Reaction paths of the water-assisted solvolysis of N,N-dimethylformamide

Density functional theory calculations were conducted on the title reactions with explicit inclusion of a variety of water molecules, H-CO-NMe2+MeOH+(H2O)n-->H-CO-OMe+HNMe2+(H2O)n. Geometries of transition states, reactant-like complexes and product-like ones were determined by the use of RB3LYP/6-31G(d) SCRF=dipole. Concerted paths were examined with n=0-3. Their Gibbs activation energies are larger than the experimental value. Stepwise paths were also investigated with n=2-4. The n=4 model has the energy close to the experimental value. However, when the catalytic water molecules were added to the n=4 one, the stepwise path was switched to the concerted one. A systematic comparison of the concerted path with n=2+1, 2+2, 2+3, 2+4, 2+5, 2+4+4, and 2+5+5 models was made, and the water-dimer based reaction path was found to be most favorable. The contrast between the concerted path of the amide solvolysis (and hydrolysis) and the stepwise one of the ester hydrolysis was discussed in terms of the frontier-orbital theory.     

Diamagnetic currents in the neutral He atoms

The mechanism of the occurrence of intraatomic diamagnetic currents in the neutral He atoms with microscopic sizes is investigated. It is found that most of all electrons can form electron pairs originating from attractive Coulomb interactions between two electrons with opposite spins occupying the 1s atomic orbital in the neutral He atom at 298 K. Intraatomic diamagnetic currents in the neutral He atoms with microscopic sizes can be explained by such electron pairing. The transition temperature Tc(He),(1s) value at which intraatomic diamagnetic currents can disappear in each He atom is estimated. The Tc(He),(1s) values for the neutral He atoms with microscopic sizes are estimated to be much larger than the superconducting transition temperatures Tc,BCS values for the conventional superconductors with macroscopic sizes. This result can be understood from continuous energy levels of electronic states in conventional superconductivity with macroscopic sizes, and from discrete energy levels of electronic states in the neutral He atoms with microscopic sizes. The energy difference between the occupied and unoccupied orbitals decreases with an increase in material size and thus the second-order perturbation effect becomes more important with an increase in material size. Therefore, the mechanism of the occurrence of intraatomic diamagnetic current in the neutral He atoms suggested in this research would not be true for materials with large sizes. The dependence of electronic properties on temperature in the diamagnetic currents in the neutral He atoms with microscopic sizes is studied and compared with that in the conventional superconductivity with macroscopic sizes.     

A Novel Amperometric Transducer Electrode with Iridium‐Niobium Binary Alloys

We have discovered that an Ir‐23Nb binary alloy more effectively oxidizes hydrogen peroxide than Ir, Ir‐13Nb, Ir‐17Nb, Ir‐30Nb, Ir‐43Nb, Ir‐62Nb, or Nb. The oxidation capability was determined via cyclic voltammogram measurements of pH‐buffer and hydrogen peroxide. We ascertained that applying a 0.7 V potential produces hydrogen peroxide currents of 9.2 μA/mm² Ir‐23Nb, 5.3 μA/mm² Ir, 5.1 μA/mm² Ir‐17Nb, 3.7 μA/mm² Ir‐13Nb, 2.0 μA/mm² Ir‐30Nb, 1.5 μA/mm² Ir‐43Nb, 0.6 μA/mm² Ir‐62Nb, and 0.13 μA/mm² Nb. These results indicate that the effective oxidation of Ir‐23Nb for hydrogen peroxide might be due to its fcc+L1₂ two‐phase structure and that Ir‐23Nb can be used as an amperometric transducer material.     

Proton transfers along hydrogen bonds in the tautomerization of purine

Tautomerization of purine in the water cluster was investigated by the use of DFT calculations. The correlation between the reaction paths and the number of water molecules (n) was examined. For n = 3 and n = 4, concerted reaction paths were obtained. However, for n = 5, a stepwise path including an ion pair intermediate was found with small activation energies. The n = 4 + 3 and n = 4 + 3 + 9 models were calculated to give further small activation energies, where n = 4 constitutes the reaction center and +3 and +3 + 9 denote the number of catalytic water molecules. The combination of the in-plane deprotonation at the N9 site and the out-of-plane protonation at the N7 site makes the n = 4 model probable. Three protonated n = 4 + 3 + 9 routes, a, b, and c, composed of purineH(+)(H(2)O)(4+3+9) were investigated. The n = 4 + 3 moiety is also included in the three routes, and the route c (with the N1 protonation) was found to be most favorable. The purine tautomerization was found to involve the Zundel cation in the ion pair intermediate.     

