Jean Jules Fifen

Revision of the Thermodynamics of the Proton in Gas Phase

Proton transfer is ubiquitous in various physical/chemical processes, and the accurate determination of the thermodynamic parameters of the proton in the gas phase is useful for understanding and describing such reactions. However, the thermodynamic parameters of such a proton are usually determined by assuming the proton as a classical particle whatever the temperature. The reason for such an assumption is that the entropy of the quantum proton is not always soluble analytically at all temperatures. Thereby, we addressed this matter using a robust and reliable self-consistent iterative procedure based on the Fermi-Dirac formalism. As a result, the free proton gas can be assumed to be classical for temperatures higher than 200 K. However, it is worth mentioning that quantum effects on the gas phase proton motion are really significant at low temperatures (T ≤ 120 K). Although the proton behaves as a classical particle at high temperatures, we strongly recommend the use of quantum results at all temperatures, for the integrated heat capacity and the Gibbs free energy change. Therefore, on the basis of the thermochemical convention that ignores the proton spin, we recommend the following revised values for the integrated heat capacity and the Gibbs free energy change of the proton in gas phase and, at the standard pressure (1 bar): ΔH0→T = 6.1398 kJ mol(-1) and ΔG0→T = -26.3424 kJ mol(-1). Finally, it is important noting that the little change of the pressure from 1 bar to 1 atm affects notably the entropy and the Gibbs free energy change of the proton.

Solvation Energies of the Proton in Methanol

pKas, proton affinities, and proton dissociation free energies characterize numerous properties of drugs and the antioxidant activity of some chemical compounds. Even with a higher computational level of theory, the uncertainty in the proton solvation free energy limits the accuracy of these parameters. We investigated the thermochemistry of the solvation of the proton in methanol within the cluster-continuum model. The scheme used involves up to nine explicit methanol molecules, using the IEF-PCM and the strategy based on thermodynamic cycles. All computations were performed at B3LYP/6-31++G(dp) and M062X/6-31++G(dp) levels of theory. It comes out from our calculations that the functional M062X is better than B3LYP, on the evaluation of gas phase clustering energies of protonated methanol clusters, per methanol stabilization of neutral methanol clusters and solvation energies of the proton in methanol. The solvation free energy and enthalpy of the proton in methanol were obtained after converging the partial solvation free energy of the proton in methanol and the clustering free energy of protonated methanol clusters, as the cluster size increases. Finally, the recommended values for the solvation free energy and enthalpy of the proton in methanol are -257 and -252 kcal/mol, respectively.

Structures of protonated methanol clusters and temperature effects

The accurate evaluation of pKas, or solvation energies of the proton in methanol at a given temperature is subject to the determination of the most favored structures of various isomers of protonated (H(+)(MeOH)n) and neutral ((MeOH)n) methanol clusters in the gas phase and in methanol at that temperature. Solvation energies of the proton in a given medium, at a given temperature may help in the determination of proton affinities and proton dissociation energies related to the deprotonation process in that medium and at that temperature. pKas are related to numerous properties of drugs. In this work, we were interested in the determination of the most favored structures of various isomers of protonated methanol clusters in the gas phase and in methanol, at a given temperature. For this aim, the M062X/6-31++G(d,p) and B3LYP/6-31++G(d,p) levels of theory were used to perform geometries optimizations and frequency calculations on various isomers of (H(+)(MeOH)n) in both phases. Thermal effects were retrieved using our homemade FORTRAN code. Thus, we accessed the relative populations of various isomers of protonated methanol clusters, in both phases for temperatures ranging from 0 to 400 K. As results, in the gas phase, linear structures are entropically more favorable at high temperatures, while more compact ones are energetically more favorable at lower temperatures. The trend is somewhat different when bulk effects are taken into account. At high temperatures, the linear structure only dominates the population for n ≤ 6, while it is dominated by the cyclic structure for larger cluster sizes. At lower temperatures, compact structures still dominate the population, but with an order different from the one established in the gas phase. Hence, temperature effects dominate solvent effects in small cluster sizes (n ≤ 6), while the reverse trend is noted for larger cluster sizes.

