Dual binding mode of s-triazine to anions and cations

Ab initio calculations were performed on complexes between cations and s-triazine, which has a small and positive quadrupole moment. Minimum energy pi-complexes were found between s-triazine and cations. Minimum pi-complexes with anions were previously reported. This ability of s-triazine to form stable complexes with either anions or cations is studied using several theoretical methods. A likely explanation of this duality is the stabilization obtained from the ion-induced polarization. [structure: see text]     

Organocuprate conjugate addition: structural features of diastereomeric and supramolecular pi-intermediates

In the reaction pathway of conjugate additions with organocuprate reagents, Cu(I) pi-complexes and Cu(III) sigma-complexes have been identified as central, NMR-detectable intermediate species. However, no experimental evidence for the structures of pi-intermediates with extensive chiral enones or the principal aggregation level and aggregate structure of pi-complexes in diethyl ether has been available so far. Furthermore, the structural characteristics of pi-complexes which are essential for their high reactivities and diastereoselectivities have not yet been rationalized experimentally. Therefore, the pi-intermediates of 4,4a,5,6,7,8-hexahydro-4a-methyl-naphthalen-2(3H)-one and Me2CuLi or Me2CuLi x LiX (X = I, CN) in diethyl ether are investigated in detail. For the first time, the formation of two intermediate cuprate enone pi-complexes on both sides of the double bond is observed. In addition, the conformation of the enone adopted in the major beta-face pi-complex rationalizes the exclusive syn addition observed in the synthetic product. For the investigation of the aggregation level and structure, a NMR screening of pi-complexes with Me2CuLi x LiX (X = I, CN) and three achiral enones is performed, which simplifies the spectra by the generation of enantiotopic pi-complexes. Thus, NMR diffusion experiments on cuprate intermediates and the detection of scalar couplings across copper without isotope labeling are possible for the first time. Extensive NMR studies, including those of cyclohexanone complexes, show that, in principle, salt-free dimethylcuprate is able to complex the carbonyl group. However, in the presence of salt, the carbonyl-complexing aggregates are composed of salt and cuprate moieties. These mixed aggregates cause the formation of large supramolecular pi-intermediate structures which control their reactivity. The pi-complexing cuprate units show a bent geometry as a general structural feature that is unaffected by the presence or kind of salt and the type of enone. Thus, the high diastereoselectivity and the reactivity of organocuprate 1,4-addition reactions are for the first time rationalized on the basis of structural characteristics of selected pi-intermediates.     

Predication of second-order optical nonlinearity of [(Bu2t Im)AuX] (X=halogen) using time-dependent density-functional theory combined with sum-over-states method

The dipole polarizability and second-order polarizability of recently synthesized (1,3-di-ter-butylimidazol-2-ylidine) gold complexes [(Bu2t Im)AuX] (X=halogen) were investigated by using time-dependent density-functional theory combined with sum-over-states method. We have discovered that these complexes possess remarkably larger molecular second-order polarizability compared with the organometallic and organic complexes. The value of the second-order polarizability increases in the order of F相似文献   

Range,strength and anisotropy of intermolecular forces in atom–molecule systems: an atom–bond pairwise additivity approach

A new method is proposed to calculate bond energies and equilibrium distances in atom–molecule van der Waals complexes which arises from a balancing between long-range attraction and asymptotic tail of the repulsion. The method, based on correlation formulas between the polarizability of the interacting partners and the main interaction parameters, is an extension of an approach originally developed for atom–atom cases. The basic idea exploits the concept of bond polarizability additivity to represent both the molecular repulsion, in terms of a size which is mainly ascribed to the molecular bonds nearest to the probe atom, and the molecular attraction as due to multi-dispersion centers delocalized on the molecular frame. The method, mainly tested on hydrocarbon–rare gas complexes, can be considered as the starting point for the study of systems of higher complexity.     

