Molecular structure of (cyanomethoxy)(dimethylamino)methane as studied by gas-phase electron diffraction and quantum-chemical calculations

The molecular structure of (cyanomethoxy)(dimethylamino)methane in the gas phase is investigated by means of gas-phase electron diffraction and quantum-chemical studies for the first time. It is shown that in the gaseous state, (cyanomethoxy)(dimethylamino)methane is a mixture of gauche-anti (65%) and anti-gauche (35%) conformers. The slight influence of the anomeric effect on the compound’s structural parameters was noted.     

3-Cyclopropyl-1,2-dimethyldiaziridine: synthesis and study of molecular structure by gas electron diffraction method

The molecular structure and conformational behavior of 3-cyclopropyl-1,2-dimethyldiaziridine have been for the first time experimentally studied by gas-phase electron diffraction and quantum chemical calculations. The two most stable conformers at 298 K possess anti and gauche mutual ring orientation (with prevalence of the anti conformer) whereas only one anti conformer is observed in solution. The determined structural parameters of gaseous 3-cyclopropyl-1,2-dimethyldiaziridine have been compared with those for 3,3-bidiaziridine structural analogues in the crystal phase. The simple and convenient procedure for the synthesis of 3-cyclopropyl-1,2-dimethyldiaziridine comprising cyclopropane and diaziridine rings in one molecule was developed. The standard enthalpy of formation of 3-cyclopropyl-1,2-dimethyldiaziridine in the gas phase was calculated using Gaussian-4 theory, yielding value of 281.9?±?5.0 kJ/mol.     

Semiexperimental equilibrium structure of 1-methylisatin from gas-phase electron diffraction data and structural changes in isatin due to 1-methyl and 5-fluoro substituents as predicted by coupled cluster computations

ABSTRACT

The molecular structure of 1-methylisatin (1) has been studied by gas-phase electron diffraction (GED) and quantum chemical computation up to the coupled cluster (CCSD(T)) level of theory. The semiexperimental equilibrium structure (C_s point group symmetry) has been determined from the GED data taking into account anharmonic vibrational corrections calculated in curvilinear coordinates with the B2PLYP force field. To observe fine structural effects due to the presence of different kinds of substituents, the accurate molecular structures of isatin (2) and 5-fluoroisatin (3) were also computed at the CCSD(T) level. The (O=)C–C(=O) carbon–carbon bonds of the pyrroline moiety in 1–3 are found to be remarkably longer than the typical single C–C bond. The electron donating methyl group causes a decrease of the C?N?C angle and an elongation of the N?C bond lengths in the pyrroline ring by 0.7° and up to 0.008?Å, respectively, whereas the electron withdrawing fluorine atom increases the ipso CCC angle by 2.5° in comparison to that in unsubstituted isatin.     

Mechanism of Photoinduced Addition of Water and Methanol to the Double Bond of 1,2-Dihydroquinolines

The kinetics and mechanism of photoinduced addition of water and methanol to the double bond of 2,2,4-trimethyl-1,2-dihydroquinoline (DHQ) bearing different substituents in the 6- and 8-positions and N-methylated DHQ were studied by flash photolysis. The reaction affords corresponding Markovnikov adducts, 4-hydroxy- and 4-methoxy-1,2,3,4-tetrahydroquinolines. On the basis of time-resolved experiments and the measurement of quantum yields of the reaction in media of different acidities or basicities, a general scheme of the reaction is suggested, which involves the formation of two intermediate species. The principal rate constants, activation parameters, and their dependence on substituents were determined.     

Upgraded X-ray topography and microtomography beamline at the Kurchatov synchrotron radiation source

An upgraded X-ray Topography and Microtomography (XRT-MT) station is described, the parameters of the optical schemes and detectors are given, and the experimental possibilities of the station are analyzed. Examples of tomographic reconstructions are reported which demonstrate spatial resolutions of 2.5 and 10 μm at fields of view of 2.5 and 10 mm, respectively.     