Roles of radical characters of pristine and nitrogen-substituted hydrographene in dioxygen bindings

We investigate by means of density functional theory (DFT) calculations how hydrogen-terminated graphenes (hydrographenes) with and without nitrogen impurities interact with dioxygen. The current study aims at searching whether hydrographenes can be utilized as cathode catalysts in fuel cell with a focus on dioxygen binding, the first step in oxygen reduction reaction (ORR). If hydrographenes have a nanometer-size rhombic structure with zigzag edges, unpaired electrons are localized at their edges with or without the nitrogen impurities. Spin localization comes from frontier orbitals of the nanometer-size hydrographenes whose amplitudes appear only at their edges. Due to their radical characters, dioxygen can bind to an edge carbon atom of the hydrographenes under the condition where fuel cell is usually operated. There are two types of dioxygen binding into a hydrographene: one is a Pauling fashion where one C-O bond is formed and the other is a bridging fashion with two formed C-O bonds. In the bridging fashion, the formation of the two C-O bonds activates dioxygen, and then radical characters of the oxygen atoms completely disappear. In contrast, the Pauling fashions retain an unpaired electron on the oxygen atom that does not participate to the C-O bond formation. The existence of radical oxygen atoms would facilitate the next step in ORR (the initial proton transfer to an adsorbed dioxygen), whereas such facilitative effects cannot be seen in its absence. According to DFT calculations, the Pauling-type bindings are always energetically preferred over the bridging-type bindings. In particular, the C→N substitution enhances the preferences of the Pauling-type binding over the bridging-type binding compared with the pristine case. Accordingly DFT calculations demonstrate that radical characters of edge carbons of a nanometer-sized rhombic hydrographene play a crucial role in dioxygen bindings in a Pauling fashion that would be responsible for enhancing the catalytic activity in fuel cell.     

A theoretical study of hydrogen adsorption on Li, Be, Na, and Mg atoms attached to aromatic hydrocarbons

The binding energy of a hydrogen molecule on metal atoms (Li, Be, Na, and Mg) attached to aromatic hydrocarbon molecules (benzene and anthracene) was calculated using an ab initio molecular orbital method at the MP2(FC)/cc-pVTZ level with basis set superposition error (BSSE) correction. The energy tended to become more negative as the metal atom had a more positive charge and a smaller radius. The energies of Li₂C₆H₆-H₂, Li₂C₁₄H₁₀-H₂, Na₂C₁₄H₁₀-H₂, and MgC₁₄H₁₀-H₂ were −2.7 to −2.2, −4.0 to −3.1, −2.8 to −0.3, and −1.3 kcal/mol, respectively. Most of these energies were more negative than those on the hydrocarbons without metal atoms (ca. −1 kcal/mol). Analyzing the Lennard–Jones type potential with the parameters determined by the MP2 calculations, it was found that these energies mainly consisted of the induction force caused by the positive charge of the metal atom and the dispersion force from the nearest C₆-ring. The energy of BeC₁₄H₁₀-H₂ was more negative (−8.6 kcal/mol) than of the other complexes. The hydrogen molecule in this complex had a comparatively longer H–H distance and a more positive H₂ charge than the others. These data suggest that the hydrogen adsorption on this complex involves a charge transfer process in addition to physisorption interactions. The hydrogen binding energies in some Li₂C₁₄H₁₀-H₂ systems (∼−4.0 kcal/mol) and BeC₁₄H₁₀-H₂ are promising to operate hydrogen storage/release at ambient temperature with moderate pressure.     

The electronic structures of polyacene and polyphenanthrene

The electronic structures of polyacene (PA) and its geometrical isomer, polyphenanthrene (PP) are studied on the basis of the one-dimensional tight-binding SCF-CO (self consistent field-crystal orbital) method. PA and PP can also be regarded as the laddered trans-polyacetylene and cis-polyacetylene, respectively. The geometry of each polymer is optimized from the energetic point of view. Although being energetically less stable than PP, PA has a small band gap and, furthermore, turns out to have a considerable dopant-philic nature. Hence PA will be a promising candidate as a new electrically conductive material because of its “intrinsically” metallic nature and thermal stability.     

MO study of counter anions of TMTSF: Electronic structure of NO−3, BF−4, ClO−4 and FSO−3

The electronic structure and geometrical structure of NO^-₃, BF^-₄, ClO^-₄ and FSO^-₃ anions are studied by means of the ab initio molecular orbital method. According to the results obtained, the central atoms of these anions are all positively charged and the surrounding atoms negatively charged. The bonding nature of ClO^-₄ and FSO^-₃ is similar to that of XF^-₆ (X = P, As and Sb) previously studied, and has a coordination-like character. However, BF^-₄ and NO^-₃ show a covalent-like character. The ion radii of these anions are determined from the total density contour maps obtained by the calculation.     

Ab initio effective core potential studies on polymers

Ab initio crystal orbital calculation with the effective core potential (ECP) approximation is performed on infinite poly-yne, all-trans-polyethylene, and all-trans-polysilane. The optimized bond lengths of poly-yne are predicted to be 1.130 Å and 1.321 Å with the split valence LP-31G basis set and agree fairly well with 4-31G results, 1.166 Å and 1.339 Å. The energy band structures of poly-yne and all-trans-polyethylene obtained from ECP calculations are in reasonable agreement with those from the all electron calculations. The fully optimized geometries of all-trans-polysilane are also predicted with the LP-31G basis set asr _SiSi = 2.264 Å,r _SiH = 1.493 Å, \(\sphericalangle\) SiSiSi = 118.97 °, and \(\sphericalangle\) HSiH = 100.35 °. The computational time for calculations of polysilane is found roughly to be comparative to that of polyethylene under ECP approximations.