Molecular structure and relative stability of trans and cis isomers of formanilide: gas-phase electron diffraction and quantum chemical studies

Molecular structure of formanilide is determined by gas-phase electron diffraction (GED) augmented by quantum chemical calculations (B3LYP/cc-pVTZ and MP2/cc-pVTZ) and literature microwave (MW) data. The combined GED and MW data are well reproduced for the mixture of trans and cis isomers with the relative abundance of 59 ± 5 and 41 ± 5 %, respectively, at T = 410 K. The trans isomer (C _s symmetry) is planar, while the cis isomer (C ₁ symmetry) has the twisted structure with the amide group rotated by 36.7 ± 2.7° with respect to the phenyl ring. In accord with theoretical calculations, the amide bond –NH–C(O)– is planar in trans formanilide and a somewhat nonplanar in cis isomer. Accurate structural parameters were obtained by a simultaneous fit of the rotational constants reported in the literature and GED intensities obtained in this study. The N–C(O) and N–C_Ph bond dissociation energies in formanilide are calculated using Gaussian-4 method. It is revealed that the strength of N–C(O) bond in formanilide is 50 kJ/mol less than that in benzamide. On the contrary, the strength of adjacent bond (N–C_Ph) increases by 35 kJ/mol compared to aniline. It is rather unexpectedly that the bond strength weakening does not result in the bond elongating, and vice versa.     

Predicting cis/trans structure of alkene based on infrared spectra.

The application of chemometrics to analyze the information of the cis/trans structure of alkenes in infrared spectra (IR) is introduced. For data from the OMNIC IR spectral database, two feature selection methods, Fisher ratios and genetic algorithm-partial least squares (GA-PLS), and two classification methods, support vector machine (SVM) and probabilistic neural network (PNN), have been used to obtain optimization classifiers. At last, some spectra from other IR databases are used to evaluate the optimization classifiers. It has been demonstrated that both the SVM and PNN optimization classifiers could give preferable predictive results about the cis and trans structures of alkene.     

Synthesis,crystal structure,and properties of a tetrairon cluster based on 2-methyl-8-hydroxyquinoline

The synthesis, structure, and magnetic properties of a tetrairon(III) complex [Fe₄(μ₂-O)₂Cl₄(L)₄]·0.5H₂O (1, L = 2-methyl-8-hydroxyquinoline), which was prepared under solvothermal conditions and characterized by IR spectra, elemental analysis, TG, and single-crystal X-ray crystal diffraction, are presented. Each Fe center adopts a distorted trigonal bipyramid geometry and is coordinated with three oxygen atoms, one nitrogen atom, and one chlorine atom. Each 2-methyl-8-hydroxyquinoline is coordinated to two Fe^III ions with μ₂-η²:η¹ coordination mode. The magnetic properties of the complex are dominated by antiferromagnetic exchange interactions.     

Theoretical studies on excited state proton transfer tautomerism reaction and spectroscopic properties of 8-hydroxyquinoline monomers and dimers

By means of ab initio HF methods, the ground state structures of 8-hydroxyquinoline (8-HQ) monomers and dimers were optimized using the 6-311+g* and 6-31G basis sets, respectively. The lowest singlet excited states of 8-HQ monomers and dimers have been studied by the single-excitation configuration interaction (CIS) approach at the same level. In the studies of the potential energy surface, it was found that all the stable configurations corresponded to enol form. The UV-vis and fluorescence spectra of 8-HQ monomers and dimers under a solvent effect condition were also calculated using the TD-B3LYP/6-31+G* method based on the HF- and CIS-optimized geometries. The computed absorption and fluorescence spectral characteristics for monomers and dimers were in good agreement with previously reported experimental values. The results also show that 8-HQ has very poor fluorescence in solvents.     

