Theory of late-transition-metal alkyl and heteroatom bonding: analysis of Pt, Ru, Ir, and Rh complexes

Density functional and correlated ab initio methods were used to calculate, compare, and analyze bonding interactions in late-transition-metal alkyl and heteroatom complexes (M-X). The complexes studied include: (DMPE)Pt(CH(3))(X) (DMPE = 1,2-bis(dimethylphosphino)ethane), Cp*Ru(PMe(3))(2)(X) (Cp* = pentamethylcyclopentadienyl), (DMPE)(2)Ru(H)(X), (Tp)(CO)Ru(Py)(X) (Tp = trispyrazolylborate), (PMe(3))(2)Rh(C(2)H(4))(X), and cis-(acac)(2)Ir(Py)(X) (acac = acetylacetonate). Seventeen X ligands were analyzed that include alkyl (CR(3)), amido (NR(2)), alkoxo (OR), and fluoride. Energy decomposition analysis of these M-X bonds revealed that orbital charge transfer stabilization provides a straightforward model for trends in bonding along the alkyl to heteroatom ligand series (X = CH(3), NH(2), OH, F). Pauli repulsion (exchange repulsion), which includes contributions from closed-shell d(π)-p(π) repulsion, generally decreases along the alkyl to heteroatom ligand series but depends on the exact M-X complexes. It was also revealed that stabilizing electrostatic interactions generally decrease along this ligand series. Correlation between M-X and H-X bond dissociation energies is good with R(2) values between 0.7 and 0.9. This correlation exists because for both M-X and H-X bonds the orbital stabilization energies are a function of the orbital electronegativity of the X group. The greater than 1 slope when correlating M-X and H-X bond dissociation energies was traced back to differences in Pauli repulsion and electrostatic stabilization.     

Hydrogen-bonding studies of amino acid side-chains with DNA base pairs

The interactions of the amino acid side-chains arginine (ARG), aspartic acid (ASP), asparagine (ASN), lysine (LYS) and serine (SER) with nucleic acid base pairs have been investigated using theoretical methods. The interaction energy of the short intermolecular N–H?···?N, N–H?···?O, O–H?···?O, O–H?···?N, C–H?···?O and C–H?···?N hydrogen bonds present in both isolated base pairs and complexes and its role in providing stability to the complexes have been explored. The homonuclear interactions are found to be stronger than the heteronuclear interactions. An improper hydrogen bond has been observed for some of the N–H?···?O and N–H?···?N hydrogen-bond interactions with the contraction of the N–H bond varying from 0.001 to 0.0260?Å and the corresponding blue shift of the stretching frequency by 4–291?cm^?1. Localized molecular orbital energy decomposition analysis (LMOEDA) reveals that the major contributions to the energetics are from the long-range polarization (PL) interaction, and the short-range attractive (ES, EX) and repulsive (REP) interactions. The Bader's atoms in molecules (AIM) theory shows good correlation for the electron density and its Laplacian at the bond critical points (BCP) with the N–H?···?N and N–H?···?O hydrogen-bond lengths in the complexes, and gives a proper explanation for the stability of the structure. The charge-transfer from the proton acceptor to the antibonding orbital of the X–H bond in the complexes was studied using natural bond orbital (NBO) analysis.     

Loss of hydrogen upon exposure of thiol to gold clusters at low temperature

Gold-acetone-dodecanethiol and gold-acetone-phenylethanethiol colloids were prepared by the solvated metal atom dispersion method. Hydrogen evolution occurred at fairly low temperature, when the melting acetone-gold solvate encountered the thiol molecules, due to S-H bond scission. The gas inside the reactor was analyzed by gas chromatography, and the moles of H(2) was determined by calibration curves obtained from a series of known concentration samples. The gold-to-thiolate ratio was calculated using thermal gravimetric analysis. The average particle diameter was also calculated using the gold-to-thiolate ratio.     

