Syntheses,X‐ray crystal structures,and emission properties of diprotonated tetrapyridylpyrazine and triprotonated terpyridine

A series of metal‐free compounds, that is, planar diprotonated tetraH₂(PF₆)₂ ( 1 ), planar monoprotonated bppzHPF₆ ( 2 ), nonplanar diprotonated bppzH₂(PF₆)₂ ( 3 ), and planar triprotonated terpyH₃Cl(PF₆)₂ ( 4 ), were prepared and characterized by electrospray ionization mass spectrometry, Ultraviolet–visible spectroscopy and cyclic voltammetry. Abbreviations used are tetra = tetra‐2‐pyridylpyrazine, bppz = 2,3‐bis(2‐pyridyl)pyrazine, and terpy = 2,2′:6′,2″‐terpyridine. The X‐ray crystal structures of the four compounds 1 , 2 , 3 , and 4 were determined. Both protonated pyridine rings are hydrogen bonded intramolecularly to the adjacent pyridine ring in compounds 1 and 3 . The π–π* absorption bands in the ultraviolet region for 1 , 2 , 3 , and 4 in acetonitrile were red‐shifted relative to those of the corresponding neutral unprotonated compounds. All the cyclic voltammetry for the protonated species, 1 , 2 , 3 , and 4 , showed the first reduction waves from ?0.37 to ?1.18 V, that were more positive than those of the neutral ones. Density functional theory was applied to interpret the planarity in 1 . The attachment of two protons to the two terminal tetra nitrogens in 1 leads to the remarkable emission (Φ = 0.031). The attachment of three protons to the three terpy nitrogens in 4 also gives the large quantum yield (Φ = 0.48). Copyright © 2015 John Wiley & Sons, Ltd.     

Electronic structure of As4S5 as a local structural model for amorphous As2S3 film

We have studied the electronic structure of As₄S₅ molecule as a model of the local pyramidal structure in amorphous As₂S₃ film. Emphasis has been put on the analysis of the behaviour of a trapped electron in this model concerning the consequent structural relaxation. It has been found that As₄S₅ species has affinity for an excess electron, and that the attachment of an excess electron promotes the cleavages of specific As-S bonds.     

Experimental and theoretical analyses of azulene synthesis from tropones and active methylene compounds: reaction of 2-methoxytropone and malononitrile

A representative azulene formation from an active troponoid precursor (2-methoxytropone) and an active methylene compound (malononitrile) has been analyzed both experimentally and theoretically. (2)H-Tracer experiments using 2-methoxy[3,5,7-(2)H(3)]tropone (2-d(3)) and malononitrile anion give 2-amino-1,3-dicyano[4,6,8-(2)H(3)]azulene (1-d(3)) in quantitative yield. New and stable (2)H-incorporated reaction intermediates have been isolated, and main intermediates have been detected by careful low-temperature NMR measurements. The detection has been guided by mechanistic considerations and B3LYP/6-31(+)G(d) calculations. The facile and quantitative one-pot formation of azulene 1 has been found to consist of a number of consecutive elementary processes: (a) The troponoid substrate, 2-methoxytropone (2), is subject to a nucleophilic substitution by the attack of malononitrile anion (HC(CN)(2)(-)) to form a Meisenheimer-type complex 3, which is rapidly converted to 2-troponylmalononitrile anion (5). (b) The anion 5 is converted to an isolable intermediate, 2-imino-2H-cyclohepta[b]furan-3-carbonitrile (6), by the first ring closure in the reaction. (c) A nucleophilic addition of the second HC(CN)(2)(-) toward the imine 6 at the C-8a position produces the second Meisenheimer-type adduct 7. (d) The second ring closure leads to 1-carbamoyl-1,3-dicyano-2-imino-2,3-dihydroazulene (11). A base attacks the imine 11, which results in generation of a conjugate base 12 of the final product, azulene 1.     

Platinum clusters on vacancy-type defects of nanometer-sized graphene patches

Density functional theory calculations found that spin density distributions of platinum clusters adsorbed on nanometer-size defective graphene patches with zigzag edges deviate strongly from those in the corresponding bare clusters, due to strong Pt-C interactions. In contrast, platinum clusters on the pristine patch have spin density distributions similar to the bare cases. The different spin density distributions come from whether underlying carbon atoms have radical characters or not. In the pristine patch, center carbon atoms do not have spin densities, and they cannot influence radical characters of the absorbed cluster. In contrast, radical characters appear on the defective sites, and thus spin density distributions of the adsorbed clusters are modulated by the Pt-C interactions. Consequently, characters of platinum clusters adsorbed on the sp2 surface can be changed by introducing vacancy-type defects.     

