Strong effect of methyl group on the strength of ionic hydrogen bond between C2H2 and H3O+

The effect of methyl group on the strength of the ionic hydrogen bond between C₂H₂ and H₃O⁺ has been studied with quantum chemical calculations at the UMP2/6‐311++G(d,p) level. The presence of a methyl group in the proton acceptor results in an energetic increase of 6.02 kcal/mol, increased by about 39%, whereas that in the proton donor leads to an energetic decrease of 2.18 kcal/mol, decreased by 14%. The charge analyses indicate that the methyl group in the proton acceptor is electron‐donating and that in the proton donor is electron‐withdrawing. The former plays a positive contribution to the formation of ionic hydrogen bond and the latter plays a negative contribution to the formation of ionic hydrogen bond. The weakening effect of solvent on the role of methyl group in the ionic hydrogen bond has also been studied at the UB3LYP/6‐311++G(d,p) level. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

Nonadditivity of methyl group in single‐electron hydrogen bond of methyl radical‐water complex

The nonadditivity of methyl group in the single‐electron hydrogen bond of the methyl radical‐water complex has been studied with quantum chemical calculations at the UMP2/6‐311++G(2df,2p) level. The bond lengths and interaction energies have been calculated in the four complexes: CH₃? H₂O, CH₃CH₂? H₂O, (CH₃)₂CH? H₂O, and (CH₃)₃C? H₂O. With regard to the radicals, tert‐butyl radical forms the strongest hydrogen bond, followed by iso‐propyl radical and then ethyl radical; methyl radical forms the weakest hydrogen bond. These properties exhibit an indication of nonadditivity of the methyl group in the single‐electron hydrogen bond. The degree of nonadditivity of the methyl group is generally proportional to the number of methyl group in the radical. The shortening of the C···H distance and increase of the binding energy in the (CH₃)₂CH? H₂O and (CH₃)₃C? H₂O complexes are less two and three times as much as those in the CH₃CH₂? H₂O complex, respectively. The result suggests that the nonadditivity among methyl groups is negative. Natural bond orbital (NBO) and atom in molecules (AIM) analyses also support such conclusions. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

On the reactions of methyl radicals with nitrilotris(methylenephosphonic-acid) complexes in aqueous solutions

Abstract

The reactions of methyl radicals with M^II-(ntp)(H₂O)₂ complexes were studied. The formation of an unstable intermediate (ntp)(H₂O)M^III-CH₃ is observed. This reaction is an equilibrium process, i.e. the M^III-C bond decomposes via homolysis. The (ntp)(H₂O)M^III-CH₃ complexes isomerize to a more stable form. The results compared to those obtained in analogous reactions of the M^II-(nta)(H₂O)₂ complexes were shown to be similar with one exception – for the nta complexes no isomerization process is observed.     

H2O3 as a reactive oxygen species: formation of 8-oxoguanine from its reaction with guanine

Reaction of guanine with H2O3 in the absence and presence of a water molecule leading to the formation of 8-oxoguanine (8-oxoG) was investigated. Initial calculations were performed using imidazole (Im) as a model for the five-membered ring of guanine. The reactant, intermediate, and product complexes as well as transition states were obtained in gas phase at the B3LYP/6-31+G* and B3LYP/AUG-cc-pVDZ levels of theory. In all the cases, except for the reactions involving imidazole, single-point energy calculations were performed in gas phase at the MP2/AUG-cc-pVDZ level of theory. Solvation calculations in aqueous media were carried out using the polarizable continuum model (PCM) of the self-consistent reaction field (SCRF) theory. Vibrational frequency analysis and intrinsic reaction coordinate (IRC) calculations were performed to ensure that the transition states connected the reactant and product complexes properly. Zero-point energy (ZPE)-corrected total energies and Gibbs free energies at 298.15 K in gas phase and aqueous media were obtained. When a reaction of H2O3 in place of H2O2 with guanine is considered, the major barrier energy which is encountered at the first step is almost halved showing that H2O3 would be much more reactive than H2O2. Considering the reaction schemes investigated here and the observed fact that H2O3 is dissociated easily under ambient conditions, it appears that H2O3 would serve as an effective reactive oxygen species.     

