Approximate ab initio energies by systematic molecular fragmentation

A scheme is introduced for generating a hierarchy of molecular fragmentations by which the total electronic energy can be approximated from the energies of the fragments. Higher levels in the hierarchy produce molecular fragments of larger size and approximate the total electronic energy more reliably. A correction to account for nonbonded interactions is also presented. The accuracy of the approach is tested for a number of examples, and shown to be essentially independent of the level of ab initio theory employed. The computational cost increases linearly with the size of the molecule.     

A study of the special features of stress relaxation in irradiated polytetrafluoroethylene

As a result of experimental studies of stress relaxation in polytetrafluoroethylene irradiated with various doses (from 2×10⁴ to 1×10⁶ rads), data have been obtained which make it possible to judge the rate of occurrence of relaxation processes in the polymer. A number of special features have been revealed which distinguish the character of occurrence of relaxation processes in the irradiated and nonirradiated material.Plastics Scientific-Research Institute, Moscow. Translated from Mekhanika Polimerov, No. 1, pp. 148–150, January–February, 1972.     

Synthesis and antiviral activity of nucleoside analogs based on 1,2,4-triazolo[3,2-<Emphasis Type="Italic">c</Emphasis>][1,2,4]triazin-7-ones

Nucleoside analogs containing hydroxybutyl, hydroxyethoxymethyl, allyloxymethyl, and propargyloxymethyl fragments were synthesized based on 1,2,4-triazolo[3,2-c][1,2,4]triazin-7-ones isosteric to purine bases. Some of the compounds obtained inhibit in vitro reproduction of influenza and respiratory syncytial virus infection.     

Selective (15)N-labeling and analysis of (13)C-(15)N J couplings as an effective tool for studying the structure and azide-tetrazole equilibrium in a series of tetrazolo[1,5-b][1,2,4]triazines and tetrazolo[1,5-a]pyrimidines

Two general methods for the selective incorporation of an (15)N-label in the azole ring of tetrazolo[1,5-b][1,2,4]triazines and tetrazolo[1,5-a]pyrimidines were developed. The first approach included treatment of azinylhydrazides with (15)N-labeled nitrous acid, and the second approach was based on fusion of the azine ring to [2-(15)N]-5-aminotetrazole. The synthesized compounds were studied by (1)H, (13)C, and (15)N NMR spectroscopy in both DMSO and TFA solution, in which the azide-tetrazole equilibrium is shifted to tetrazole and azide forms, respectively. Incorporation of the (15)N-label led to the appearance of (13)C-(15)N J coupling constants (J(CN)), which can be measured easily using either 1D (13)C spectra with selective (15)N decoupling or with amplitude modulated 1D (13)C spin-echo experiments with selective inversion of the (15)N nuclei. The observed J(CN) patterns permit unambiguous determination of the type of fusion between the azole and azine rings in tetrazolo[1,5-b][1,2,4]triazine derivatives. Joint analysis of J(CN) patterns and (15)N chemical shifts was found to be the most efficient way to study the azido-tetrazole equilibrium.     

Study of thin Ge films with amorphous and nanocrystalline phases via the techniques of EXAFS spectroscopy and AFM

The local atomic structure and surface morphology of thin semiconductor films of Ge have been studied via extended X-ray absorption fine structure spectroscopy and atomic force microscopy. The films have been obtained by thermal evaporation of a material in an ultrahigh vacuum at different substrate temperatures. The films contain both amorphous and nanocrystalline phases. The percentage of the phases depends on the condensation temperature. The classical linear dependence of grain sizes on condensation temperature T is violated at T=100°C.     

Spin-spin coupling constants 13C-15N and 1H-15N in the investigation of azido-tetrazole tautomerism in a series of 2-azidopyrimidines

A new method was developed for the investigation of an azido-tetrazole equilibrium based on using a complex analysis of ¹³C-¹⁵N and ¹H-¹⁵N spin-spin coupling constants. The use of this approach became possible due to the selective inclusion of ¹⁵N isotopes into the structures of 2-azidopyrimidines and their cyclic analogs tetrazolo[1,5-a]pyrimidines.     

Properties and structural features of nanodisperse powder fillers

Comparative characteristics of polymer materials based on epoxy binder reinforced by disperse and fiber fillers are given. The structural features of nanopowders of aluminum oxide and copper are considered with the aim to use them as nanofillers of polymer materials. The dependence of structures of nanopowders under investigation on the change of their specific surface is shown. The fractional composition of aluminum oxide is analyzed, and particle sizes of fine and coarse fractions are found.     

Effect of the substrate crystalline structure on the magnetic, electrical, and crystallographic characteristics of La0.35Nd0.35Sr0.3MnO3 epitaxial films

A study has been made of the electrical resistivity ρ, magnetoresistance Δρ/ρ, and magnetization of La_0.35Nd_0.35Sr_0.3MnO₃ epitaxial films on ZrO₂(Y₂O₃), SrTiO₃, LaAlO₃, and MgO substrates. The first film can exist in four equivalent crystallographic orientations in the sample plane, while the other three have only one orientation. The maxima of ρ and Δρ/ρ of the first film are broadened considerably in the vicinity of the Curie point T _C compared to the three others, the magnitude of ρ itself being larger by 1.5 orders of magnitude, and a large negative magnetoresistance (|Δρ/ρ| ～ 10% in a field of 8.4 kOe) is observed at temperatures 80≤T≤200 K. In all films, the magnetic moment per molecule at 5 K is ～46% smaller than the pure spin value, due to the existence of magnetically disordered regions. The larger value of ρ of the film deposited on ZrO₂(Y₂O₃) is due to the electrical resistance of the boundaries separating regions with different crystallographic orientations, and the magnetoresistance is associated with polarized carriers tunneling through the boundaries coinciding with domain walls. The low-temperature magnetoresistance in fields above technical saturation is caused by the strong p-d exchange coupling within spin-ordered regions.