Conformational analysis of the complexes of copper(ii) with two identical α-amino acid ligands,relative stability of the cis and trans isomers

Conclusions The energy closeness of the cis and trans isomers of the (Ala)₂Cu complexes, and consequently the possibility of the presence of the cis isomers in the equilibrium mixture, was shown. The differences in the energies of the cis and trans isomers increase in the complexes with the N-mono- and N,N-dimethyl-substituted alanine. The coexistence of both isomers is thereby possible for the monosubstituted compounds, but the disubstituted cis isomers are virtually unrealized. According to the conformational calculations, the formation of the optically active compounds containing ligands of one configurational series is more preferred among the complexes considered.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 11, pp. 2484–2487, November, 1987.     

Synthesis,structure, and photoluminescence properties of Zn(II) complexes with 2-styryl derivatives of 8-hydroxyquinoline

Novel Zn(II) complexes with 2-styryl-8-hydroxyquinoline were synthesized and characterized by ¹H, ¹⁹F, and ¹³C NMR spectroscopy and X-ray diffraction analysis. Photoluminescence properties of the ligands and complexes in solutions were studied. The effect of substituents in the quinoline and phenylethenyl fragments on spectral characteristics was discussed.     

Electronic structure and relative stability of 1:1 Cu-O2 adducts from difference-dedicated configuration interaction calculations

A computational strategy to analyze Cu-O(2) adducts based on the use of difference-dedicated configuration interaction (DDCI) calculations is presented. The electronic structure, vertical gaps and nature of the metal-O(2) interaction, and the extension of the charge transfer between both fragments have been investigated. Relative stabilities between isomers are determined from triplet states CCSD(T) calculations. The key point of the here proposed strategy rests on the use of a rationally designed active space, containing only those orbitals, which optimize the interaction pathways between LCu and O(2) fragments. The procedure has been tested on a broad set of model and synthetic biomimetic systems, the results compared with previous theoretical evaluations and/or available experimental data. Our study indicates that this strategy can be considered as an alternative approach to multireference second-order perturbation theory methods to deal with this type of systems with remarkable biradical nature.     

First-principle computational study on the full conformational space of L-threonine diamide, the energetic stability of cis and trans isomers

First-principle computations were carried out on the conformational space of trans and cis peptide bond isomers of HCO-Thr-NH2. Using the concept of multidimensional conformational analysis (MDCA), geometry optimizations were performed at the B3LYP/6-31G(d) level of theory, and single-point energies as well as thermodynamic functions were calculated at the G3MP2B3 level of theory for the corresponding optimized structures. Two backbone Ramachandran-type potential energy surfaces (PESs) were computed, one each for the cis and trans isomers, keeping the side chain at the fully extended orientation (chi1=chi2=anti). Similarly, two side chain PESs for the cis and trans isomers were generated for the (phi=psi=anti) orientation corresponding to approximately the betaL backbone conformation. Besides correlating the relative Gibbs free energy of the various stable conformations with the number of stabilizing hydrogen bonds, the process of trans-->cis isomerization is discussed in terms of intrinsic stabilities as measured by the computed thermodynamic functions.     

Synthesis,crystal structures and spectroscopic studies on trans and cis isomers of Co(II) complexes with 1-benzyl-2-hydroxymethylimidazole

By carrying out the synthesis in a special way, two novel cobalt(II) isomers of trans(O)-[Co(1-Bz-2-CH₂OHIm)₄](NO₃)₂ (1) and cis(O)-[Co(1-Bz-2-CH₂OIm)₄](NO₃)₂·1.5H₂O (2) have been separated. The crystal structures of the Co(II) isomers show the triclinic space group P1̄ (1) and the monoclinic space group C2/c (2). The coordination geometry around the Co atom is approximately octahedral (1) or very distorted octahedral (2) and the Co(II) ions are surrounded by four nitrogen atoms of the four imidazole rings and two oxygen atoms of the hydroxymethyl group. Two of the ligands act as a monodentate and two as a bidentate, forming the five–membered chelate ring with the central ion. The structural data obtained for the Co(II) isomers were confirmed by IR and UV–Vis spectroscopic methods.     

