-1 catalysts in propylene epoxidation to PO from the theoretical point of view.

Ceramide-1-phosphate transfer protein enhances lipid transport by disrupting hydrophobic lipid-membrane contacts.

Cellular distributions of the sphingolipid ceramide-1-phosphate (C1P) impact essential biological processes. C1P levels are spatiotemporally regulated by ceramide-1-phosphate transfer protein (CPTP), which efficiently shuttles C1P between organelle membranes. Yet, how CPTP rapidly extracts and inserts C1P into a membrane remains unknown. Here, we devise a multiscale simulation approach to elucidate biophysical details of CPTP-mediated C1P transport. We find that CPTP binds a membrane poised to extract and insert C1P and that membrane binding promotes conformational changes in CPTP that facilitate C1P uptake and release. By significantly disrupting a lipid's local hydrophobic environment in the membrane, CPTP lowers the activation free energy barrier for passive C1P desorption and enhances C1P extraction from the membrane. Upon uptake of C1P, further conformational changes may aid membrane unbinding in a manner reminiscent of the electrostatic switching mechanism used by other lipid transfer proteins. Insertion of C1P into an acceptor membrane, eased by a decrease in membrane order by CPTP, restarts the transfer cycle. Most notably, we provide molecular evidence for CPTP's ability to catalyze C1P extraction by breaking hydrophobic C1P-membrane contacts with compensatory hydrophobic lipid-protein contacts. Our work, thus, provides biophysical insights into how CPTP efficiently traffics C1P between membranes to maintain sphingolipid homeostasis and, additionally, presents a simulation method aptly suited for uncovering the catalytic mechanisms of other lipid transfer proteins.

Experimental, theoretical, computational and spectroscopic analysis in binary liquid mixtures containing 1-propanol and C-1 to C-4 alkyl acetates (T = 298.15–318.15 K): Physicochemical properties and molecular interaction studies

The molecular interactions between 1-propanol (1P) (1) and alkyl acetates (AAc) (C1-C4) (2) were examined by understanding of thermophysical properties of given binary liquid mixtures. Density (ρ), viscosity (η) and speed of sound (u) were measured for 1P, AAc and their binary mixture at T = 298.15–318.15 K with interval of 5 K at 0.1 MPa. Experimental data of ρ, η and u were used to evaluate VmE (excess molar volume), uE (excess ultrasonic speed), Δη (deviation in viscosity), ksE (excess molar isentropic compressibility), G∗E (excess Gibbs free energy of activation of viscous flow), VfE (excess free volume), LfE (excess intermolecular free length) and VaE (excess available volume) and correlated with R.K. (Redlich-Kister polynomial) equation. VmE data was also examined in term of Graph theoretical approach (GTA) and Prigogine-Flory-Patterson (PFP) theory. Δη were analyzed by Bloomfield and Dewan model (BFD) and also examined in terms of Graph theoretical approach (GTA). The u data was examined by different correlations like Van Dael, Nomoto, JFLT (Jacobson’s free length theory), Impedance dependence and CFT (Schaaff’s collision factor theory). FT-IR spectroscopic studies provide important information for understanding the intermolecular interactions between 1P (1) and AAc (2) components in the binary mixture which is also recognized by GTA. Further, DFT studies indicated the strength of molecular interactions and supported the graph theory in predicting probable associative molecular species in the liquid mixtures.

Theory for Heterogeneous Electron Transfer Kinetics on Nanocorrugated Atomic Stepped Metal Electrodes

We develop a novel mean-field theoretical approach for the outer-sphere heterogeneous electron transfer (OS-HET) rate constant at an atomically stepped nanocorrugated metal surface. Theory accounts for the contributions from the nature of the metal, local curvature of the nanocorrugated atomic step, the density of atomic steps and kinks, dipolar solvents, and the frontier molecular orbital of electroactive molecules. Our theoretical approach develops a novel model for the free energy of activation obtained using the alignment of the Fermi energy level in the metal with one of the frontier molecular orbitals of the electroactive species. The activation free energy is obtained as a product of a modified work function (WF) and the fractional electronic charge of activation for the alignment of levels. Metal surface local curvatures at atomic steps are modeled as a hyperbolic tangent functional with a random nanocorrugated step edge. The influence of step density and edge fluctuations (or kinks) on the activation free energy is included through the surface average mean curvature and the mean-square gradient dependent modified WF. The theoretical results highlight the step density-dependent nonmonotonic variation in WF, fractional electronic charge, and activation free energy. The increase in step density shifts the Fermi energy level toward LUMO and away from HOMO energy levels. This causes the reduced (for the LUMO) and enhanced (for the HOMO) activation free energy for ET kinetics compared to the basal plane. The dichotomy in the OS-HET kinetics at the atomically stepped metal results in anomalous enhancement and suppression for the LUMO and HOMO, respectively. The strong influence of kinks along the step edge on the HET rate constant is predicted up to 6 decades (compared to the basal surface of Pt metal). Finally, theory shows agreement with the reported experimental data of WF and exchange current density for single crystal Pt electrodes.

