An extreme values of the function <Emphasis Type="Italic">S</Emphasis>(<Emphasis Type="Italic">T</Emphasis>) in short intervals

We obtain new estimates for the maximum and minimum of the argument of the Riemann zetafunction on very short segments of the critical line. These results are based on the Riemann hypothesis.     

Unified Approach to Modeling of Generation of High- and Low-Frequency Spectrum Components under Ionization of a Dielectric Medium by a Few-Cycle Pulse

It is shown that the model of interaction of high-power few-cycle optical pulses with media, which is based on the density matrix formalism, is capable of simultaneously considering generation of longwave radiation in the infrared and terahertz range and describing the broadening of the radiation spectrum into the high-frequency region. The results of the mathematical modeling are compared to the known experimental data.     

Heterocyclization of electrophilic alkenes with tetranitromethane revisited: regiochemistry and the mechanism of nitroisoxazole formation

Revised regiochemistry for the heterocyclization of electrophilic alkenes with tetranitromethane (TNM) in the presence of triethylamine, providing rapid access to nitroisoxazoles, is reported. The formation of 5-nitroisoxazoles previously incorrectly assigned as 3-nitro regioisomers, has now been established unambiguously by X-ray crystallography. Empirical computations with ACD/CNMR Predictor, based both on hierarchical ordering of spherical environments (HOSE) and an algorithm of artificial neural networks (ANN), and also Density Functional Theory computations of the ¹³C NMR chemical shifts for the 3- versus 5-nitroisoxazoles are shown to consistently match the spectra of the experimentally observed 5-regioisomers.     

Phosphorylation of Conjugated Enamines

Abstract

Introduction of benzylidene group at α-position of enamines derived from cyclic ketones substantially increases strength of C-P bond thus permitting further syntheses without splitting of the C-P bond. A wide set of phosphorylated derivatives of type (I) were prepared and their properties were studied. Combination of an enamine moiety with other nucleophilic centers such as C or N in a molecule allows to carry out cyclisation giving five- and six-membered phosphorus-containing heterocycles of types (II, III).     

Amino acid hydration in aqueous ammoniu chloride and sodium chloride solutions

Partial volumes $\bar V^0$ of amino acids in aqueous NH₄Cl and NaCl solutions are discussed. The salts have different effects on water structure. The contributions of the charged NH ₃ ⁺ and COO^? groups of amino acids are found. Structural characteristics of hydrated complexes are calculated: partial volumes of water inside and outside the hydration sphere and hydration numbers. The same value of $\bar V^0$ (NH ₃ ⁺ , COO^?) is achieved at a higher NH₄Cl concentration. The two salt systems with the same $\bar V^0$ (NH ₃ ⁺ , COO^?) have similar values of the partial volumes of water and hydration numbers.     

Direct Evidence of Significantly Different Chemical Behavior and Excited‐State Dynamics of 1,7‐ and 1,6‐Regioisomers of Pyrrolidinyl‐Substituted Perylene Diimide

Novel bay‐functionalized perylene diimides with additional substitution sites close to the perylene core have been prepared by the reaction between 1,7(6)‐dibromoperylene diimide 6 (dibromo‐PDI) and 2‐(benzyloxymethyl)pyrrolidine 5 . Distinct differences in the chemical behaviors of the 1,7‐ and 1,6‐regioisomers have been discerned. While the 1,6‐dibromo‐PDI produced the corresponding 1,6‐bis‐substituted derivative more efficiently, the 1,7‐dibromo‐PDI underwent predominant mono‐debromination, yielding a mono‐substituted PDI along with a small amount of the corresponding 1,7‐bis‐substituted compound. By varying the reaction conditions, a controlled stepwise bis‐substitution of the bromo substituents was also achieved, allowing the direct synthesis of asymmetrical 1,6‐ and 1,7‐PDIs. The compounds were isolated as individual regioisomers. Fullerene (C₆₀) was then covalently linked at the bay region of the newly prepared PDIs. In this way, two separate sets of perylene diimide–fullerene dyads, namely single‐bridged (SB‐1,7‐PDI‐C₆₀ and SB‐1,6‐PDI‐C₆₀) and double‐bridged (DB‐1,7‐PDI‐C₆₀ and DB‐1,6‐PDI‐C₆₀), were synthesized. The fullerene was intentionally attached at the bay region of the PDI to achieve close proximity of the two chromophores and to ensure an efficient photoinduced electron transfer. A detailed study of the photodynamics has revealed that photoinduced electron transfer from the perylene diimide chromophore to the fullerene occurs in all four dyads in polar benzonitrile, and also occurs in the single‐bridged dyads in nonpolar toluene. The process was found to be substantially faster and more efficient in the dyads containing the 1,7‐regioisomer, both for the singly‐ and double‐bridged molecules. In the case of the single‐bridged dyads, SB‐1,7‐PDI‐C₆₀ and SB‐1,6‐PDI‐C₆₀, different relaxation pathways of their charge‐separated states have been discovered. To the best of our knowledge, this is the first observation of photoinduced electron transfer in PDI‐C₆₀ dyads in a nonpolar medium.     

Increase in the coercivity of an ensemble of (DyPr)–(CoFe)–B microparticles during their dispersion in a polymer matrix

In ensembles of annealed ferrimagnetic alloy (DyPr)–(CoFe)–B microparticles in the free state and fixed in polymer, the coercivity differs more than by an order of magnitude. The contributions of orientation locking of magnetic axes of particles and the dipole magnetic interaction between particles to the coercivity and saturation magnetization field of an ensemble are discussed. In ensembles of unannealed microparticles, the effect of their locking and dispersing in polymer is significantly weaker.     

Low-temperature heat capacity upon the transition from paramagnetic to ferromagnetic Heusler alloys Fe<Subscript>2</Subscript> <Emphasis Type="Italic">Me</Emphasis>Al (<Emphasis Type="Italic">Me</Emphasis> = Ti,V, Cr,Mn, Fe,Co, Ni)

The heat capacity of band magnets Fe₂MeAl (Me = Ti, V, Cr, Mn, Fe, Co, Ni) ordered in crystal structure L2₁ has been measured in the range 2 K ≤ T ≤ 50 K. The dependences of the Debye temperature Θ_D, the Sommerfeld coefficient γ, and the temperature-independent contribution to heat capacity C₀ on the number of valence electrons z in the alloys have been determined.