Synthesis,crystal structure,and photophysical properties of a double open cubane-like Cd(II) complex based on 2-substituted-8-hydroxyquinoline

A 2-substituted-8-hydroxyquinoline (E)-2-[2-(3-thienyl)ethenyl]-8-quinolinol (HL) was synthesized and characterized by ESI-MS, NMR spectroscopy, and elemental analysis. Using solvothermal method, a tetranuclear complex [Cd₄L₆Br₂]·6DMF (1) was fabricated by assembly of Cd(II) with HL. X-ray structural analysis shows that 1 exhibits a double open cubane-like core structure, which is bridged by six 8-hydroxyquinolinate-based ligands. The supramolecular structure of 1 features a 3-D porous solid constructed by aromatic stacking interactions, C–H···π interactions and C–H···O hydrogen bonds. The assembly of cadmium salts and HL in solution was investigated by UV–vis and photoluminescence. We also studied the thermal stability and photophysical properties (fluorescent emission, lifetime, and quantum yield) of 1. The results show that 1 emits yellow luminescence in the solid state.     

Infrared laser spectroscopy of imidazole complexes in helium nanodroplets: monomer, dimer, and binary water complexes

Infrared laser spectroscopy has been used to characterize imidazole (IM), imidazole dimer (IMD), and imidazole-water (IMW) binary systems formed in helium nanodroplets. The experimental results are compared with ab initio calculations reported here. Vibrational transition moment angles provide conclusive assignments for the various complexes studied here, including IM, one isomer of IMD, and two isomers of the IMW binary complexes.     

Preparation and properties of an 8-hydroxyquinoline silica gel, synthesized via mannich reaction

Summary A new method for the preparation of an 8-hydroxyquinoline-and an 8-hydroxy-5-sulphonic-acid quinoline silica gel is described. The synthesized chelating silica gels are studied by DRIFT-spectroscopy and investigated for the sorption of copper(II), nickel(II) and zinc(II). The molar coverage is determined by elemental analysis and by the development of a molecular coating model.     

Syntheses,crystal structures,and properties of four transition metal complexes based on 5-nitro-8-hydroxyquinoline

Four new complexes, [M(C₉H₅N₂O₃)₂(H₂O)₂]·CH₃OH (M?=?Zn(II), Cd(II) and Mn(II)) (1–3) and [Cu(C₉H₅N₂O₃)₂] (4), have been synthesized by reactions of 5-nitro-quinoline-8-yl acetate (NQA) and corresponding metal salts at room temperature and characterized by elemental analysis, IR, thermal gravimetric analysis (TGA), fluorescence measurement and single crystal X-ray diffraction. 1–3 are mononuclear, further extended to 2-D layers by hydrogen bonds. For 4, Cu²⁺ is coordinated by two N atoms and two O atoms from two NQ ligands. TGA indicates that 1–4 have good thermal stabilities. Furthermore, 1 and 2 show excellent luminescence in the solid state at room temperature.     

Electronic structure, spectroscopic properties, and reactivity of molybdenum and tungsten nitrido and imido complexes with diphosphine coligands: influence of the trans ligand

A series of molybdenum and tungsten nitrido, [M(N)(X)(diphos)2], and imido complexes, [M(NH)(X)(diphos)2)]Y, (M = Mo, W) with diphosphine coligands (diphos = dppe/depe), various trans ligands (X = N3-, Cl-, NCCH3) and different counterions (Y-= Cl-, BPh4-) is investigated. These compounds are studied by infrared and Raman spectroscopies; they are also studied with isotope-substitution and optical-absorption, as well as emission, spectroscopies. In the nitrido complexes with trans-azido and -chloro coligands, the metal-N stretch is found at about 980 cm(-1); upon protonation, it is lowered to about 920 cm(-1). The 1A1 --> 1E (n --> pi) electronic transition is observed for [Mo(N)(N3)(depe)2] at 398 nm and shows a progression in the metal-N stretch of 810 cm(-1). The corresponding 3E --> 1A (pi --> n) emission band is observed at 542 nm, exhibiting a progression in the metal-N stretch of 980 cm(-1). In the imido system [Mo(NH)(N3)(depe)2]BPh4, the n --> pi transition is shifted to lower energy (518 nm) and markedly decreases in intensity. In the trans-nitrile complex [Mo(N)(NCCH3)(dppe)2]BPh4, the metal-N(nitrido) stretching frequency increases to 1016 cm(-1). The n --> pi transition now is found at 450 nm, shifting to 525 nm upon protonation. Most importantly, the reduction of this nitrido trans-nitrile complex is drastically facilitated compared to its counterparts with anionic trans-ligands (Epred = -1.5 V vs Fc+/Fc). On the other hand, the basicity of the nitrido group is decreased (pKa{[Mo(NH)(NCCH3)(dppe)2](BPh4)2} = 5). The implications of these findings with respect to the Chatt cycle are discussed.     

