Magnetic surface levels

As is well known, the energy spectrum of conduction electrons in a metal in a magnetic field is split into the Landau levels. These levels give rise to several phenomena whose essence is in the oscillatory dependence of some property of the metal characteristics on the strength of the magnetic field, in the range of strong and medium fields. Of these the most famous are the de Haas-van Alphen effect, and the Shubnikov-de Haas effect. Electronic transitions between the Landau levels give rise to the cyclotron resonance.

In a surface layer of a metal placed in a weak magnetic field another system of levels appears for electrons moving along shallow arcs, with their ends resting upon the surface of the metal. These levels originate from the quantized periodic motion of electrons along such ‘skipping’ trajectories due to a specular reflection at the metal surface. The spectrum of the system of magnetic surface levels manifests itself in an oscillatory dependence of the surface impedance on a weak magnetic field. The oscillations are due to a resonant absorption of microwave radiation in transitions of electrons between the magnetic levels occurring at discrete values of the magnetic field.

This new quantum effect discovered in several metals in both the normal and superconducting states should, in principle, be common to all the conductors. Studies of the effect are being extended rapidly, and one foresees a discovery of some new phenomena due to the surface bound states of charged quasiparticles, arising in a conductor when exposed to a weak magnetic field.     

Polymer hydrogels with the memory effect for immobilization of drugs

Hydrogels with the memory effect are synthesized from crosslinked copolymers of 2-hydroxyethyl methacrylate and functional monomers (acrylic acid or dimethylaminoethyl methacrylate) via the method of template synthesis with the use of the drug cephazoline as a matrix. It is shown that the hydrogels show an increased sorption activity against the target drug and a slow rate of drug release from the hydrogel (the memory effect) under certain conditions. In combination with cephazoline, these hydrogels may be regarded as a new prolonged-drug-release system for the treatment of infectious diseases.     

Molecular structures of acetylene derivatives of tin

The geometry and force fields of the bis(trimethylstannyl)acetylene molecule (a conformer withD _3d symmetry corresponding to a minimum of the total energy of the molecule) were calculated by the RHF and MP2(fc) methods. The effective core potential in SBK form with the optimized 31G^* valence basis set was employed in the case of Sn atoms. The 6–31G^** and 6–311G^** basis sets were used for carbon and hydrogen atoms. Vibrational spectra of the light and perdeuterated isotopomers of bis(trimethylstannyl)acetylene were interpreted using the procedure of scaling the quantum-chemical force fields. For Part 5, see Ref. 1. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 616–626, April, 2000.     

Efficiency of post-Hartree-Fock force field for the interpretation of vibrational spectra ofN,N-dimethylnitramine

Harmonic force fields for the molecule ofN,N-dimethylnitramine were calculated in the RHF/6-31G^* and MP2/6-31G^** approximations. Scaling of the force fields obtained made it possible to reliably interpret the vibrational spectra of light and perdeuterated compounds reported in the literature. The assignment is confirmed by good reproducibility of experimental isotope shifts upon¹⁵N-amino- and¹⁵N-nitrosubstitution. The frequencies of intramolecular vibrations in far IR and Raman spectra as well as in neutron inelastic scattering spectra for the light and perdeuterated samples of solidN,N-dimethylnitramine were identified using the force field calculated with the inclusion of electron correlation (MP2). Although general structures of the force fields calculated in the RHF and MP2 approximations are similar, considerable differences in the force constants of the NO and NN stretching vibrations and especially in the constants of the NO_str/NO_str and NO_str/NN_str interactions remain even after scaling the force fields.     