ESI‐MS/MS analysis of protonated N‐methyl amino acids and their immonium ions

Methylation is one of the important posttranslational modifications of biological systems. At the metabolite level, the methylation process is expected to convert bioactive compounds such as amino acids, fatty acids, lipids, sugars, and other organic acids into their methylated forms. A few of the methylated amino acids are identified and have been proved as potential biomarkers for several metabolic disorders by using mass spectrometry–based metabolomics workstation. As it is possible to encounter all the N‐methyl forms of the proteinogenic amino acids in plant/biological systems, it is essential to have analytical data of all N‐methyl amino acids for their detection and identification. In earlier studies, we have reported the ESI‐MS/MS data of all methylated proteinogenic amino acids, except that of mono‐N‐methyl amino acids. In this study, the N‐methyl amino acids of all the amino acids ( 1 ‐ 21 ; including one isomeric pair) were synthesized and characterized by ESI‐MS/MS, LC/MS/MS, and HRMS. These data could be useful for detection and identification of N‐methyl amino acids in biological systems for future metabolomics studies. The MS/MS spectra of [M + H]⁺ ions of most N‐methyl amino acids showed respective immonium ions by the loss of (H₂O, CO). The other most common product ions detected were [MH‐(NH₂CH₃]⁺, [MH‐(RH)]⁺ (where R = side chain group) ions, and the selective structure indicative product ions due to side chain and N‐methyl group. The isomeric/isobaric N‐methyl amino acids could easily be differentiated by their distinct MS/MS spectra. Further, the MS/MS of immonium ions inferred side chain structure and methyl group on α‐nitrogen of the N‐methyl amino acids.     

Theoretical and experimental investigations of complexation with BF3.Et2O effects on electronic structures,energies and photophysical properties of Anil and tetraphenyl (hydroxyl) imidazol

The novel compounds (E)‐2‐(((4‐hydroxyphenyl)imino)methyl)phenol, Tetraphenyl (hydroxyl) imidazole and their corresponding Boron difluoride complexes were synthesized and characterized by spectroscopic techniques. Density functional theory calculations at B3LYP‐D3/6–311++G (d, p) level of theory were performed for the geometric parameters. The MEP surface studies were used to understand the behavior of molecules in terms of charge transfer and to determine how these molecules interact. We used the GIAO and the B3LYP‐D3 with a 6–311++ G (d, p) basis set to simulate the (¹H‐NMR and ¹⁹F‐NMR) and the IR spectra, respectively. The corresponding calculated results are in good agreement with the experimental data. The stability of the molecule arising from hyperconjugation interaction and charge delocalization were analyzed using NBO analysis. FMOs revealed the occurrence of charge transfer within the molecule. The complexation using BF₃.Et₂O was also found to have remarkable effects on the electrochemical properties of the studied molecules, where (b) and (d) present lower chemical stability, higher reactivity and higher polarizability than (a) and (c), respectively. Moreover, the energy gap of (a) and (c) decreased after complexation using BF₃.Et₂O, indicating the reliability of the electrochemical evaluation of LUMO and HOMO energy levels. These values are the factors explaining the possible charge transfer interaction within the molecule. The absorption and emission spectra of the model compound were also simulated and compared to experimental observations in the DMF solvent. The results of DFT calculations supported the structural and spectroscopic data and confirmed the structure modification of frontier molecular orbitals for BF₂ complexes as well as tunable potentials and energy levels.     