Synthesis,crystallographic and spectroscopic characterization and magnetic properties of dimer and monomer ternary copper(II) complexes with sulfonamide derivatives and 1,10-phenanthroline. Nuclease activity by the oxidative mechanism

Three dinuclear and one mononuclear copper(II)-1,10-phenanthroline ternary complexes, [Cu(L1)(phen)(OH)]₂ (1), [Cu(L2)(phen)(OH)]₂·3H₂O (2), [Cu(L3)(phen)(OH)]₂ (3) and [Cu(L4)₂(phen)(H₂O)] (4), with thiadiazole sulfonamide derivative ligands: HL1 (N-(5-ethyl-1,3,4-thiadiazol-2-yl)naphthalene-1-sulfonamide), HL2 (N-(5-ethylthio)-1,3,4-thiadiazol-2-yl)-4-methylbenzenesulfonamide), HL3 (N-(5-ethyl-1,3,4-thiadiazol-2-yl)benzenesulfonamide) and HL4 (N-(5-ethyl-1,3,4-thiadiazol-2-yl)-4-methylbenzenesulfonamide) have been synthesized and characterized. In the four complexes each copper atom is five-coordinated. The structure of complexes 1, 2 and 3 consists of a dimeric unit with a C2 symmetry axis, where both coppers are bridged by two hydroxo anions. Magnetic measurements show that the dimer complexes are ferromagnetic according to the Cu–O–Cu angles. Cleavage experiments using pUC18 plasmid DNA in the presence of H₂O₂/ascorbic acid as an activating agent show that the title complexes are potent artificial chemical nucleases, the order of efficiency being 3 > 2 ∼ 1 > 4. Control cleavage experiments indicated that the dimer complexes are stronger artificial nucleases than the [Cu(phen)₂]²⁺ complex under the same experimental conditions, while the monomer 4 has a lower nuclease activity than the [Cu(phen)₂]²⁺ complex. The inhibition of the cleavage process in the presence of reactive oxygen intermediate scavengers suggests that the hydroxyl radical and the superoxide anion are reactive species for the breakage of the DNA strands.     

Synthesis,spectroscopic, thermal,crystal structure properties and characterization of new Hofmann-type-like clathrates with 4-aminopyridine and water

In this study, synthesis of two new heteronuclear tetracyanonickelate(II) clathrates based on 4-aminopyridine (4AP) and guest water (H₂O) molecule and investigation of their structural properties were reported. These clathrates were characterized by using vibration spectroscopy, elemental, thermal analysis and single crystal X-ray diffraction (SC-XRD) techniques. Examining the elemental and spectral data of these clathrates, it was observed that the formulas [Zn(II)(4AP)₂Ni(µ-CN)₂(CN)₂]·6H₂O and [Cu(II)(4AP)₄Ni(µ-CN)₂(CN)₂]·H₂O were defined their structures. General information about the structural properties of these clathrates in single crystal form has been obtained by considering the changes in the characteristic peaks of the cyanide group and the 4AP that formed them. The thermal behaviors of these clathrates were obtained by examining the temperature-dependent changes of their masses. The magnetic susceptibilities of these clathrates in single crystal form were measured with a Gouy balance. According to the data obtained using SC-XRD technique, the heterometallic [Zn(II)(4AP)₂Ni(µ-CN)₂(CN)₂]·6H₂O compound has Cmcm and the heterometallic [Cu(II)(4AP)₄Ni(µ-CN)₂(CN)₂]·H₂O compound has crystal structures in the C2 / c space group.