Thermodynamics of the Electron Revisited and Generalized

The accurate evaluation of redox potentials in various media and the ability of electron transfer in some biological or chemical reactions are subject to the determination of the accurate gas phase thermodynamic data of the electron. These data are also useful to describe with a high accuracy the movement of the electron in a stellar core. However, these data were not available at all temperatures, and the available data were not sufficiently accurate. I addressed this matter using a robust and reliable self-consistent iterative procedure which determines the entropy of a gas phase free electron and, thereafter, allows the calculation of its heat capacity, enthalpy, and free energy. Extremely accurate analytic expressions of the aforementioned thermodynamic parameters were provided at all temperatures. The thermodynamic parameters of the gas phase electron are now known at all temperatures (integer or noninteger) in the standard atmosphere with a high accuracy. Analytic expressions proposed for the thermodynamic parameters are highly advisable where iteratively computed data are unavailable. Note that at room temperature (T = 298.15 K), the values recommended for the thermodynamic parameters of the gas phase electron are S = 22.6432 J mol(-1) K(-1), CP = 17.1062 J mol(-1) K(-1), ΔH = 3.1351 kJ mol(-1), and ΔG = -3.6160 kJ mol(-1).

DFT study of the effect of solvent on the H-atom transfer involved in the scavenging of the free radicals ●HO2 and ●O2 − by caffeic acid phenethyl ester and some of its derivatives

AbstractH-atom transfer from caffeic acid phenethyl ester (CAPE), MBC (3-methyl-2-butenyl caffeate), BC (benzoic caffeate), P3HC (phenethyl-3-hydroxycinnamate), and P4HC (phenethyl-4-hydroxycinnamate) to the selected free radicals ●HO2 and ●O2− was studied. Such a transfer can proceed in three different ways: concerted proton-coupled electron transfer (CPCET), electron transfer followed by proton transfer (ET-PT), and proton transfer followed by electron transfer (PT-ET). The latter pathway is sometimes competitive with SPLET (sequential proton loss electron transfer) in polar media. Analyzing the thermodynamic descriptors of the reactions of CAPE and its derivatives with co-reactive species—in particular, the free energies of reactions, the activation barrier to the CPCET mechanism, and their rate constants—appears to be the most realistic method of investigating the H-atom transfers of interest. These analyses were performed via DFT calculations, which agree well with the data acquired from experimental studies (IC50) and from CBS calculations. The CPCM solvation model was used throughout the work, while the SMD model—employed as a reference—was used only for CAPE. The main conclusion drawn from the analysis was that SPLET is the mechanism that governs the reaction of phenolic acids with ●HO2, while PT-ET governs the reaction of phenols with ●O2−. In kinetic investigations of the CPCET process, the rate constant decreases as the solvent polarity increases, so the reaction velocity slows down. Graphical Abstract3D RPES of the reaction of CAPE with HO_2 radical (left), and free energies of the reaction of CAPE with O_2 radical in various media (right)

Structures and spectroscopy of protonated ammonia clusters at different temperatures

The accurate determination of the solvation energies of a proton in ammonia is based on the precise knowledge of the structures of neutral and protonated ammonia clusters. In this work, we have investigated all the possible and stable structures of protonated ammonia clusters H+(NH3)n=2-9, along with their isomeric distribution at a specific temperature. New significant isomers are reported here for the first time and show that the structures of protonated ammonia clusters are not only branched linear as assumed by all previous authors. Branched linear structures are the only ones responsible for the population of protonated ammonia clusters for n = 4-6 at any temperature. However, for larger cluster sizes, these types of structures compete with branched cyclic, double cyclic, branched double cyclic and triple cyclic structures depending on the temperature. In addition, we have shown that protonated ammonia clusters are all Eigen structures and the first solvation shell of the related ammonium ion core is saturated by four ammonia molecules. We have also carried out a study of the hydrogen bond network of protonated ammonia clusters establishing the stability rule governing the various isomers of each cluster from estimated energies of the hydrogen bond types in H+(NH3)n=2-9. With all these results, a route for the accurate determination of the solvation energies of a proton in ammonia at a given temperature could be conceivable.

Structures and spectroscopy of medium size protonated ammonia clusters at different temperatures, H+(NH3)10–16

Structures of protonated ammonia clusters (H+(NH3)n) are very important for the determination of pKas and solvation energies of the proton in ammonia. In this work, their structures were investigated at M06-2X/6-31++g(d,p) level of theory, for n=10-16 and for temperatures ranging from 0 to 400 K. In the cluster community, this is the first theoretical study on the protonated ammonia clusters larger than the nonamer. We noted that the population of the investigated clusters is reproduced by branched cage or cage like structures at low temperatures, while branched linear and branched cyclic or branched double cyclic isomers are the only isomers responsible for the population at higher temperatures. In these isomers, the proton is highly and entirely solvated at the center of the cluster. In addition, protonated ammonia clusters are all Eigen structures and the first solvation shell of the related ammonium ion core is saturated by four ammonia molecules. Moreover, infrared (IR) spectra of all isomers have been investigated and these spectra show good agreement with the experiment. This allowed us to assign experimental peaks and to provide the constitution of the populations of the various clusters.

