Molecular recognition of chloroform by divergent polymorphic transitions in tert-butylthiacalix[4]arene tetrasubstituted with N-(2-hydroxyethyl)carbamoylmethoxy groups in a lower rim

The genuine molecular recognition of chloroform by a tert-butylthiacalix[4]arene derivative was observed, where the host remembers the guest above its elimination point showing two polymorphic transitions with opposite heat effects.     

Thermochemistry of drugs: experimental and theoretical study of analgesics

Structural Chemistry - Acetanilides are broadly used in the pharmaceutical industry. Thermochemical data on vapor pressures, solid-gas, liquid-gas, and solid-liquid phase transitions, as well as on...     

Almost periodic solutions of differential equations with piecewise constant argument of generalized type

We consider the existence and stability of an almost periodic solution of the following hybrid system:

(1)

where if θ_i≤t<θ_i+1,i=…−2,−1,0,1,2,…, is an identification function, θ_i is a strictly ordered sequence of real numbers, unbounded on the left and on the right, p_j,j=1,2,…,m, are fixed integers, and the linear homogeneous system associated with (1) satisfies exponential dichotomy. The deviations of the argument are not restricted by any sign assumption when existence is considered. A new technique of investigation of equations with piecewise argument, based on integral representation, is developed.     

Unraveling the electronic structures of low-valent naphthalene and anthracene iron complexes: X-ray, spectroscopic, and density functional theory studies

Naphthalene and anthracene transition metalates are potent reagents, but their electronic structures have remained poorly explored. A study of four Cp*-substituted iron complexes (Cp* = pentamethylcyclopentadienyl) now gives rare insight into the bonding features of such species. The highly oxygen- and water-sensitive compounds [K(18-crown-6){Cp*Fe(η(4)-C(10)H(8))}] (K1), [K(18-crown-6){Cp*Fe(η(4)-C(14)H(10))}] (K2), [Cp*Fe(η(4)-C(10)H(8))] (1), and [Cp*Fe(η(4)-C(14)H(10))] (2) were synthesized and characterized by NMR, UV-vis, and (57)Fe M?ssbauer spectroscopy. The paramagnetic complexes 1 and 2 were additionally characterized by electron paramagnetic resonance (EPR) spectroscopy and magnetic susceptibility measurements. The molecular structures of complexes K1, K2, and 2 were determined by single-crystal X-ray crystallography. Cyclic voltammetry of 1 and 2 and spectroelectrochemical experiments revealed the redox properties of these complexes, which are reversibly reduced to the monoanions [Cp*Fe(η(4)-C(10)H(8))](-) (1(-)) and [Cp*Fe(η(4)-C(14)H(10))](-) (2(-)) and reversibly oxidized to the cations [Cp*Fe(η(6)-C(10)H(8))](+) (1(+)) and [Cp*Fe(η(6)-C(14)H(10))](+) (2(+)). Reduced orbital charges and spin densities of the naphthalene complexes 1(-/0/+) and the anthracene derivatives 2(-/0/+) were obtained by density functional theory (DFT) methods. Analysis of these data suggests that the electronic structures of the anions 1(-) and 2(-) are best represented by low-spin Fe(II) ions coordinated by anionic Cp* and dianionic naphthalene and anthracene ligands. The electronic structures of the neutral complexes 1 and 2 may be described by a superposition of two resonance configurations which, on the one hand, involve a low-spin Fe(I) ion coordinated by the neutral naphthalene or anthracene ligand L, and, on the other hand, a low-spin Fe(II) ion coordinated to a ligand radical L(?-). Our study thus reveals the redox noninnocent character of the naphthalene and anthracene ligands, which effectively stabilize the iron atoms in a low formal, but significantly higher spectroscopic oxidation state.     

Entrainment by Chaos

A new phenomenon, the entrainment of limit cycles by chaos, which results from the appearance of cyclic irregular behavior, is discussed. In this study, sensitivity is considered as the main ingredient of chaos to be captured, and the period-doubling cascade is chosen for extension. Theoretical results are supported by simulations and discussions regarding Chua’s oscillators, entrainment of toroidal attractors by chaos, synchronization, and controlling problems. It is demonstrated that the entrainment cannot be considered as generalized synchronization of chaotic systems.     

