Titus V. Albu

Catalytic Effect of Platinum on Oxygen Reduction An Ab Initio Model Including Electrode Potential Dependence

The effects of bonding to a platinum atom are calculated for the reduction of oxygen to water. The electron-correlation corrected MP2 method is used, and the electrode potential is modeled by variations in values for the electron affinities of the reaction centers. Potential-dependent transition state structures and activation energies are reported for the one-electron reactions Pt-O 2 + H + (aq) + e - (U) → Pt-OOH [i] Pt-OOH + H + (aq) + e - (U) → Pt-(OHOH) [ii] Pt-(OHOH) + H + (aq) + e - (U) → Pt-OH + H 2 O [iii] Pt-OH + H + (aq) + e - (U) → Pt-OH 2 [iv] This is the predicted lowest energy pathway. An alternative, where step (ii) is replaced by Pt-OOH + H + (aq) + e - (U) → Pt-O + H 2 O [v] is excluded by the high activation energy calculated for it, though reduction of Pt-O to Pt-OH Pt-O + H + (aq) + e - (U) → Pt-OH [vi] has a very low activation energy. Compared to uncatalyzed outer-Helmholtz-plane values, bonding to the Pt has the effect of decreasing the calculated high reduction activation energies for O 2 and H 2 O 2 . Bonding to Pt also decreases the HOO. and increases the HO- activation energy values. The reverse reaction, oxidation of H 2 O to O 2 , is also discussed in light of these results. The issues of potential-dependent double-layer potential drops and adsorbate bond polarizations are discussed, and it is pointed out that the results of this study can be used to estimate the effects of such potential drops.

Ab initio approach to calculating activation energies as functions of electrode potential: Trial application to four-electron reduction of oxygen

Abstract A self-consistent ab initio approach to calculating Arrhenius activation energies for electron transfer reactions outside the Helmholtz plane and their dependences on electrode potential is introduced. The methodology is applied to the four steps of oxygen reduction to water in acidic aqueous electrolyte.

Studies of model dependence in an ab initio approach to uncatalyzed oxygen reduction and the calculation of transfer coefficients

Abstract In a recent study [J. Am. Chem. Soc. 121 (1999) 11855], an ab initio approach to calculate potential dependent activation energies was applied in studying the outer-sphere O 2 reduction and H 2 O oxidation. The purpose of this paper is to examine influences of changes in the calculational methodology and the reactant structural models. The first step in the overall four-electron reduction of O 2 to water, O 2 (g) + H + ( aq )+ e − (U)⇌ HOO(aq) is the focus of this work. U is the electrode potential and H + (aq) is modeled by the [HOH 2 (OH 2 ) 2 ] + cluster. For an electrode potential of 0.727 V on the hydrogen scale, the findings of this study are: 1. Determining the transition state structures constrained to using the product OOH angle is a satisfactory approximation. 2. The calculated activation energies are reduced for the forward reaction and increased for the reverse reaction when the hydronium ion structure is relaxed along the reaction coordinate. 3. Calculated reduction activation energies using the 6-31G** basis set are highest for the HF calculations, intermediate for MP2 calculations and lowest for B3LYP density functional calculations. Adding diffuse functions lowers all of the values. 4. Increasing the model size by coordinating another water molecule to the transferring proton increases the activation energy for the forward reaction. In addition to the above, the transfer coefficients in the Butler–Volmer equation relating current density to overpotential are calculated and discussed.

Dopants in Diamond Nanoparticles and Bulk Diamond Density Functional Study of Substitutional B, N, P, SB, S, PN, O, NN, and Interstitial H

We show how calculated ionization potentials and electron affinities of doped diamond nanoparticles can he used to predict the donor and acceptor excitation energies of doped n-type and p-type semiconducting bulk diamond. The method uses good quality quantum chemical calculations on small clusters of diamond and doped diamond. Excitation energies are calculated based on differences in total energies of neutral and positively or negatively charged clusters. When charges are free in the bulk, as for an electron associated with substitutional B, a simple particle-in-a-box kinetic energy of cluster confinement is taken into account. Optical and thermal excitation energies are calculated for dopants using the B3LYP hybrid density functional method with the 6-31G basis set and substituted C 44 H 42 nanocrystals in the hulk structure and with structure optimization around the defects. Based on eight experimentally determined values, the errors of this method are 0.6 eV and less. Calculations at this level allow screening for potential shallow donor and shallow acceptor defects. A classical approach to estimating the cluster confinement energy based on charging the cluster in a polarized dielectric continuum is tested and shown to he less successful.

A theoretical investigation on the isomerism and the NMR properties of thiosemicarbazones

Hybrid density functional theory calculations at the mPW1PW91/6-31+G(d,p) level of theory have been used to investigate the optimized structures and other molecular properties of five different series of thiosemicarbazones. The investigated compounds were obtained from acenaphthenequinone, isatin and its derivatives, and alloxan. The focus of the study is the isomerism and the NMR characterization of these thiosemicarbazones. It was found that only one isomer is expected for thiosemicarbazones and methylthiosemicarbazones, while for dimethylthiosemicarbazones, two isomers are possible. All investigated thiosemicarbazones exhibit a hydrazinic proton that is highly deshielded and resonates far downfield in the proton NMR spectra. This proton is a part of a characteristic sixmembered ring, and its NMR properties are a result of its strong, intermolecular hydrogen bond. The relationships between the calculated 1H and 13C NMR chemical shifts and various geometric parameters are reported.