Tetracyanoethylene pi-complex chemistry. Indirect spectrophotometric determination of Diels-Alder-active 1,3-dienes

An indirect spectrophotometric method, based on the rapid Diels-Alder reaction between cisoid 1,3-dienes and tetracyanoethylene (TCNE) and the destruction of an aromatic-TCNE pi-complex, was developed to determine eleven 1,3-dienes in the 0.05-1.00 x 10(-3)M range. These dienes were: cyclopentadiene; 1,3-cyclohexadiene; trans-1,3-pentadiene; 2,4-dimethyl-1,3-pentadiene; trans-2-methyl-1,3-pentadiene; 2-methyl-1,3-butadiene; 9-methylanthracene; 9,10-dimethylanthracene; 1,6-diphenyl-1,3,5-hexatriene; 2,3-dimethyl-1,3-butadiene; and 1,4-diphenyl-1,3-butadiene. Three 1,3-dienes were determined in the 0.05-1 x 10(-4)M range: cyclopentadiene, trans-2-methyl-1,3-pentadiene, and anthracene. The limit of detection for cyclopentadiene in carbon tetrachloride solutions is 0.11 microg/ml. Fourteen 1,3-dienes were found to form stable pi-complexes and could not be determined by the proposed method. For these 1,3-dienes, the spectra of some of the complexes are reported; in addition, relative equilibrium constants for the pi-complexes of 2,5-dimethyl-2,4-hexadiene, cis-1,3-pentadiene, 4-methyl-1,3-pentadiene, and 1,3-cyclo-octadiene were estimated. An explanation of the transient colour in the 1,3-diene-TCNE Diels-Alder reaction is suggested.     

P-Heterocyclic carbenes as potential ligands in the design of new metathesis catalysts. A computational study

Density functional calculations are reported concerning the olefin metathesis characteristics of a variety of P-heterocyclic carbene (PHC) complexes. The calculations employ model catalysts of the type (PMe3)(PHC)Cl2Ru=CH2, the PHC ligands being 1,3-dihydro-1,3-diphosphol-2-ylidene PH, 1,3-diphenyl-1,3-diphosphol-2-ylidene PPH, and 1,4-dihydro-1,4-diphosphol-2-azol-5-ylidene PNH. Complexes with N-heterocyclic carbenes (NHC) are included for comparison. Associative and dissociative reaction pathways are considered, the latter ones representing the favored reaction mechanisms. Calculations show that the rate determining step is ring opening of a ruthena-cyclobutane intermediate. In comparison with NHC model catalysts, the PHC compounds have lower phosphine dissociation energies, and also form weaker pi-complexes with an olefinic substrate. Compared to the initially formed pi-complexes, the ruthena-cyclobutane is more stable for PHC- than for NHC-catalysts. The catalytic activity of model PHC-compounds in comparison with NHC-compounds is discussed on the basis of the calculated reaction profiles. In this context, different models for enhanced reactivity of NHC-based catalysts that have been proposed in the literature are considered as well. It is demonstrated that the nature of the substituent of the carbene phosphorus not only exhibits a steric influence on the course of the reaction, but a significant stereoelectronic effect as well. Further, agostic interactions in ruthena-cyclobutane intermediates are investigated.     

Rapid-injection NMR study of iodo- and cyano-Gilman reagents with 2-cyclohexenone: observation of pi-complexes and their rates of formation

Rapid-injection is a very useful technique for the preparation of temperature-sensitive and air-sensitive compounds in the cold, nitrogen-filled probe of an NMR spectrometer. We have used this method to prepare solutions of pi-complexes from 2-cyclohexenone and prototypical cuprates Me2CuLi.LiI and Me2CuLi.LiCN, and we have assigned structures on the basis of 1H and 13C NMR. In each case two pi-complexes were observed, and in the former, their rates of formation were measured by rapid-injection 1H NMR and EXSY spectroscopy. These results provide insights into the normal and anomalous conjugate addition reactions of organocuprates.     

