Structural constraints for the formation of macrocyclic rhombimines

Rhombimines, macrocyclic tetraimines, have been obtained by the condensation of enantiomerically pure trans-1,2-diaminocyclohexane with aromatic dialdehydes connected by a one-atom bridge. The efficiency of the cyclocondensation is dependent upon the nature of the dialdehyde bridge atom: low selectivity was observed and rationalized by computational analysis for sp² hybridized bridge atoms. Unusual triple-split exciton Cotton effects were measured and calculated for highly symmetrical, tetrachromophoric rhombimines.     

Criteria for the formation of complexes in solutions by molecules

The causes for the non-fulfillment of correlations describing the changes of various properties of molecules (Y) in the transition from the gaseous phase to solutions have been discussed; they are based on concepts of the discrete structure of the surroundings of the molecules of the dissolved substance by solvent molecules and on multi-particle interactions of molecules in solutions, describing Y by universal interactions of molecules in solution. It is shown that these correlations are not fulfilled in the case of solutions containing in the molecules the group A-H (A=O, S, NC), capable of forming intermolecular hydrogen bonds, and also in the case of systems where donor-acceptor complexes can be formed. Some features have been formulated of universal interactions of molecules in solutions and compared to specific features in complex formation.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 22, No. 6, pp. 729–733, November–December, 1986.     

Structural investigations of alumino-metalloaminocarbene complexes

[Al(NMe₂)₃]₂ combines with Fe(CO)₅ and Mn₂(CO)₁₀ forming bimetallic compounds Fe₂(CO)₈[C(NMe₂)OAl(NMe₂)₂]₂ and Mn₂(CO)₉[C(NMe₂)OAl₂-(NMe₂)₅]. X-ray diffraction analyses reveal the formation of aluminometallocarbene compounds with central connectivities, MC[NMe₂)][OAl(NMe₂)(μ-NMe₂)₂].     

Structural chemistry of azo complexes

Summary Complexes of Co^II, Ni^II, Cu^II, Pd^II, and Pt^IV with 2,4,5-trihydroxybenzoic acid and its substituted phenylazoderivatives have been prepared, together with some mixed-metal (Co, Cu or Ni, Cu) complexes of itsm-nitro,m-carboxy andm-hydroxyarylazo derivatives. The stereochemistries and the modes of bonding of the complexes were elucidated by spectral and magnetic susceptibility measurements.Abstracted from her Ph.D. thesis.     

Structural control of the photodynamics of boron-dipyrrin complexes

Boron-dipyrrin chromophores containing a 5-aryl group with or without internal steric hindrance toward aryl rotation have been synthesized and then characterized via X-ray diffraction, static and time-resolved optical spectroscopy, and theory. Compounds with a 5-phenyl or 5-(4-tert-butylphenyl) group show low fluorescence yields (approximately 0.06) and short excited-singlet-state lifetimes (approximately 500 ps), and decay primarily (>90%) by nonradiative internal conversion to the ground state. In contrast, sterically hindered analogues having an o-tolyl or mesityl group at the 5-position exhibit high fluorescence yields (approximately 0.9) and long excited-state lifetimes (approximately 6 ns). The X-ray structures indicate that the phenyl or 4-tert-butylphenyl ring lies at an angle of approximately 60 degrees with respect to the dipyrrin framework whereas the angle is approximately 80 degrees for mesityl or o-tolyl groups. The calculated potential energy surface for the phenyl-substituted complex indicates that the excited state has a second, lower energy minimum in which the nonhindered aryl ring rotates closer to the mean plane of the dipyrrin, which itself undergoes some distortion. This relaxed, distorted excited-state conformation has low radiative probability as well as a reduced energy gap from the ground state supporting a favorable vibrational overlap factor for nonradiative deactivation. Such a distorted conformation is energetically inaccessible in a complex bearing the sterically hindered o-tolyl or mesityl group at the 5-position, leading to a high radiative probability involving conformations at or near the initial Franck-Condon form of the excited state. These combined results demonstrate the critical role of aryl-ring rotation in governing the excited-state dynamics of this class of widely used dyes.     

