Conformations of poly{G}-poly{C} pi stacks with high hole mobility

Charge transfer properties of DNA depend strongly on the pi stack conformation. In the present paper, we identify conformations of homogeneous poly-{G}-poly-{C} stacks that should exhibit high charge mobility. Two different computational approaches were applied. First, we calculated the electronic coupling squared, V(2), between adjacent base pairs for all 1 ps snapshots extracted from 15 ns molecular dynamics trajectory of the duplex G(15). The average value of the coupling squared is found to be 0.0065 eV(2). Then we analyze the base-pair and step parameters of the configurations in which V(2) is at least an order of magnitude larger than . To obtain more consistent data, approximately 65,000 configurations of the (G:C)(2) stack were built using systematic screening of the step parameters shift, slide, and twist. We show that undertwisted structures (twist<20 degrees) are of special interest, because the pi stack conformations with strong electronic couplings are found for a wide range of slide and shift. Although effective hole transfer can also occur in configurations with twist=30 degrees and 35 degrees, large mutual displacements of neighboring base pairs are required for that. Overtwisted conformation (twist> or =38 degrees) seems to be of limited interest in the context of effective hole transfer. The results may be helpful in the search for DNA based elements for nanoelectronics.     

A theoretical study of the vibrational structure in the \documentclass{article}\pagestyle{empty}\begin{document}$\tilde{A}^{2}A_{1} - \tilde{X}^{2}B_{1}$\end{document} transition of PH2

The potential energy curves of the bending mode of PH₂ in the ground ²B₁ and the excited ²A₁ states have been calculated using ab initio SCF -MO wave-functions. The radical is found, in agreement with experiment, to be bent in the two states. The calculated results clarify the interpretations of the anomalous behaviour of observed vibrational intervals, and offer an explanation of the ‘quasilinearity’; of the radical in the ²A₁ state.     

On the calculation of arbitrary multielectron molecular integrals over Slater‐type orbitals using recurrence relations for overlap integrals. III. Auxiliary functions \documentclass{article}\pagestyle{empty}\begin{document}$Q_{nn'}^{q}$\end{document} and \documentclass{article}\pagestyle{empty}\begin{document}$G_{-nn'}^{q}$\end{document}

The auxiliary functions $Q_{nn'}^{q}(p,pt)$ and $G_{-nn'}^{q}(p_{a},p,pt)$ which are used in our previous paper [Guseinov, I. I.; Mamedov, B. A. Int J Quantum Chem 2001, 81, 117] for the computation of multicenter electron‐repulsion integrals over Slater‐type orbitals (STOs) are discussed in detail, and the method is given for their numerical computation. The present method is suitable for all values of the parameters p_a, p, and pt. Three‐ and four‐center electron‐repulsion integrals are calculated for extremely large quantum numbers using relations for auxiliary functions obtained in this paper. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001     

Control of pi-stacking for highly emissive poly(p-phenylenevinylene)s: synthesis and photoluminescence of new tricyclodecane substituted bulky poly(p-phenylenevinylene)s and its copolymers

In the present work, we have demonstrated a facile approach to increase the luminescence of the poly (p-phenylenevinylene)s via controlling the molecular aggregates induced by pi-stacking. We have synthesized new bulky tricyclodecane (TCD) substituted PPVs: poly(2-methoxy-5-tricyclodecanemethyleneoxy-1,4-phenylenevinylene) (MTCD-PPV), poly(bis-2,5-tricyclodecanemethyleneoxy-1,4-phenylenevinylene) (BTCD-PPV), and a series of symmetrically substituted bulky PPV copolymers (P-1-P-7) covering the entire composition range from 0 to 100 mol %. The structures of the monomers and polymers were confirmed by 1H NMR and FTIR, and the molecular weights were determined by gel permeation chromatography. The composition analysis by NMR revealed that the bulky monomer was highly reactive and the incorporation of bulky units in MEH-PPV increased irrespective of the feed ratio. The polymers possess good solubility, high molecular weights, good thermal stability, and so forth. The molecular weights of the PPV copolymers were also significantly affected by the bulky substitution: the higher the incorporation of bulky units, the lower the molecular weight. The absorption and emission studies revealed that there was no influence on the MEH-PPV by TCD substitution in solution whereas in the solid state the photoluminescence intensity of PPV increased more than 10 times. The luminescence increase in PPV was observed throughout the entire bulk and was not confined to any particular domain in the polymer. The bulky PPV copolymers showed that both the luminescence intensity (in film) and quantum yields (in solution) increased with an increase in the extent of BTCD incorporation in the MEH-PPV and attained a maximum for 50% BTCD. The TCD unit has thus proved to be an efficient bulky susbstituent for PPV as it controls the pi-stack-induced molecular aggregates in the polymer chains by increasing the interchain distances. The new bulky PPV copolymers are highly soluble, thermally stable, and highly luminescent besides being economically cheap compared to the other materials reported so far for the bulkier approach in pi-conjugated materials.     