Stability and reactivity of all-metal aromatic and antiaromatic systems in light of the principles of maximum hardness and minimum polarizability

It is demonstrated that among various possible isomers of all-metal aromatic compounds such as Al(4)(2-) and their complexes the most stable isomer with the minimum energy is the hardest and the least polarizable. A similar situation is observed for different isomers of all-metal antiaromatic compounds such as Al(4)(4-) and their complexes. It is shown that linear Al(4)(4-) is energetically more stable than its cyclic isomer. The reaction energies associated with the complexation processes highlight the stability of those complexes. The difference in energy, hardness, and polarizability between a cyclic molecule and its linear counterpart convincingly shows that an aromatic molecule exhibits negative changes in energy and polarizability but positive changes in hardness as expected from the principles of minimum energy, minimum polarizability, and maximum hardness. Although the aromaticity of Al(4)(2-) is unequivocally established through this study, the antiaromaticity picture in the case of Al(4)(4-) is shown to be poorly understood;however, the present analysis sheds light on this controversy.     

Weak molecular complexes as studied by NMR spectroscopy of oriented molecules

Weak molecular interactions such as those in pyridine—iodine, benzene—iodine and benzene—chloroform systems oriented in thermotropic liquid crystals have been studied from the changes of the order parameters as a result of complex formation. The results indicate the formation of at least two types of charge transfer complexes in pyridine—iodine solutions. The pi-complexes in benzene—chloroform and benzene—iodine mixtures have also been detected. No detectable changes in the inter-proton distances in these systems were observed.     

Synthesis and liquid-crystalline properties of copper(II) bis[4-heptyloxy-N-(aryl)benzaldimine-2-olates]

We describe the synthesis and liquid-crystalline properties of copper complexes with Schiffs bases. We show that when going from ligands to complexes, the mesophase interval narrows until it disappears and the phase transition temperature increases. In contrast to the case for ligands, no linear relationship is observed between the thermal stability of complexes and the electronic properties. The variation in the clearing point in the series of complexes with alkyl substituents is consistent with the even-odd contributions of the polarizability of the radicals to the total polarizability of the compounds.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 12, pp. 2833–2838, December, 1989.     

Structural trends in transition metal cation-acetylene complexes revealed through the C-H stretching fundamentals

Metal cation-acetylene complexes (M = V, Fe, Co, Ni) are produced in molecular beams and studied with infrared photodissociation spectroscopy in the C-H stretching region. Each complex has two vibrational bands corresponding to the symmetric and asymmetric stretches of acetylene that are shifted to the red of these vibrations in the isolated acetylene molecule. Density functional theory reveals the sources of the red-shifted vibrations and their relative magnitudes. Fe+, Co+, and Ni+ form pi-complexes with acetylene, while V+(C2H2) is a metallacycle.     

Eta3-pyranyl and eta3-pyridinyl molybdenum pi-complexes as chiral scaffolds for the enantioselective construction of substituted oxa- and aza[3.3.1]bicyclics: first enantio- and regiocontrolled [5+3] cycloaddition reactions

The first Mo-mediated [5+3] cycloadditions are reported. 3-Substituted pyranyl and pyridinyl molybdenum pi-complexes participate in enantiocontrolled [5+3] cycloadditions and provide a new and efficient synthetic approach to oxa- and aza[3.3.1]bicyclics of high enantiomeric purity. The reaction proceeds in good to excellent yields and with complete regio- and endo-selectivities; it diverts to a [2+3] cycloaddition pathway when 2-substituted heterocycle pi-complexes are used. This methodology, coupled with a variety of general demetalation protocols, holds much promise in synthetic applications.     

EPR parameters of radical ions and polarizability effect

The literature data on substituent influence on the g factors, hyperfine coupling (hfc) constants (a) of the EPR signals and the spin densities (ρ) have been analyzed for 25 series of the organic radical cations and radical anions as well as of the transition metal complexes. The g, a, and ρ values were first established to depend not only on the inductive and resonance effects but also on the polarizability of substituents which can be characterized by the σ(α) constants. The polarizability effect is caused by the partial charge on the radical center. This effect consists in an electrostatic attraction between the charge and the dipole moments induced by this charge in the substituents. The polarizability contribution ranges up to 92%.     