Synthesis of 2‐(N‐arylimino)pyrrolide nickel complexes and polymerization of methyl methacrylate

The synthesis, characterization and methyl methacrylate polymerization behaviors of 2‐(N‐arylimino)pyrrolide nickel complexes are described. The nickel complex [NN]₂Ni ( 1 , [NN] = [2‐C(H)NAr‐5‐tBu‐C₄H₂N]^?, Ar = 2,6‐iPr₂C₆H₃) was prepared in good yield by the reaction of [NN]Li with trans‐[Ni(Cl)(Ph)(PPh₃)₂] in THF. Reaction of [NN]Li with NiBr₂(DME) yielded the nickel bromide [NN]Ni(Br)[NNH] ( 2 ). Complexes 1 and 2 were characterized by ¹H NMR and IR spectroscopy and elemental analysis, and by X‐ray single crystal analysis. Both complexes, upon activation with methylaluminoxane, are highly active for the polymerization of methyl methacrylate to give high molecular weight polymethylmethacrylate with narrow molecular distributions. Copyright © 2009 John Wiley & Sons, Ltd.     

Unimolecular gas-phase reactions of the methyl acetoacetate radical cation. Oxygen atom permutation via ion-molecule complexes

The reactions of metastable decomposing methyl acetoacetate (a mixture of keto a ad enol tautomers) are reported and discussed. The unimolecular fragmentations of the tautomers are different. The metastable decomposing radical cation of the keto form displays four specific ions: [M –CO]⁺˙, [M – CH₂O]⁺˙, [M – CH₂CO]⁺˙ and m/z 43. The results derived from D-, ¹³C- and ¹⁸O-labelled precursors together with thermochemical data have been used to study the mechanisms. Experimental results indicate that an unexpected isomerization occurs before dissociation. It formally corresponds to oxygen atom permutation of the two carbonyl groups without participation of the carbon atoms. This remarkable process is interpreted in terms of a mechanism involving ion-molecule complexes.     

A charge‐transfer salt composed of methyl viologen and hexacyanidoferrate(II)

The methyl viologen dication, used under the name Paraquat as an agricultural reagent, is a well‐known electron‐acceptor species that can participate in charge‐transfer (CT) interactions. The determination of the crystal structure of this species is important for accessing the CT interaction and CT‐based properties. The title hydrated salt, bis(1,1′‐dimethyl‐4,4′‐bipyridine‐1,1′‐diium) hexacyanidoferrate(II) octahydrate, (C₁₂H₁₄N₂)₂[Fe(CN)₆]·8H₂O or (MV)₂[Fe(CN)₆]·8H₂O [MV²⁺ is the 1,1′‐dimethyl‐4,4′‐bipyridine‐1,1′‐diium (methyl viologen) dication], crystallizes in the space group P 2₁/c with one MV²⁺ cation, half of an [Fe(CN)₆]⁴⁻ anion and four water molecules in the asymmetric unit. The Fe^II atom of the [Fe(CN)₆]⁴⁻ anion lies on an inversion centre and has an octahedral coordination sphere defined by six cyanide ligands. The MV²⁺ cation is located on a general position and adopts a noncoplanar structure, with a dihedral angle of 40.32 (7)° between the planes of the pyridine rings. In the crystal, layers of electron‐donor [Fe(CN)₆]⁴⁻ anions and layers of electron‐acceptor MV²⁺ cations are formed and are stacked in an alternating manner parallel to the direction of the −2a + c axis, resulting in an alternate layered structure.     

Mechanistic insight on (E)‐methyl 3‐(2‐aminophenyl)acrylate cyclization reaction by multicatalysis of solvent and substrate

The reaction mechanism of (E)‐methyl 3‐(2‐aminophenyl)acrylate ( A ) with phenylisothiocyanate ( B ) as well as the vital roles of substrate A and solvent water were investigated under unassisted, water‐assisted, substrate A ‐assisted, and water‐ A ‐assisted conditions. The reaction proceeds with four processes via nucleophilic addition, deprotonation and protonation, intramolecular cyclization with hydrogen transfer, and keto–enol tautomerization. According to the different H‐shift mode, two possible types of H‐shift P1 and P2 are carefully investigated to identify the most preferred pathway, differing in the ? NH₂ group deprotonation and ? CH group of A protonation processes. It is found that substrate A and water not only act as reactant and solvent, but also as catalyst, proton shuttle, and stabilizer in effectively lowering the energy barrier. Therefore, the results demonstrate that the strong donating and accepting ability of ? NH₂ group on A and the presence of bulk water are the keys to the title reaction proceed. © 2016 Wiley Periodicals, Inc.     