Are They Linear,Bent, or Cyclic? Quantum Chemical Investigation of the Heavier Group 14 and Group 15 Homologues of HCN and HNC

The singlet potential‐energy surface (PES) of the system involving the atoms H, X, and E (the (H, X, E) system) in which X=N–Bi and E=C–Pb has been explored at the CCSD(T)/TZVPP and BP86/TZ2P+ levels of theory. The nature of the X? E bonding has been analyzed with charge‐ and energy‐partitioning methods. The calculations show that the linear isomers of the nitrogen systems lin ‐HEN and lin ‐HNE are minima on the singlet PES. The carbon compound lin ‐HCN (HCN=hydrogen cyanide) is 14.9 kcal mol^?1 lower in energy than lin ‐HNC but the heavier group 14 homologues lin ‐HEN (E=Si–Pb) are between 64.8 and 71.5 kcal mol^?1 less stable than the lin ‐HNE isomers. The phosphorous system (H, P, E) exhibits significant differences concerning the geometry and stability of the equilibrium structures compared with the nitrogen system. The linear form lin ‐HEP of the former system is much more stable than lin ‐HPE . The molecule lin ‐HCP is the only minimum on the singlet PES. It is 78.5 kcal mol^?1 lower in energy than lin ‐HPC , which is a second‐order saddle point. The heavier homologues lin ‐HPE , in which E=Si–Pb, are also second‐order saddle points, whereas the bent ‐HPE structures are the global minima on the PES. They are between 10.3 (E=Si) and 36.5 kcal mol^?1 (E=Pb) lower in energy than lin ‐HEP . The bent ‐HPE structures possess rather acute bending angles H‐P‐E between 60.1 (E=Si) and 79.7° (E=Pb). The energy differences between the heavier group 15 isomers lin ‐HEX (X=P–Bi) and the bent structures bent ‐HXE become continuously smaller. The silicon species lin ‐HSiBi is even 3.1 kcal mol^?1 lower in energy than bent ‐HBiSi . The bending angle H‐X‐E becomes more acute when X becomes heavier. The drastic energy differences between the isomers of the system (H, X, E) are explained with three factors that determine the relative stabilities of the energy minima: 1) The different bond strength between the hydrogen bonds H? X and H? E. 2) The electronic excitation energy of the fragment HE from the X ²Π ground state to the ⁴Σ^? excited state, which is required to establish a E≡X triple bond in the molecules lin ‐HEX . 3) The strength of the intrinsic X? E interactions in the molecules. The trends of the geometries and relative energies of the linear, bent, and cyclic isomers are explained with an energy‐decomposition analysis that provides deep insight into the nature of the bonding situation.     

Oxide ion conductivity and relaxation in CaREZrNbO7 (RE = La, Nd, Sm, Gd, and Y) system

CaREZrNbO₇ (RE = La, Nd, Sm, Gd and Y) system changed from fluorite (F)-type to pyrochlore (P)-type structure when the ionic radius ratios, r(Ca²⁺–RE³⁺)_av/r(Zr⁴⁺–Nb⁵⁺)_av were larger than 1.34. Thus, the La, Nd, and Sm compounds have a cubic P-type structure and the Gd and Y ones have a defect F-type structure. The electrical conductivity was measured using complex-plane impedance analysis over a wide temperature (300–750 °C) and frequency (1 Hz–1 MHz) ranges. The conductivity relaxation phenomenon was observed in these compounds and the relaxation frequencies were found to show Arrhenius-type behavior and activation energies were in good agreement with those obtained from high temperature conductivity plots. These results support the idea that the relaxation process and the conductivity have the same origin. The ionic conductivity of CaREZrNbO₇ (RE = La, Nd, Sm, Gd and Y) system showed the maximum at the phase boundary between the F-type and P-type phases. On the other hand, the activation energy for the conduction decreased in the F-type phase and increased in the P-type phase with increasing ionic radius ratio. Among the prepared compounds, CaGdZrNbO₇ showed the highest ionic conductivity of 9.47 × 10^− 3 S/cm at 750 °C which was about twice as high as that observed in Gd₂Zr₂O₇ (4.2 × 10^− 3 S/cm at 800 °C). The grain morphology observation by scanning electron microscope (SEM) showed well-sintered grains. AC impedance measurements in various atmospheres further indicated that they are predominantly oxide ion conductors at elevated temperatures (> 700 °C).     