Prediction of formation constants for actinide complexes in solution

A numerical treatment for predicting the formation constants of actinide complexes from the properties of the metal ion and the ligand is presented. Comparison of predicted and averaged literature values of the formation constants shows excellent agreement, and the method will be most useful for modelling transport from radioactive waste repositories.     

4-二羟基硼酰苯丙氨酸与果糖结合物的结构特性研究

使用密度泛函方法对4-二羟基硼酰苯丙氨酸与果糖结合物的结构特性进行了理论计算,确定了3种稳定构型,分析了该结构具有良好水溶性的原因.计算结果表明,前2种结构的反应活性较强,而第3种结构中果糖的加入并未过多影响其化学活性,3种结构中果糖的加入只是改变了原有分子的极性,果糖部分容易从复合结构上脱落,因而起到了一个很好的类似催化剂的作用.     

Structural chemistry of amino acid complexes

Summary Complex formation between Co^II, Ni^II and Cu^II and the amino acids (cysteine, cystine and methionine) in the solid state were investigated by elemental analyses, i.r. and electronic spectra, and magnetic susceptibilities     

Structural diversity for phosphine complexes of stibenium and stibinidenium cations

Reactions of trimethylphosphine or diphosphines with SbCl(3) in the presence of AlCl(3) or Me(3)SiSO(3)CF(3) give ligand stabilized stibenium and stibinidenium cations. The geometry at each antimony center reveals a variety of environments for antimony that describes new bonding and highlights new directions in the chemistry of the pnictogen elements.     

On the estimation of cross sections for the formation of triatomic collision complexes

The traditional procedure of associating the formation of a collision complex with the passage over a centrifugal potential barrier is reinvestigated. It is argued that the complex is better defined in terms of a spatially localized critical surface (or separation), such that the complex is strongly coupled, while the system is weakly elsewhere. Weak coupling implies that the rotational and vibrational energies of the diatomic fragment are, to a good approximation, conserved. Strong coupling implies that only total energy and angular momentum is conserved. Graphical methods are used to obtain upper bounds for the cross section within weak or strong coupling dynamics by optimizing the attraction within the given constraints. Results for a K + NaCl system and a O(¹D)+H₂ system are presented and compared with results of molecular dynamic simulations available in the literature. The weak coupling constraints are found to give a useful representation of the simulation data. Discontinuities in the optimal orientation of approach indicate the presence of orientational barriers.     

Diastereospecificity in the formation of chelated organometallic complexes

Three types of intramolecular cyclization reactions in organoiron complexes having one or more chiral centers are shown to proceed with high diastereospecificity.     

Equilibrium constants for the formation of Cu(I) halide complexes

The stoichiometric equilibrium constants, K ₃ ^* , for the formation of CuX ₃ ²⁻ from CuX ₂ ⁻ +X⁻ where X=Cl and Br, have been determined from spectral measurements. The measurements were made in NaCl and NaBr solutions from I=0.5 to 6.0m at 5, 25 and 45°C. The measured constants were extrapolated to infinite dilution using the Pitzer equations. The Pitzer parameters, β⁰, β¹ and C^φ, for the interaction of Na⁺ with CuX ₂ ⁻ and CuX ₃ ²⁻ are briefly discussed.     

A kinetic method for determining the first formation constant of cuprous complexes

Summary The oxidation with molecular oxygen of the cuprous salt, Cu(MeCN)₄ClO₄, in the presence of the different concentrations of complexing agent phenanthroline in DMSO was studied, and the first formation constant was calculated from the kinetic data.     

Structural motifs of DNA complexes in the gas phase

DNA duplexes are known to be quite stable in the condensed phase but recent mass spectrometry results have shown that DNA complexes are also stable (at least for a limited time) in the gas phase. However, very little is known about the overall shape of the complexes in a solvent-free environment and what factors influence that shape. In this article, we present recent ion mobility and molecular modeling results that address some issues concerning the gas-phase conformations of DNA duplexes. Examples include the effect of metal ions on Watson–Crick base pairing, investigating the onset of helicity in duplexes as a function of strand length, comparison of the stability of C·G and A·T base pairs, and examining the formation of quadruplex structures.     