Cu (II) tyrosinate complexes containing methyl substituted phenanthrolines: Synthesis,X‐ray crystal structures,biomolecular interactions,antioxidant activity,ROS generation and cytotoxicity

In this paper, three new copper (II) complexes, [Cu(4‐mphen)(tyr)(H₂O)]ClO₄ (1) , [Cu(5‐mphen)(tyr)(H₂O)]ClO₄·1.5H₂O (2) and [Cu (tmphen)(tyr)(NO₃)]0.5H₂O (3) (4‐mphen: 4‐methyl‐1,10‐phenanthroline, 5‐mphen: 5‐methyl‐1,10‐phenanthroline, tmphen: 3,4,7,8‐tetramethyl‐1,10‐phenanthroline and tyr: L‐tyrosine), were synthesized and characterized using elemental analyses, FT‐IR, ESI‐MS, cyclic voltammetry and single‐crystal X‐ray diffraction. It was found that the complexes adopt a distorted five‐coordinate square pyramidal geometry. The interaction of the three complexes with calf thymus DNA was also investigated using UV–visible absorption spectra, ethidium bromide and Hoechst 33258 displacement assay and thermal denaturation. The DNA cleavage activity of the complexes, monitored using gel electrophoresis, showed significant damage of the pUC19 plasmid DNA. Binding activity of bovine serum albumin (BSA) reveals that these complexes can strongly quench the fluorescence of BSA through a static quenching mechanism. The results suggested that interaction of the complexes with DNA occurred through a partial intercalation into the minor grooves of DNA. In addition, interaction of the complexes with bovine serum albumin quenched the fluorescence emission of the tryptophan residues of the protein binding constants and thermodynamic parameters were obtained from the fluorescence quenching experiments at different temperatures. Free radical scavenging activities of the complexes were determined by various in vitro assays such as 1,1‐diphenyl‐2‐picryl‐hydrazyl free radicals (DPPH˙) and H₂O₂ scavenging methods. In addition, the cytotoxicity of these complexes in vitro on tumor cell lines (Caco‐2 and MCF‐7) was examined by XTT and showed better antitumor effect on the tested cells. ROS (reactive oxygen species) and comet experiments are consistent with each other and these complexes lead to DNA damage via the production of ROS. The effect of the hydrophobic properties of the synthesized complexes on DNA and BSA binding activities were discussed.     

Anionic polymerization of methyl methacrylate initiated with late transition‐metal halides/organolithium/triisobutylaluminum systems

Anionic polymerization of methyl methacrylate (MMA) initiated with late transition‐metal halides [manganese chloride (MnCl₂), iron dichloride (FeCl₂), iron trichloride (FeCl₃), cobalt chloride (CoCl₂), or nickel bromide (NiBr₂)]/organolithium [nButyllithium (nBuLi) or phenyllithium (PhLi)]/triisobutylaluminum (iBu₃Al) systems is described. Except for the system with NiBr₂, the polymerizations of MMA afforded narrow molecular weight distribution poly(methyl methacrylate)s (PMMAs) with high molecular weights in quantitative yields at 0 °C in toluene. Matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry (MALDI‐TOF MS) analyses of the PMMAs obtained by the systems with FeCl₂, FeCl₃, and CoCl₂ revealed that the polymers had hydrogen (H) at both chain ends. Accordingly, the reaction of the transition‐metal halides with the organolithium in the presence of iBu₃Al should result in the formation of transition‐metal hydride [M‐H]^? species, which was nucleophilic enough to initiate the MMA polymerization. Because the presence of a six‐membered cyclic structure resulting from backbiting was confirmed from the MALDI‐TOF MS analyses of the PMMA obtained with the metal halide (FeCl₂, FeCl₃, or CoCl₂)/organolithium systems in the absence of iBu₃Al, the introduction of H at the ω‐chain end indicated that iBu₃Al should prevent the backbiting. However, the MnCl₂/nBuLi/iBu₃Al initiating system gave PMMAs bearing H at the α chain end and six‐membered cyclic structure at the ω chain end. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 1962–1977, 2003     