Order–disorder phase transformations in quaternary pyrochlore oxide system: Investigated by X-ray diffraction, transmission electron microscopy and Raman spectroscopic techniques

Order–disorder transformations in a quaternary pyrochlore oxide system, Ca–Y–Zr–Ta–O, were studied by powder X-ray diffraction (XRD) method, transmission electron microscope (TEM) and FT-NIR Raman spectroscopic techniques. The solid solutions in different ratios, 4:1, 2:1, 1:1, 1:2, 1:4, 1:6, of CaTaO_3.5 and YZrO_3.5 were prepared by the conventional high temperature ceramic route. The XRD results and Rietveld analysis revealed that the crystal structure changed from an ordered pyrochlore structure to a disordered defect fluorite structure as the ratios of the solid solutions of CaTaO_3.5 and YZrO_3.5 were changed from 4:1 to 1:4. This structural transformation in the present system is attributed to the lowering of the average cation radius ratio, r_A/r_B as a result of progressive and simultaneous substitution of larger cation Ca²⁺ for Y³⁺ at A sites and smaller cation Ta⁵⁺ for Zr⁴⁺ at B sites. Raman spectroscopy and TEM analysis corroborated the XRD results.     

Insight into the Excitation-Dependent Fluorescence of Carbon Dots

High quantum yield, photoluminescence tunability, and sensitivity to the environment are a few distinct trademarks that make carbon nanodots (CDs) interesting for fundamental research, with potential to replace the prevalent inorganic semiconductor quantum dots. Currently, application and fundamental understanding of CDs are constrained because it is difficult to make a quantitative comparison among different types of CDs simply because their photoluminescence properties are directly linked to their size distribution, the surface functionalization, the carbon core structures (graphitic or amorphous) and the number of defects. Herein, we report a facile one-step synthesis of mono-dispersed and highly fluorescent nanometre size CDs from a ‘family’ of glucose-based sugars. These CDs are stable in aqueous solutions with photoluminescence in the visible range. Our results show several common features in the family of CDs synthesized in that the fluorescence, in the visible region, is due to a weak absorption in the 300–400 nm from a heterogeneous population of fluorophores. Fluorescence quenching experiments suggest the existence of not only surface-exposed fluorophores but more importantly solvent inaccessible fluorophores present within the core of CDs. Interestingly, time-resolved fluorescence anisotropy experiments directly suggest that a fast exchange of excitation energy occurs that results in a homo-FRET based depolarization within 150 ps of excitation.     

Smart optical probe for ‘equipment-free’ detection of oxalate in biological fluids and plant-derived food items

A reversible ‘turn-on’ sensor has been designed for ‘naked-eye’ detection of oxalate at nanomolar concentration (～12.5?nM) at pH 7.4. The sensory system shows a highly specific response towards oxalate among a wide range of antinutrients and biologically relevant anionic species. Mechanistic investigations indicate that oxalate can turn the pink-colored solution colorless by dissociating the preformed metal complex. Additionally, high specificity and good accuracy with recovery values ranging from 93.3 to 105.0% were obtained during oxalate estimation in spiked water and human urine samples, confirming the suitability of the present method in estimating trace-level of oxalate in complex matrices. With these results, quantitative estimations of endogenous oxalate were achieved in more than twenty-five different agricultural crops. Finally, low-cost, portable paper strips were developed for on-site detection of oxalate.