Evolutional kinetics of metal-polymer complexes formation

Under the conditions of polydental binding, metal ions couple azacrown ether links of the polymer, limiting segmental chain mobility and decreasing statistical weight of the states available for complex formation. According to the rate constant dependence on conversion extent two different regimes of the complex formation reaction may be given: 1) retardation resulting from decreasing reactivity, the measured rate constant decreases by a few orders of magnitude in the course of time; 2) retardation followed by increasing reactivity, the observed rate constant remains unchangeable. The process covering conformational nonequilibrium states of macromolecular coil makes it dependent on the system prehistory which violates the law of mass action. The couplings being labile, the instantaneous value of pseudomonomolecular rate constant of the reaction is a function of conversion extent on metal and is not dependent on absolute polymer uptake by metal. The reaction rate in each point does not depend only on reagents concentration ratio but also on the character of metal ion distribution along the polymer chain obtained to the given moment. At complex formation with “hard” couplings the increase of metal concentration makes the effective retardation of the process change, whereas initial parts of the kinetic curves remain practically the same. In this system pseudomonomolecular rate constant does not seem to be a function of conversion extent, but is defined by absolute quantity of the formerly bound metal. Formal application of the law of mass action in this case is useless, it is necessary to apply distribution function on reactivity. Unlike solid phase systems nonequivalent character of reaction sites in the polymer is of evolution character, i.e.it arises in the course of the reaction on the account of polymer coil conformation change.     

Different alternatives for stability constant determination of Zr and Hf 2-methyl lactate complexes by cation exchange

Three different alternatives are applied for the determination of the stability constants of zirconium and hafnium complexes with 2-methyl lactic acid using cation exchange. Calculations based on the exchange equilibria of the highly charged metal cations and the positively charged ligand complexes rather than that of the hydrolysed complex species are worked out according to SCHUBERT and FRONAEUS, respectively. The third alternative adopted in this work, is principally based on the consideration of all complexed species produced by hydrolyses in the aqueous phase. The B-values obtained by the different methods are intercorrelated and discussed.     

Structural aspects of metal–amide complexes

An exhaustive survey of crystal structure data on simple amides and metal complexes containing monodentate amide ligands has been performed. Statistical analysis of structural features are reported as a function of the degree of alkylation of the amide functional group, the type of metal ion in the amide complex, and the type of binding to the metal ion. Average values are reported for bond lengths, bond angles, and torsional angles. Orientational preferences of the coordinated amide ligand are discussed in terms of M–O–C bond angles and M–O–C–N torsion angles.     

A novel methodology for evaluation of formation constants of complexes: example of lanthanide-Arsenazo III complexes

A novel method is proposed for the computation of formation constants of complexes. It makes use essentially of the data from the first stages of the titration and is based on the Taylor expansion of the concentration of the ligand as a function of the titrant concentration. The applicability of the methodology is examplified by the computation of the complexation constants of lanthanides (Ln) by 3,6-bis[(2-arsonophenyl)azo]-4,5-dihydroxy-2,7-naphtalenedisulphonic acid disodium salt (Arsenazo III). Arsenazo III is known to form stable complexes with a large variety of ions. The spectrophotometric study of these complexes in the visible domain is facilitated by the presence of chromophore groups in the ligand. In the present work, spectral deconvolution was used to improve the accuracy in the determination of formation constants of complexes between Arsenazo III and lanthanides. It turns out that all the 11 lanthanides studied here form the same type of complexes with this ligand. For each of them, the three formation constants correspond, respectively, to Ln to Ar ratios 1:1, 1:2 and 2:2. Their approximate values are in the ranges: K₁₁=10⁵; K₁₂=10¹⁰; K₂₂=10¹⁵. Accurate values are given in the text. The results are compared with those obtained with the classical iteration method which makes use of the whole titration curve. The two techniques give satisfactorily convergent results. The new methodology might be useful in the treatment of the competition of two ligands for the complexation of the same lanthanide, where the first stages of the titration of only one of the complexants is experimentally available.