Theoretical study on the lowest-frequency mode of the flavin ring

The dynamic aspects along the normal vibrational motions of the lowest frequencies in the oxidized, radical, and reduced states of flavin (isoalloxazine) have been studied. In comparison with the twist motions in the oxidized state, the butterfly motions in the radical and reduced states turned out to bring more significant variations to the frontier molecular orbital energies and to the charge distributions on the atoms of the pyrazine ring in isoalloxazine. It can be considered that the electron transfers from and to the isoalloxazine ring can be adjusted or controlled by these variations. In the reduced states the electron release from the molecule, and in the radical states the electron release from or acceptance by the molecule, could be impelled by the butterfly motions, while in the oxidized state the electron acceptance by the molecule could be accelerated slightly by the twist motion. Received: 30 September 1998 / Accepted: 20 January 1999 / Published online: 7 June 1999     

SN1‐SN2 and SN2‐SN3 mechanistic changes revealed by transition states of the hydrolyses of benzyl chlorides and benzenesulfonyl chlorides

Hydrolysis reactions of benzyl chlorides and benzenesulfonyl chlorides were theoretically investigated with the density functional theory method, where the water molecules are explicitly considered. For the hydrolysis of benzyl chlorides (para‐Z? C₆H₄? CH₂? Cl), the number of water molecules (n) slightly influences the transition‐state (TS) structure. However, the para‐substituent (Z) of the phenyl group significantly changes the reaction process from the stepwise (S_N1) to the concerted (S_N2) pathway when it changes from the typical electron‐donating group (EDG) to the typical electron‐withdrawing one (EWG). The EDG stabilizes the carbocation (MeO? C₆H₄? CH₂⁺), which in turn makes the S_N1 mechanism more favorable and vice versa. For the hydrolysis of benzenesulfonyl chlorides (para‐Z? C₆H₄? SO₂? Cl), both the Z group and n influence the TS structure. For the combination of the large n value (n > 9) and EDG, the S_N2 mechanism was preferred. Conversely, for the combination of the small n value and EWG, the S_N3 one was more favorable. © 2014 Wiley Periodicals, Inc.     

A theoretical deduction of the shape and size of nanocarbons suitable for hydrogen storage

We evaluated the adsorption energy of a hydrogen molecule in nanocarbons consisting of graphene sheets. The nanocarbon shapes were a pair of disks with separation 2d, a cylinder with radius d, and a truncated sphere with radius d. We obtained the adsorption energy in the form of a 10–4 Lennard–Jones function with respect to 1/d. The values of the potential depth (D) and equilibrium distance (d _e), respectively, were 94 meV and 2.89 Å for the disk pair, 158 meV and 3.14 Å for the cylinder, and 203 meV and 3.37 Å for the sphere. When d=d _e, the adsorption energy of the disk pair (cylinder) became deeper than ?0.9D, and it approached ?D when the radius (length) increased to more than twice its separation (radius). The adsorption energy of the sphere was increased from ?D to ?0.5D when the radius of the opening increased from 0 to d _e. These results suggest that porous carbon materials can increase the adsorption energy by up to ～200 meV if the carbon atoms are arranged on a spherical-like surface with ～7 Å separation. This may lead to practical hydrogen storage for fuel cells.     

Biradical processes in reactions between benzyne and tropone

The title reactions were investigated experimentally and theoretically. The benzyne--tropone pair has been known to give a Diels?CAlder [4+2] cycloadduct. However, in the present experiment using two sources of benzyne, many unexpected products were obtained. In particular, a zwitterion with a C?CCl bond is strikingly a major product, the structure of which was determined by the X-ray analysis. The bond is formed by quenching the Cl atom from the solvent CH₂Cl₂, which demonstrates that the reactions proceed via radical processes. DFT calculations revealed the elementary processes of the reactions. The presence of the novel zwitterion in the solid state has been interpreted in terms of the permanent dipole?Cdipole attractions/interactions/stabilization.