Adsorption behavior of Crys tal Violet dye in aqueous solution using Co+2 hectorite composite as adsorbent surface

This study focused on the adsorption behavior of the cationic CrystalViolet (CV) dye from aqueous solutions using a Co+2‒hectoritecomposite as an adsorbent surface. The initial and equilibrium CV dyeconcentrations were determined using a UV-Vis spectrophotometer. Theresults were discussed and presented for the impacts of pH, primary CVdye concentration, composite dosage, and temperature. The optimumconditions were found for eliminating Crys tal Violet dye from theaqueous solution at a pH 4, ideal temperature 293 K, and 0.5 g L-1of composite dose. The pseudo-second-order kinetic, intraparticlediffusion analyzed the tes ts’ data and film diffusion models. Eachmodel’s defining features have been identified, and these models werein good agreement and in charge of regulating the adsorption reaction.The adsorption operation was also thermodynamically examined todetermine thermodynamic variables such as Gibbs free energy (ΔGo),entropy (ΔSo), activation energy (Ea), and enthalpy (ΔHo). The negativevalue of Gibbs free energy (ΔGo) and enthalpy (ΔHo) indicated thatthe adsorption process was a spontaneous and exothermic reaction.While the activation energy (Ea) data which fell within the normalrange for physisorption, was showed at 22.434 kJ mol-1. The physical adsorption occurs between CV dye and adsorbent .

Semi-microscopic Theory for the Current Rectification Phenomenon in Nanogap Molecular Devices.

A semi-microscopic theory is developed for heterogeneous electron transfer (HET) kinetics based on the energy level alignment approach at self-assembled monolayer (SAM) covered metal electrodes. Theory provides the electronic and molecular property-dependent equations for the HET rate constant (k0) and the transfer coefficient (α) for potential. k0 is formulated using the activation free energy as a product of the SAM covered metal work function (WF) and fractional electronic charge exchanged at the transition state (attained through the alignment of the frontier molecular orbital (FMO) energy level of the electroactive group with the WF of metal). k0 is a function of the metal jellium electronic screening length and dielectric and of the molecular self-assembly (through its dipole moment, size, and packing density) and the FMO energies of electroactive groups. The operative potential at the transition state is governed by α, which is a function of molecular spacer length and characteristic electronic-dipolar coupling length. The current rectification phenomenon in nanogap molecular devices is theoretically analyzed using equations for k0 and α for SAM covered source and drain electrodes. Theory unravels the LUMO or HOMO dichotomy for a given metal: (i) for the HOMO assisted ET, the metal with a high WF has a high current rectification ratio (RR), while (ii) for the LUMO assisted ET, the metal with a low WF has a high current RR in asymmetrical devices. Theory predicts the reversal in current rectification by altering the dipole moment of the anchoring molecule, the HOMO/LUMO energy of the electroactive groups, and the nature of the metal. Finally, theory shows qualitative and quantitative coherence with the reported experimental current-potential response of molecular device.

Cucurbit[7]uril complexes with gabapentin: Effect on lactamization

Spectroscopic and computational techniques have been used to study the complexation of gabapentin (GAB) and its lactam form (LAC) with cucurbit[7]uril (CB7) in aqueous solution. 1H NMR and fluorescence displacement titrations demonstrated that CB7 forms 1:1 complexes with GAB and LAC, binding preferentially to LAC (ΔΔG ∼ -4 kcal.mol−1). Furthermore, 1H NMR spectra recorded over a period of two weeks revealed that the rate of GAB lactamization increased when it was encapsulated within the cavity of CB7. This is a direct result of the lower activation free energy of the lactamization process within the cavity of CB7, as computed by quantum-chemical calculations. Molecular dynamics simulations revealed the encapsulation of the cyclohexyl moiety inside the host cavity, while the NH3+ in GAB and NH in LAC interacting with the CB7 portal, in agreement with the NMR results. MM-PBSA results showed that van der Waals interactions provided major favorable contributions to the binding free energies of both guest molecules while the electrostatic contribution was significant only for GAB. MM-PBSA estimates of the binding free energies failed to reproduce the observed preferential affinity of CB7 toward LAC, while thermodynamics integration and density functional theory calculations exhibited much better agreement.

Characteristic Activity Parameters of Electron Donors and Electron Acceptors.

It is well-known that for an electron transfer reaction, the electron-donating ability of electron donors and the electron-accepting ability of electron acceptors can be quantitatively described by the oxidation potential of electron donors and the reduction potential of electron acceptors. However, for an electron transfer reaction, the electron-donating activity of electron donors and the electron-accepting activity of electron acceptors cannot be quantitatively described by a characteristic parameter of electron donors and a characteristic parameter of electron acceptors till now. In this paper, a characteristic activity parameter of electron donors and electron acceptors named as their thermo-kinetic parameter is proposed to quantify the electron-donating activity of electron donors and the electron-accepting activity of electron acceptors in electron transfer reactions. At the same time, the thermo-kinetic parameter values of 70 well-known electron donors and the corresponding 70 conjugated electron acceptors in acetonitrile at 298 K are determined. The activation free energies of 4900 typical electron transfer reactions in acetonitrile at 298 K are estimated according to the thermo-kinetic parameter values of 70 electron donors and 70 conjugated electron acceptors, and the estimated results have received good verification of the corresponding independent experimental measurements. The physical meaning of the thermo-kinetic parameter is examined. The relationship of the thermo-kinetic parameter with the corresponding redox potential as well as the relationship of the activation free energy with the corresponding thermodynamic driving force of electron transfer reactions is examined. The results show that the observed relationships between the thermo-kinetic parameters and the redox potentials as well as the observed relationships between the activation free energy and the thermodynamic driving force depend on the choice of electron donors and electron acceptors as well as the electron transfer reactions. The greatest contribution of this paper is to realize the symmetry and unification of kinetic equations and the corresponding thermodynamic equations of electron transfer reactions.