NQR parameters of complexes and polarizability effect

The literature data on substituent influence on the nuclear quadrupole resonance frequencies (ν), quadrupole coupling constants (e²Qq ? h^{? 1}), and asymmetry parameters (η) for 36 series of the H‐complexes, charge–transfer complexes, transition metal complexes and other donor–acceptor complexes have been considered, using the correlation analysis. Generally the ν, e²Qq ? h^{? 1}, and η values were first established to depend on the inductive, resonance, polarizability, and steric effects of substituents. The presence or otherwise of certain effects as well as relation between their contributions are determined by the type of series. The polarizability effect owes its existence to the appearance of an excess charge on the indicator centre as a result of the complexation. The contribution of this effect ranges up to 75%. Copyright © 2012 John Wiley & Sons, Ltd.     

Face-to-face arene-arene binding energies: dominated by dispersion but predicted by electrostatic and dispersion/polarizability substituent constants

Parallel face-to-face arene-arene complexes between benzene and substituted benzenes have been investigated at the MP2(full)/6-311G** and M05-2X/6-311G** levels of theory. A reasonably good correlation was found between the binding energies and the ∑|σ(m)| values of the substituted aromatics. It is proposed that a substituent |σ(m)| value informs on both the aromatic substituent dispersion/polarizability and the effect the substituent has on the aromatic electrostatics. Supporting this hypothesis, a combination of electrostatic (∑σ(m)) and dispersion/polarizability (∑M(r)) substituent constant terms gives an excellent, and statistically significant, correlation with the benzene-substituted benzene binding energy. Symmetry adapted perturbation theory energy decomposition calculations show the dominant attractive force is dispersion; however, the sum of all nonelectrostatic forces is essentially a constant, while the electrostatic component varies significantly. This explains the importance of including an electrostatic term when predicting benzene-substituted benzene binding energies.     

Vibrational spectroscopy and structures of Ni+(C2H2)(n) (n =1-4) complexes

Nickel cation-acetylene complexes of the form Ni(+)(C(2)H(2))(n), Ni(+)(C(2)H(2))Ne, and Ni(+)(C(2)H(2))(n)Ar(m) (n = 1-4) are produced in a molecular beam by pulsed laser vaporization. These ions are size-selected and studied in a time-of-flight mass spectrometer by infrared laser photodissociation spectroscopy in the C-H stretch region. The fragmentation patterns indicate that the coordination number is 4 for this system. The n = 1-4 complexes with and without rare gas atoms are also investigated with density functional theory. The combined IR spectra and theory show that pi-complexes are formed for the n = 1-4 species, causing the C-H stretches in the acetylene ligands to shift to lower frequencies. Theory reveals that there are low-lying excited states nearly degenerate with the ground state for all the Ni(+)(C(2)H(2))(n) complexes. Although isomeric structures are identified for rare gas atom binding at different sites, the attachment of rare gas atoms results in only minor perturbations on the structures and spectra for all complexes. Experiment and theory agree that multiple acetylene binding takes place to form low-symmetry structures, presumably due to Jahn-Teller distortion and/or ligand steric effects. The fully coordinated Ni(+)(C(2)H(2))(4) complex has a near-tetrahedral structure.     