Photoluminescence properties of a cationic trinuclear zinc(II) complex with the tetradentate Schiff base ligand 6‐methyl‐2‐({[(pyridin‐2‐yl)methyl]imino}methyl)phenolate

Metal complexes with Schiff base ligands have been suggested as potential phosphors in electroluminescent devices. In the title complex, tetrakis[6‐methyl‐2‐({[(pyridin‐2‐yl)methyl]imino}methyl)phenolato‐1:2κ⁸N,N′,O:O;3:2κ⁸N,N′,O:O]trizinc(II) hexafluoridophosphate methanol monosolvate, [Zn₃(C₁₄H₁₃N₂O)₄](PF₆)₂·CH₃OH, the Zn^II cations adopt both six‐ and four‐coordinate geometries involving the N and O atoms of tetradentate 6‐methyl‐2‐({[(pyridin‐2‐yl)methyl]imino}methyl)phenolate ligands. Two terminal Zn^II cations adopt distorted octahedral geometries and the central Zn^II cation adopts a distorted tetrahedral geometry. The O atoms of the phenolate ligands bridge three Zn^II cations, forming a dicationic trinuclear metal cluster. The title complex exhibits a strong emission at 469 nm with a quantum yield of 15.5%.     

High polymerization of methyl methacrylate with organonickel/MMAO systems

A series of nickel complexes, including Ni(acac)₂, (C₅H₅)Ni(η³‐allyl), and [NiMe₄Li₂(THF)₂]₂, that were activated with modified methylaluminoxane (MMAO) exhibited high catalytic activity for the polymerization of methyl methacrylate (MMA) but showed no catalytic activity for the polymerization of ethylene and 1‐olefins. The resulting polymers exhibited rather broad molecular weight distributions and low syndiotacticities. In contrast to these initiators, the metallocene complexes (C₅H₅)₂Ni, (C₅Me₅)₂Ni, (Ind)₂Ni, and (Me₃SiC₅H₄)₂Ni provided narrower molecular weight distributions at 60 °C when these initiator were activated with MMAO. Half‐metallocene complexes such as (C₅H₅)NiCl(PPh₃), (C₅Me₅)NiCl(PPh₃), and (Ind)NiCl(PPh₃) produced poly(methyl methacrylate) (PMMA) with much narrower molecular weight distributions when the polymerization was carried out at 0 °C. Ni[1,3‐(CF₃)₂‐acac]₂ generated PMMA with high syndiotacticity. The NiR(acac)(PPh₃) complexes (R = Me or Et) revealed high selectivity in the polymerization of isoprene that produced 1,2‐/3,4‐polymer at 0 °C exclusively, whereas the polymerization at 60 °C resulted in the formation of cis‐1,4‐rich polymers. The polymerization of ethylene with Ni(1,3‐tBu₂‐acac)₂ and Ni[1,3‐(CF₃)₂‐acac]₂ generated oligo‐ethylene with moderate catalytic activity, whereas the reaction of ethylene with Ni(acac)₂/MMAO produced high molecular weight polyethylene. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 4764–4775, 2000     

QM/MM calculation of solvent effects on absorption spectra of guanine

Electronic spectra of guanine in the gas phase and in water were studied by quantum mechanical/molecular mechanical (QM/MM) methods. Geometries for the excited‐state calculations were extracted from ground‐state molecular dynamics (MD) simulations using the self‐consistent‐charge density functional tight binding (SCC‐DFTB) method for the QM region and the TIP3P force field for the water environment. Theoretical absorption spectra were generated from excitation energies and oscillator strengths calculated for 50 to 500 MD snapshots of guanine in the gas phase (QM) and in solution (QM/MM). The excited‐state calculations used time‐dependent density functional theory (TDDFT) and the DFT‐based multireference configuration interaction (DFT/MRCI) method of Grimme and Waletzke, in combination with two basis sets. Our investigation covered keto‐N7H and keto‐N9H guanine, with particular focus on solvent effects in the low‐energy spectrum of the keto‐N9H tautomer. When compared with the vertical excitation energies of gas‐phase guanine at the optimized DFT (B3LYP/TZVP) geometry, the maxima in the computed solution spectra are shifted by several tenths of an eV. Three effects contribute: the use of SCC‐DFTB‐based rather than B3LYP‐based geometries in the MD snapshots (red shift of ca. 0.1 eV), explicit inclusion of nuclear motion through the MD snapshots (red shift of ca. 0.1 eV), and intrinsic solvent effects (differences in the absorption maxima in the computed gas‐phase and solution spectra, typically ca. 0.1–0.3 eV). A detailed analysis of the results indicates that the intrinsic solvent effects arise both from solvent‐induced structural changes and from electrostatic solute–solvent interactions, the latter being dominant. © 2009 Wiley Periodicals, Inc. J Comput Chem 2010     