Scanning Force Microscopy as Applied to Conformational Studies in Macromolecular Research

The modern state of SFM research on polymer nano‐objects including single chains is discussed in comparison with other similar high‐resolution microscopy techniques. The range of problems to be solved preferentially by SFM is highlighted. Promising methodology to describe quantitatively the morphology of macromolecular objects is proposed. The main benefits of this algorithm seem to be the apparent mathematical correctness as well as the possibility to estimate errors and the confidence of the numbers obtained. Special attention is paid to the dynamic observations of conformational transitions on a substrate in real time regime. This approach allows one to realise direct control of the adsorbed macromolecules by means of exposure to different vapours. Driving forces of the vapour‐induced reorganisation are discussed.

     

Homoleptic diphosphacyclobutadiene complexes [M(η(4)-P2C2R2)2]x- (M = Fe, Co; x = 0, 1)

The preparation and comprehensive characterization of a series of homoleptic sandwich complexes containing diphosphacyclobutadiene ligands are reported. Compounds [K([18]crown-6)(thf)(2)][Fe(η(4)-P(2)C(2)tBu(2))(2)] (K1), [K([18]crown-6)(thf)(2)][Co(η(4)-P(2)C(2)tBu(2))(2)] (K2), and [K([18]crown-6)(thf)(2)][Co(η(4)-P(2)C(2)Ad(2))(2)] (K3, Ad = adamantyl) were obtained from reactions of [K([18]crown-6)(thf)(2)][M(η(4)-C(14)H(10))(2)] (M = Fe, Co) with tBuC[triple bond]P (1, 2), or with AdC[triple bond]P (3). Neutral sandwiches [M(η(4)-P(2)C(2)tBu(2))(2)] (4: M = Fe 5: M = Co) were obtained by oxidizing 1 and 2 with [Cp(2)Fe]PF(6). Cyclic voltammetry and spectro-electrochemistry indicate that the two [M(η(4)-P(2)C(2)tBu(2))(2)](-)/[M(η(4)-P(2)C(2)tBu(2))(2)] moieties can be reversibly interconverted by one electron oxidation and reduction, respectively. Complexes 1-5 were characterized by multinuclear NMR, EPR (1 and 5), UV/Vis, and M?ssbauer spectroscopies (1 and 4), mass spectrometry (4 and 5), and microanalysis (1-3). The molecular structures of 1-5 were determined by using X-ray crystallography. Essentially D(2d)-symmetric structures were found for all five complexes, which show the two 1,3-diphosphacyclobutadiene rings in a staggered orientation. Density functional theory calculations revealed the importance of covalent metal-ligand π bonding in 1-5. Possible oxidation state assignments for the metal ions are discussed.     

Metal substitution in a Lindqvist polyoxometalate leads to improved photocatalytic performance

The photochemical properties of homo- and heterometallic molybdate-based Lindqvist polyoxometalate clusters are investigated in a comparative study and it is shown that vanadium substitution can be used as a facile synthetic tool to optimize the visible light absorption and photocatalytic activity of the cluster. The mono-vanadium substituted unit, [VMo(5)O(19)](3-) shows light absorption up to 480 nm whereas the light absorption of the molybdate analogue [Mo(6)O(19)](2-) is mainly in the UV region below 400 nm. The electronic absorption properties of both clusters are further investigated using TD-DFT calculations which show that vanadium incorporation leads to the formation of low-energy O → V LMCT transitions. In comparative photochemical dye decomposition test reactions under UV and Vis irradiation, a higher reactivity is observed for [VMo(5)O(19)](3-) together with turnover numbers of more than 1600. In addition it is shown that under anaerobic conditions, the photoreaction proceeds faster than in the presence of oxygen, suggesting that oxygen acts as a quencher in one of the photoredox steps.