Chemical effects in SERS of pyrazine adsorbed on Au-Pd bimetallic nanoparticles: a theoretical investigation

Chemical enhancement in surface-enhanced Raman scattering (SERS) of pyrazine adsorbed on Au-Pd nanoclusters is investigated by using density functional theory. Changing Pd content in the bimetallic clusters enables modulation of the direct chemical interactions between the pyrazine and the clusters. The magnitude of chemical enhancement is correlated well with the induced polarizability for the complexes with low Pd content, which fails for the complexes with high Pd content. Furthermore, the dependence of chemical enhancement on cluster size and coupling is also described by the induced polarizability. Additionally, the chemical enhancement in the cluster-molecule-cluster junction is found to account for as much as 10(3), which suggests that a chemical mechanism might be more important than previously believed, in particular for Au-Pd bimetallic nanoparticle aggregates.     

Reactivity of homoallylic substituted adamantylideneadamantanes with bromine. Substituent effects on the stability of the ionic and nonionic intermediates

Sterically congested adamantylideneadamantanes (1b-g) (X = Br, Cl, F, OH, OEt, OCOCH(3)), homoallylically substituted with equatorial groups (X), react with bromine in 1,2-dichloroethane to give a stable bromonium ion intermediate or a substitution product depending on the nature of the substituent and on the bromine concentration. The nature of the substituent markedly affects the formation constant of the 1:1 pi-complexes, as well as of the formation constant and reactivity of bromonium ion intermediates. The different reactivity of the ionic intermediates, which depends on the nature of substituents, is attributed to bromonium or bromocarbenium character of the intermediate, with the support of theoretical investigations. Ab initio calculations on 1:1 adamatylideneadamantane-Br(2) complexes (2a-f) show that the substituent affects the stability of these species through electrostatic and dispersion effects. Solvent effects may also contribute to modulate the relative stability of these species.     

Mechanistic pathways of the hydroxyl radical reactions of quinoline. 2. Computational analysis of hydroxyl radical attack at C atoms

Density functional theory (DFT) calculations are employed to compare the mechanism of the *OH attacks at all carbon atoms in quinoline. The computational analysis of the energy surface for the reaction of *OH with quinoline reveals that the formation of OH adducts proceeds through exothermic formation of pi-complexes/H-bonded complexes. The gas-phase reactions have activation energies ranging from <1.3 kcal/mol for the attack at positions C3 through C8 to 8.6 kcal/mol for the attack at the C2 position. Solvation, as described by the CPCM cavity model, lowers these activation barriers so that the attack at all carbon atoms except C2 is effectively barrierless. The *OH attack at C2 in solution is significantly different than at all other quinoline positions because it involves the only transition structure with energy higher than that of the starting materials and with an energetic barrier of 5.1 kcal/mol. The specific solvation approach also corroborates this finding because the attack at C2 was shown to have an energy barrier of 2.3 kcal/mol compared to the barrierless attack at C5. These results are in agreement with our recent experimental studies but differ from literature reports on the degradation of quinoline using the photo-Fenton reaction.     

Hydrogen bonding to pi-systems of indole and 1-methylindole: is there any OH...phenyl bond?

The weak hydrogen-bonded complexes between proton donors and the pi-cloud of indole and 1-methylindole (MI) are investigated theoretically by three different methods: DFT/B3LYP, MPW1B95, and MP2. This study addresses the question as to whether the 1:1 complex can only form between the proton and the pi-cloud of the pyrrole part of indole or if there also exists a 1:1 complex between the proton and the pi-cloud of the phenyl ring. For the water-indole system, the more elaborate MP2 and MPW1B95 methods yield only one minimum with a hydrogen bond to the pyrrole part and weak secondary interactions to the phenyl ring, in agreement with a recent criticism by Van Mourik (Chem. Phys. 2004, 304, 317-319) that the B3LYP functional is unable to account for the dispersion interaction. However, for the 1:1 complexes between MI and 2-propanol, all three methods indicate that both the five-membered and the six-membered rings of the indole chromophore can form pi-complexes. For the MI-trifluoroethanol (TFE) system, it is shown that the ethanol conformation is specific for the interaction site: for the complex to the five-membered ring, TFE is in the cis-gauche conformation, while for the complex to the six-membered ring site, it is in the trans conformation. These results are discussed as a function of local interactions in the systems.