Simulate methylation reaction of arsenic(III) with methyl iodide in an aquatic system

The methylation reaction of inorganic arsenic occurring in aquatic systems was studied by HPLC‐HG‐AFS as a method to separate and detect soluble methylarsenic species. Transformation from inorganic arsenic to methylarsenic was essential for major changes in toxicity to organisms. Monomethylarsenic [AsOCH₃(OH)₂] was the only product in the methylation reaction of inorganic arsenic(III) with methyl iodide (MeI). This process can be described as an oxidative carbonium‐ion transfer, with MeI acting as a methyl donor. From a thermodynamic point of view, the activity of the carbonium ion and pH were the two major influencing factors. The pH dependence of redox potential of As(III) was the reason for the effect of pH on methylation of arsenic. The influences of salinity and concentration of the methyl donor may be explained by their effects on the activity of carbonium. Moreover, kinetics experiments demonstrated that the methylation reaction was first‐order for both As(III) and methyl iodide. First‐order reaction rates were also calculated at different pH, salinity and MeI, and were found to be in the range 0.0026–0.0123 h^?1. The methylation rate varied largely under different reaction conditions. Copyright © 2005 John Wiley & Sons, Ltd.     

An Evaluation of Ligand Properties of Neutral and Anionic Tris(imidazol‐2‐yl)phosphines

In order to study the applicability of tris(imidazol‐2‐yl)phosphine (PIm₃) as a possible charge‐variable ligand, new neutral N‐butyl and N‐benzyl derivatives and d⁰‐metal complexes thereof were prepared and characterized as reference compounds for planned complexes with high valent metals. In addition, an anionic ligand precursor was characterized by X‐Ray analysis and its reactivity towards transition metal halides assayed.     

金属离子(Na~+、K~+、Ca~(2+)、Mg~(2+)、Zn~(2+))与鸟嘌呤异构体配合物的稳定性

在B3LYP/6-311++G**水平上用极化连续介质模型(PCM)系统研究了金属离子(M+/2+=Na+,K+,Ca2+,Mg2+,Zn2+)和十三种鸟嘌呤异构体形成的配合物GnxM+/2+(n为鸟嘌呤异构体的编号,x表示M+/2+与鸟嘌呤异构体的结合位点)在气(g)液(a)两相中的稳定性顺序.着重探讨了液相中配合物的稳定性差异,并且从溶质-溶剂效应、结合能、形变能及异构体的相对能量等几个方面分析了造成稳定顺序发生变化的原因.报道了溶液中这五种金属离子与鸟嘌呤异构体结合形成的六种基态配合物:aG1N2,N3Na+,aG1N2,N3K+,aG1O6,N7Ca2+,aG1N2,N3Mg2+(aG1O6,N7Mg2+),aG2N3,N9Zn2+.可以看出,除了在Zn2+配合物中鸟嘌呤异构体为G2外,构成其余四种金属离子配合物的鸟嘌呤异构体都是G1,但结合位点不同.同时对气相中各类配合物稳定性也进行了系统的排序,并报道了几种较稳定的配合物,如:gG3N1,O6K+,gG5N1,O6K+,gG3N1,O6Ca2+/Mg2+,gG4O6,N7Ca2+/Mg2+.     