Structural,magnetic and magnetocaloric properties of $$\hbox {Ni}_{54}\hbox {Mn}_{14}\hbox {Ga}_{27}\hbox {Fe}_{5}$$ Ni 54 Mn 14 Ga 27 Fe 5 : the impact of annealing time

Journal of Thermal Analysis and Calorimetry - The structural, magnetic and magnetocaloric properties of $$\hbox {Ni}_{54}\hbox {Mn}_{14}\, \hbox {Ga}_{27}\hbox {Fe}_{5}$$ Heusler alloy as a...     

On the anomaly of the quasiclassical product distributions of the \hbox{OH} +\hbox{CO} \rightarrow\hbox{H} +\hbox{CO}_2 reaction

A grid empowered molecular simulator (GEMS) embodying in a single workflow the ab initio treatment of elementary chemical processes has been extended to four atom reactions. GEMS has been used to carry out a massive quasiclassical investigation for the $\hbox{OH} +\hbox{CO} \rightarrow\hbox{H} +\hbox{CO}_2$ reaction on the most recently proposed potential energy surface. The type of potential energy surface used and the possibility of running the simulations on the grid have allowed us to keep the error of the order of a few percent at all values of the collision energy and to estimate accurately the dependence of the reaction cross section on the collision energy. The accuracy of the calculations has allowed to unequivocally single out the fact that the calculated center-of-mass angular distribution is clearly isotropic and radically differs from the asymmetric forward?Cbackward structure obtained from the experiment. However, when the laboratory frame analogues are compared, the difference almost vanishes.     

Hierarchical TiO2 microspheres: synergetic effect of {001} and {101} facets for enhanced photocatalytic activity

Well-faceted nanocrystals of anatase TiO(2) with specific reactive facets have attracted extraordinary research interest due to their many intrinsic shape-dependent properties. In this work, hierarchical TiO(2) microspheres consisting of anatase nanosheets or decahedrons were synthesized by means of a facile hydrothermal technique; meanwhile, the percentage of {001} facets can be tuned from 82 to 45%. Importantly, by investigating the photo-oxidation reactions for ˙OH radical generation and photoreduction reactions for hydrogen evolution, the TiO(2) microspheres consisting of nano-decahedrons with 45% {001} facets show superior photoreactivity (more than 4.8-times) compared to the nanosheets with 82% {001} facets. By analyzing the results of scanning electron microscopy (SEM), photoluminescence (PL) and first-principles density functional theory (DFT) calculations, a model of charge separation between the well-formed {001} and {101} facets is proposed, and the enhanced photocatalytic efficiency is largely attributed to the efficient separation of photogenerated charges among the crystal facets co-exposed.     

Density functional theory study of the structures and stabilities of CuO\documentclass{article}\pagestyle{empty}\begin{document}$_{3}^{-}$\end{document} and CuO3

Hybrid density functional theory calculations on the structures, vibrational frequencies, electron binding and dissociation energies, and bonding properties of CuO$_{3}^{-}$ and CuO₃ species have been carried out. Stable isomers containing an O₃ subunit and composed of O₂ bound to CuO have been located on the potential energy hypersurfaces of CuO$_{3}^{-}$ and CuO₃. The isomers formed by O₂ bonded to CuO in side‐on and end‐on coordination are more stable than those containing an O₃ subunit. © 2001 John Wiley & Sons, Inc. Int J Quant Chem 81: 162–168, 2001     