Hybrid density functional theory investigation of a series of alloxan-based thiosemicarbazones and semicarbazones

Recently, the synthesis and the NMR characterization of a series of eight alloxan-based thiosemicarbazones and semicarbazones were reported. These compounds exhibit a strongly hydrogenbonded hydrazinic proton that is a part of a characteristic six-membered ring. This proton is highly deshielded and resonates far downfield in the proton NMR spectra. In this report, mPW1PW91/6-31+G(d,p) calculations have been used to investigate the structure and other molecular properties of this series of eight compounds. The relationship between the 1H and 13C NMR chemical shifts and various geometric parameters was investigated, and linear relationships for proton peaks that are involved in hydrogen-bond interactions were found.

Force field variations along the torsional coordinates of CH3OH and CH3CHO

Abstract We examine the question of reduced Hamiltonians for the torsion–rotation problem and in particular the question of the continuity of frequencies obtained by projecting out a large-amplitude torsional coordinate. We find that suitable results for torsion–rotation analysis can be obtained using either a rectilinear or a curvilinear formalism.

Synthetic, structural, and spectroscopic studies of mixed sandwich Ru(II) complexes involving η6-p-cymene with monodentate fluorine-containing phosphines or phosphites

Abstract Syntheses, structures, spectroscopy, and calculated structures for several Ru(II) complexes involving p-cymene and either fluorine-containing phosphines or phosphites are reported. The complexes are readily prepared by ligand substitution reactions from [{Ru(p-cymene)Cl2}2] to produce [Ru(p-cymene)(L)Cl2] {L = P(C6H4-p-F)3 (1), P(C6H4-p-CF3)3 (2), P(C6H3(m-CF3)2)3 (3), PPh3 (4), P(OCH2CF3)3 (5), P(OCH(CF3)2)3 (6), and POMe3 (7)}. The structures for all seven complexes are supported by UV–vis and multi-nuclear NMR spectroscopy. Five complexes are characterized by single-crystal X-ray crystallography (1, 3, 5–7) and exhibit a distorted octahedral structure involving three donors from one phosphine or phosphite ligand, two chlorides, and the facially coordinating η6-p-cymene ligand. Electronic structure theory computations have been performed on 1–7 along with the theoretical [Ru(p-cymene){P(C6F5)3}Cl2] (8). For all complexes, the HOMO is primarily Ru (dxy or ) in character. The LUMO shifts from primarily Ru (dxz or dyz) to primarily phosphine ligand π* as the number of fluorines increases.

Modifications of ribonuclease A induced by p-benzoquinone

The nature of ribonuclease A (RNase) modifications induced by p-benzoquinone (pBQ) was investigated using several analysis methods. SDS-PAGE experiments revealed that pBQ was efficient in producing oligomers and polymeric aggregates when RNase was incubated with pBQ. The fluorescence behavior and anisotropy changes of the modified RNase were monitored for a series of incubation reactions where RNase (0.050 mM) was incubated with pBQ (0.050, 0.25, 0.50, 1.50 mM) at 37 °C in phosphate buffer (pH 7.0, 50 mM). The modified RNase exhibited less intense fluorescence and slightly higher anisotropy than the unmodified RNase. UV-Vis spectroscopy indicated that pBQ formed covalent bonds to the modified RNase. Confocal imaging analysis confirmed the formation of the polymeric RNase aggregates with different sizes upon exposure of RNase to high concentrations of pBQ. The interaction between the modified RNase and salts affecting biomineralization of salts was also investigated by scanning electron microscopy. Overall, our results show that pBQ can induce formation of both RNase adducts and aggregates thus providing a better understanding of its biological activity.

A comparison study on ribonuclease A modifications induced by substituted p-benzoquinones.

In this paper, we present our investigation on ribonuclease A (RNase) modifications induced by 1,4-benzoquinone (PBQ), 2-methyl-1,4-benzoquinone (MBQ), and 2-chloro-1,4-benzoquinone (CBQ). The goal of the study was to evaluate quinone-induced protein modifications as well as substituent effects, utilizing several techniques such as SDS-PAGE, fluorescence spectroscopy, microscopy, and LC-ESI(+)-QTOF-MS. SDS-PAGE experiments revealed that all quinones modify RNase through oligomerization as well as polymeric aggregation; with CBQ functioning as the most efficient quinone while MBQ was least efficient. The fluorescence emission was found to be less intense and the anisotropy values were found to be slightly higher for the modified RNase compared to the unmodified RNase. UV-Vis spectroscopy indicated that all three quinones formed adducts in which they were covalently linked to RNase. Confocal imaging analysis showed that the presence of CBQ resulted in massive RNase aggregation, while PBQ-treated RNase formed much smaller aggregates. MBQ-treated RNase exhibited micrographic features that closely resembled those of the unmodified RNase. LC-ESI(+)-QTOF-MS studies indicated the nature of PBQ- and CBQ-induced RNase modifications are complex mainly due to simultaneously occurrence of both adduct formation and oligomerization. Kinetic studies on quinone reactivity toward lysine revealed the rank order of CBQ>PBQ≫MBQ, based on the second-order rate constants. We also utilized scanning electron microscopy in order to investigate the effect of modified RNase on the biomineralization of salts.