Structure, antioxidative potency and potential scavenging of OH and OOH of phenylethyl-3,4-dihydroxyhydrocinnamate in protic and aprotic media: DFT study

DFT methods including B3LYP, B3PW91 and M05-2x associated to 6-31+G(d,p) were used for the structural and antioxidant potency studies of phenylethyl-3,4-dihydroxy-hydrocinnamate (PDH). Solvents were employed according to their protric and aprotic character. So, calculated structures agree with the experimental data. O4H4 is propitious to scavenge radicals whatever the medium except in water where O3H3 and O4H4 are competitive. The explicit solvents of dichloromethane (DCM) and water present a disparity of OH bond dissociation enthalpy and free energy (BDE and BDFE). These parameters are low in continuum except in water. The ionization potentials (IP) and potential affinities (PA) are low in solvents. BDE, IP and PA are each, approximatively constant in mixed solvent treatment in water using n-H2O (n=3,5,8). Elsewhere, H-atom transfer (HAT) mechanism is favoured in vacuum and DCM, whereas sequential proton loss electron transfer (SPLET) is likely in protic solvents. A discord between HAT and SPLET in benzene is observed. The PDH compound is more antioxidant and resistant to oxidation than caffeic acid phenethyl ester (CAPE). The potential of scavenging of OH and OOH whatever the reaction channel shows that they decay rapidly in any media through HAT. PDH is easily deprotonated in the protic solvents and the resulting product is the most antioxidant and the least resistant to oxidation.

Solvation energies of the proton in ammonia explicitly versus temperature

We provide in this work, the absolute solvation enthalpies and the absolute solvation free energies of the proton in ammonia explicitly versus temperature. As a result, the absolute solvation free energy of the proton remains quite constant for temperatures below 200 K. Above this temperature, it increases as a linear function of the temperature: ΔGam(H+,T)=-1265.832+0.210 T. This indicates that a temperature change of 100 K would induce a solvation free energy change of 21 kJ mol-1. Thus, ignoring this free energy change would lead to a bad description of hydrogen bonds and an unacceptable error higher than 3.7 pKa units. However, the absolute solvation enthalpy of the proton in ammonia is not significantly affected by a temperature change and, the room temperature value is -1217 kJ mol-1. The change of the solvation enthalpy is only within 3 kJ mol-1 for a temperature change up to 200 K.

Anticonvulsant effects of Senna spectabilis on seizures induced by chemicals and maximal electroshock

Senna spectabilis (Fabaceae) is one of the medicinal plants used in Cameroon by traditional healers to treat epilepsy, constipation, insomnia, anxiety. The present study aimed to investigate the anticonvulsant effects of Senna spectabilis decoction on seizures induced by maximal electroshock (MES), pentylenetetrazole (PTZ), pilocarpine (PC) and its possible action mechanisms in animal models using flumazenil (FLU), methyl-ß-carboline-3-carboxylate (BC) and bicuculline (BIC). Senna spectabilis decoction (106.5 and 213.0mg/kg) antagonized completely tonic-clonic hind limbs of mice induced by MES. The lowest plant dose (42.6mg/kg) provided 100% of protection against seizures induced by PTZ (70mg/kg). Administration of different doses of the plant decoction antagonized seizures induced by PC up to 75%, causing a dose dependent protection and reduced significantly the mortality rate induced by this convulsant. Both FLU and BC antagonize strongly the anticonvulsant effects of this plant and are unable to reverse totally diazepam or the plant decoction effects on inhibiting seizures. The animals did not present any sign of acute toxicity even at higher doses of the plant decoction. In conclusion, Senna spectabilis possesses an anticonvulsant activity. We showed that its decoction protects significantly mice against seizures induced by chemicals and MES, delays the onset time and reduces mortality rate in seizures-induced. It also appears that the oral administration of the decoction of S. spectabilis is more active than the intraperitoneal administration of the ethanolic extract on inhibiting seizures induced by MES and PTZ. Moreover, the plant decoction could interact with GABAA complex receptor probably on the GABA and benzodiazepines sites.