Synthesis and Characterization of 1,3,2‐Bis (arylamino) phospholidine, 2‐oxide Derivatives: Crystal Structures of $\rm Cl(O)\overline{PN(p-Me-C_{6}H_{4})CH_{2}CH_{2}N}(p-Me-C_{6}H_{4})$ and $\rm ((CH_{2}C_{6}H_{5})(O)\overline{PN(p-Me-C_{6}H_{4})CH_{2}CH_{2}N}(p-Me-C_{6}H_{4})$

Using phosphoryl chloride as a substrate, a family of 1,3,2‐bis(arylamino) phospholidine, 2‐oxide of the general formula ; (X=Cl, 6a ; X=NMe₂, 1b ; X=N(CH₂C₆H₅)(CH₃), 2b ; X=NHC(O)C₆H₅, 3b ; X=4Me‐C₆H₄O, 4b ; X=C₆H₅O, 5b ; X=NHC₆H₁₁, 6b ; X=OC₄H₈N, 7b ; X=C₅H₁₀N, 8b ; X=NH₂, 9b ; X=F, 10b and Ar=4Me‐C₆H₄) was prepared and characterized by ¹H, ¹⁹F, ³¹P and ¹³C NMR and IR spectroscopy, and elemental analysis. A general and practical method for the synthesis of these compounds was selected. The structures of 6a and 2b were determined by single‐crystal X‐ray diffraction techniques. The low temperature NMR spectra of 2b revealed the restricted rotation of P‐N bond according to two independent molecules in crystalline lattice.     

Ultraviolet photochemistry of ethane: implications for the atmospheric chemistry of the gas giants

Chemical processing in the stratospheres of the gas giants is driven by incident vacuum ultraviolet (VUV) light. Ethane is an important constituent in the atmospheres of the gas giants in our solar system. The present work describes translational spectroscopy studies of the VUV photochemistry of ethane using tuneable radiation in the wavelength range 112 ≤ λ ≤ 126 nm from a free electron laser and event-triggered, fast-framing, multi-mass imaging detection methods. Contributions from at least five primary photofragmentation pathways yielding CH₂, CH₃ and/or H atom products are demonstrated and interpreted in terms of unimolecular decay following rapid non-adiabatic coupling to the ground state potential energy surface. These data serve to highlight parallels with methane photochemistry and limitations in contemporary models of the photoinduced stratospheric chemistry of the gas giants. The work identifies additional photochemical reactions that require incorporation into next generation extraterrestrial atmospheric chemistry models which should help rationalise hitherto unexplained aspects of the atmospheric ethane/acetylene ratios revealed by the Cassini–Huygens fly-by of Jupiter.

The vacuum ultraviolet photodissociation dynamics of ethane provide clues for modelling the atmospheric chemistry of the gas giants.     

Quasi-classical trajectory insight into stereodynamics for reaction $$\hbox {He}+\hbox {D}_{2}^{+}\rightarrow \hbox {HeD}^{+}+\hbox {D}$$ with the improved potential energy surface

In this work, a theoretical study on the detailed vector correlation for the reaction \(\hbox {He}+\hbox {D}_{2}^{+}\rightarrow \hbox {HeD}^{+}+\hbox {D}~(\hbox {v}, \hbox {j})\) has been carried out at the collision energy of 23.06 kcal/mol with different rotational states of j \(=\) 0–5 and vibrational states v \(=\) 1–5 by use of the quasi-classical trajectory calculation on an improved potential energy surface. The features of \({{\varvec{P}}}\mathbf{(}{\varvec{\theta }}_{{\varvec{r}}}{} \mathbf{)}\) distributions describing k and \(\mathbf{j}^\prime \) correlations were discussed. In addition, in order to get full knowledge of stereodynamics of the system, the distributions of dihedral angle \({{\varvec{P}}}\mathbf{(}{\varvec{\varphi }}_{{\varvec{r}}}{} \mathbf{)}\) and the polarization-dependent differential cross-sections (PDDCSs) were also reported. It has been demonstrated that both product alignment and polarization are sensitive to the reagent vibrational and rotational number. Furthermore, the dynamics behavior of the reaction is independently changed with respect to j under a certain v except for the product alignment effect \(({\varvec{P}}({\varvec{\theta }}_{{\varvec{r}}}))\), while it exhibits a generally regular trend concerning v when j is invariable.     