Two nickel(II) bis[(pyridin‐2‐yl)methyl]amine complexes with homophthalic and benzene‐1,2,4,5‐tetracarboxylic acids

Two new Ni^II complexes involving the ancillary ligand bis[(pyridin‐2‐yl)methyl]amine (bpma) and two different carboxylate ligands, i.e. homophthalate [hph; systematic name: 2‐(2‐carboxylatophenyl)acetate] and benzene‐1,2,4,5‐tetracarboxylate (btc), namely catena‐poly[[aqua{bis[(pyridin‐2‐yl)methyl]amine‐κ³N,N′,N′′}nickel(II)]‐μ‐2‐(2‐carboxylatophenyl)aceteto‐κ²O:O′], [Ni(C₉H₆O₄)(C₁₂H₁₃N₃)(H₂O)]_n, and (μ‐benzene‐1,2,4,5‐tetracarboxylato‐κ⁴O¹,O²:O⁴,O⁵)bis(aqua{bis[(pyridin‐2‐yl)methyl]amine‐κ³N,N′,N′′}nickel(II)) bis(triaqua{bis[(pyridin‐2‐yl)methyl]amine‐κ³N,N′,N′′}nickel(II)) benzene‐1,2,4,5‐tetracarboxylate hexahydrate, [Ni₂(C₁₀H₂O₈)(C₁₂H₁₃N₃)₂(H₂O)₂]·[Ni(C₁₂H₁₃N₃)(H₂O)₃]₂(C₁₀H₂O₈)·6H₂O, (II), are presented. Compound (I) is a one‐dimensional polymer with hph acting as a bridging ligand and with the chains linked by weak C—H...O interactions. The structure of compound (II) is much more complex, with two independent Ni^II centres having different environments, one of them as part of centrosymmetric [Ni(bpma)(H₂O)]₂(btc) dinuclear complexes and the other in mononuclear [Ni(bpma)(H₂O)₃]²⁺ cations which (in a 2:1 ratio) provide charge balance for btc⁴⁻ anions. A profuse hydrogen‐bonding scheme, where both coordinated and crystal water molecules play a crucial role, provides the supramolecular linkage of the different groups.     

Intermolecular interactions in uracil–nitrous acid complexes: structures,binding energy,topological properties,and nuclear magnetic resonance study

Molecular interactions between uracil and nitrous acid (U–NA) [C₄N₂O₂H₄? NO₂H] have been studied using B3LYP, B3PW91, and MP2 methods with different basis sets. The optimized geometries, harmonic vibrational frequencies, charge transfer, topological properties of electron density, nucleus‐independent chemical shift (NICS), and nuclear magnetic resonance one‐ and two‐bonds spin–spin coupling constants were calculated for U–NA complexes. In interaction between U and NA, eight cyclic complexes were obtained with two intermolecular hydrogen bonds N(C)HU…N(O) and OH_NA…O_U. In these complexes, uracil (U) simultaneously acts as proton acceptor and proton donor. The most stable complexes labeled, UNA1 and UNA2, are formed via NH bond of U with highest acidity and CO group of U with lowest proton affinity. There is a relationship between hydrogen bond distances and the corresponding frequency shifts. The solvent effect on complexes stability was examined using B3LYP method with the aug‐cc‐pVDZ basis set by applying the polarizable continuum model (PCM). The binding energies in the gas phase have also been compared with solvation energies computed using the PCM. Natural bond orbital analysis shows that in all complexes, the charge transfer takes place from U to NA. The results predict that the Lone Pair (LP)(O)_U → σ*(O? H) and LP(N(O)_NA → σ*(N(C)? H)_U donor–acceptor interactions are most important interactions in these complexes. Atom in molecule analysis confirms that hydrogen bond contacts are electrostatic in nature and covalent nature of proton donor groups decreases upon complexation. The relationship between spin–spin coupling constant (^1hJ_H…_Y and ^2hJ_H…_Y) with interaction energy and electronic density at corresponding hydrogen bond critical points and H‐bonds distances are investigated. NICS used for indicating of aromaticity of U ring upon complexation. © 2013 Wiley Periodicals, Inc.