Thermal strains in focusing channels of the stations for X-ray diffraction analysis in the Sibir-2 storage ring

The thermal load caused by the absorption of synchrotron radiation in X-ray optical elements of the Belok and RSA stations leads to optics elements heating and induces strains upon simultaneous cooling. The heating of the cooled first crystal in the double-crystal monochromator causes its bending and increases the reflected beam divergence, which, in turn, results in the monochromatic beam intensity loss [1]. Numerical simulation makes it possible to more accurately determine the strains, choose the optimal monochromator design, estimate the vertical sizes of the focal spot and wavelength resolution in the focusing channel, correctly design the system for cooling the mirror at the channel input, and choose a design providing the minimum temperature of the beam-limiting slit knives.     

Millimetric electromagnetic radiation of a lightning return stroke

The paper reports results of recording the pulsed microwave radiation of return lightning strokes using a high–time–resolution, 8–mm–wavelength radiometer. The procedure of recording pulsed radiations with millimetric wavelength is briefly described. The complex shape of the recorded pulses is shown to be due to the features of the current system within the antenna spot at the initial stage of a lightning return stroke.     

Doppler-shifted cyclotron resonance in tungsten plates with atomic pure surface

The surface resistance of thin monocrystalline W plates as a function of the constant magnetic field H directed along the normal to the sample surface is studied in the r.f. spectrum region. The sample surface was cleaned in high vaccum (10^-11 torr) or coated with the monomolecular impurity film. The oscillating with the magnetic field part R_osc due to the Doppler-shifted cyclotron resonance is studied. The doppleron oscillation amplitude is found to depend on the surface state and increases with the crystal cleaning. The observed changes are caused by the increase of the specular reflection coefficient for resonance electrons. With the deviation of the magnetic field from the normal to the plate surface, the doppleron wave undergoes a collisionless magnetic Landau damping and the signal amplitude decreases down to values comparable with that of Gantmakher-Kaner oscillations. Cleaning of the surface (and related increase of specularity) gives rise to a further decrease of the doppleron amplitude and appearance of additional interference maxima induced by the Gantmakher-Kaner effect.     

Gas phase electron diffraction analysis andab initio calculations of the molecular structure of 3,3-dimethyl-1-oxa-3-silacyclopentane

3,3-Dimethyl-1-oxa-3-silacyclopentane was synthesized, and its molecular structure was determined by gas phase electron diffractometry using ab initio calculations (GAUSSIAN-92, RHF method, 6-31G^* basis set). The six models derived from the gas phase electron diffraction analysis equally adequately (R≈3%) describe experimental data, whereas quantum chemical calculations lead to only one (O-envelope) conformer. The stereochemical data and the results of ab initio calculations confirm that the O-envelope is the best conformation; its geometrical parameters are as follows (bond lengths r_a in Å, and angles in degrees; total error in parentheses in units of the least significant digit): r(Si-C_cyc) 1.891(3) and 1.904; r(Si-C_Me) 1.877 and 1.879; r(C-C) 1.556(7); r(C-O) 1.431(3) and 1.445; r(C_cyc-H) 1.11(2); r(C_Me-H) 1.10(2); ϕ(O−C−C−Si) −33.0(14); ∠C_cycSiC_cyc 92.2(5); −33.0(14); ∠C_MeSiC_Me 107.2(8); ∠SiCC 102.6(6); ∠CCO 109.9(11); ∠HC_cycH 110.5(47); ∠SiC_MeH 111.6(14). Method for the synthesis of 3,3-dimethyl-1-oxa-3-silacyclopentane is given, and generalized notation for five-membered ring confor-mations is suggested. Moscow State University. Tromse University, Norway. Irkutsk Institute of Organic Chemistry, Siberian Branch, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 37, No. 4, pp. 689–707, July–August, 1996. Translated by L. Smolina