Solid-state Structures and Solution Behavior of Alkali Salts of the [\hbox{Nb}_{6}\hbox{O}_{19}]^{8-} Lindqvist Ion

The hexaniobate Lindqvist ion has long been known as the dominant specie in alkaline niobium oxide solutions. Recent advances in heteropolyniobate chemistry continue to be greatly aided by use of alkali salts as soluble precursors; in particular, potassium, sodium and lithium hexaniobate salts. We report here the solid-state characterization and solution behavior of Li, K, Rb and Cs Lindqvist salts. Synthesis and single-crystal X-ray diffraction data is reported for nine new hexaniobate salts. These structures differ in the number of charge-balancing alkali cations, protonation of the clusters, relative arrangement of the clusters and alkali metal cations, amount of lattice water and its mode of interaction with other lattice species. Trends of alkali-cluster bonding are observed as a function of alkali radius. Protonation of the clusters in the solid-state is influenced by the method of crystallization of the salt. Lability of the cluster oxygens is observed by solution ¹⁷O NMR experiments. Rates of isotopic enrichment of the bridging oxygen, terminal oxygen and bridging hydroxyl cluster sites are compared for aqueous solutions of Li, K, Rb and Cs hexaniobate salts. Parameters influencing the oxo-ligand exchange rates of the salts are discussed relative to their use as heteropolyniobate precursors.This paper is dedicated to Professor Michael T. Pope on the event of his retirement to acknowledge his fruitful career in polyoxometalate chemistry.     

Revisiting the generator coordinate approximation for calculating the ro‐vibrational energies of H\documentclass{article}\pagestyle{empty}\begin{document}$_{3}^{+}$\end{document}

The Generator Coordinate Approximation, a relatively recent approximation formulated to solve systems of three or more bodies, is tested for its accuracy and viability by applying it to calculate the ro‐vibrational energies of the triatomic system H$_{3}^{+}$. We employ in this work a recently formulated basis called the Numerically Generated‐Discrete Variable Representation for the wave function and test it against the well‐known Finite Element Method basis. Comparison of the two results and with other results shows a tentative superiority of the Numerically Generated‐Discrete Variable Representation. In addition, many new physical properties of the Generator Coordinate Approximation were discovered. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 2028–2039, 2001     

Hot fusion of fission fragments for the synthesis of doubly magic nucleus $${}_{126}^{310} X^{184}$$ 126 310 X 184

Journal of Radioanalytical and Nuclear Chemistry - The worldwide population of elephants shows a rapid decline due to trafficking of ivory. Hence, a method is needed for the age determination to...     

Fe-N-O structure and bonding in six-coordinate {FeNO}6 porphyrinates containing imidazole: implications for reactivity of coordinated NO

We report density functional theory calculations on six-coordinate ferric-NO ({FeNO}6) porphyrinates that contain either imidazole or imidazolate as the trans axial ligand. Our results show that the sensitivities of the Fe-NO and N-O stretching frequencies to cis and trans influences are directly correlated. In other words, as one decreases so does the other for both the imidazole and the imidazolate complexes. This correlation is opposite that of the isoelectronic ferrous-CO systems, whose Fe-CO and C-O frequencies are well-known to be inversely correlated. Based on the results of our calculations, the molecular origin of the direct correlation in {FeNO}6 porphyrinates can be explained by trends in the electron density distributions within the HOMO or HOMO-1, which exhibits Fe-NO and N-O pi-antibonding character. Variability in the Fe-N-O pi-antibonding character of the HOMO or the HOMO-1 modulates the angleFeNO as well as the Fe-NO and N-O bond strengths in concert. Orbital interactions in the six-coordinate FeIIINO porphyrin complexes are compared and contrasted with those of the isoelectronic FeIICO analogues, and an overall view of {FeNO}6 bonding in these complexes is set forth.     

Small Ligated Organometallic $${{\hbox {Pd}}}_{n}$$ Clusters ($$n= 4{-}12$$n=4-12): A DFT Investigation

DFT calculations were carried out at the BP86/LANL2DZ level of theory to probe the structures and relative stabilities of small and medium size ligated palladium clusters. Results show that optimized geometries satisfactorily mimic experimental structural data. Observed cluster electron counts generally deviate from the expected Wade–Mingos electron counts because of the presence of several non-conical T-shaped \({\hbox {ML}}_{3}\) fragments in the clusters. These species are thermodynamically stable with substantial HOMO–LUMO gaps.