Gas phase electron diffraction study of tetravinyltin Sn(CHCH2)4

The tetravinyltin molecule has been studied by gas phase electron diffraction. The r_a structure analysis is based on the assumptions that a single conformer occurs in Sn(CHCH₂)₄ and that tin has a tetrahedral bond configuration. The preferred model (S₄ symmetry) predicts all four vinyl groups to be intermediate between staggered and eclipsed conformations. The structure refinement gives the following parameters (bond lengths, r_a, in nm, valence angles in degrees):

, SnCC = 121.9(0.6), CC = 0.1349(8), SnCH = 116.8(4.5),<C—H?_av. = 0.1098(14). The uncertainties given in parentheses represent three times the standard deviation values.The observed shortening of the

bond in Sn(CHCH₂)₄ from

in SnMe₄ (by 0.0027 nm) is equal to the shortening that occurs on going from

in ethane to

in propylene. With the corresponding Si and Ge derivatives, this effect is less pronounced.     

Nonempirical quantum chemical calculation for nitramide,its chloro- and methyl-substituted derivatives. I. Structure of equilibrium forms

We have carried out quantum chemical calculations for a series of N-nitramines (H₂NNO₂, MeNHNO₂, CINHNO₂, MeNClNO₂, Me₂NNO₂) using the GAUSSIAN-90 program by the restricted Hartree—Fock method, taking into account electron correlation according to Møller—Plesset second-order perturbation theory in a standard 6–31G^* basis. In this paper, we present the geometric parameters of the equilibrium forms of the molecules obtained as a result of full optimization. The calculation allowed us to establish that with an increase in electronegativity of the substituents on the amine nitrogen atom, we observe coordinated lengthening of the NN bond, shortening of the N=O bond, and increase in the pyramidal character of the configuration of the bonds of the amine nitrogen atoms. We observed a close to linear relationship between the length of the NN bond and the magnitude of the angle of deviation of this bond from the plane of the ONO atoms. The greatest difference in NN bond lengths (0.06 Å) occurs between the Me₂NNO₂ and ClNHNO₂ molecules.Moscow State University. Translated from Zhurnal Strukturnoi Khimii, Vol. 34, No. 1, pp. 4–11, January–February, 1993.     

Molecular structures of acetylene derivatives of tin: Part II. Gas phase electron diffraction study ofbis(trimethylstannyl)acetylene,Me3SnCCSnMe3

A gas phase electron diffraction study of bis(trimethylstannyl)acetylene is reported. The r_a structure refines to the following parameters (bond lengths in nm, valence angles in degrees):

The numbers in parentheses are three times the standard deviation values.The difference between the

bond lengths in the bis(trimethyl-stannyl) acetylene molecule is less significant than with trimethylstannylacetylene. The observed departure from linearity in the Sn-CC-Sn fragment is seemingly due to the shrinkage effect.     

Ab initio investigation of tautomeric stability,molecular structure,and internal rotation of methylphosphonic dicyanide,methoxydicyanophosphine, and their isocyano analogs

The equilibrium geometric parameters and structures of the transition states of internal rotation for MeP(O)(CN)₂, McOP(CN)₂, and their isocyano analogs, MeP(O)(NC)₂ and MeOP(NC)₂, have been calculated by theab initio SCF method and with inclusion of electron correlation effects according to the second-order Muuller-Plesset perturbation theory (MP2). At both levels the 6-31G* basis set has been used. The estimation of relative stability of these tautomeric forms depends largely on the calculation level. The total energies of the cyanides calculated by the MP2 method are 25–30 kcal mol^–1 lower than those of the corresponding isocyanides. The oxo-tautomeric forms containing four-coordinate phosphorus are 15–25 kcal mol^–1 more stable than the three-coordinate phosphorus aci-derivatives. The internal rotation potential curves of the aci-forms are characterized by a deep minimum for thetrans-arrangement of the methoxy group and phosphorus lone electron pair. Two additional less clearly pronounced minima are located symmetrically on both sides of the weak maximum, which corresponds to thecis-arrangement. The equilibrium oxo-form structures have a staggered configuration of the methyl group with respect to the phosphorus atom bonds.Translated from izvestiyaAkademii Nauk. Seriya Khimicheskaya, No. 5, pp. 1104–1115, May, 1996.