Infrared studies of conformational isomerism in some gem-chloronitrosoalkanes

The IR spectra of 2-chloro-2-nitrosopropane, 2-chloro-2-nitrosobutane and 2-chloro-3-methyl-2-nitrosobutane have been studied in the gas phase and solid phase, and in solutions of varying polarity. The spectra reveal clear evidence for the hindered internal rotation of the NO functional group about the CN bond in a splitting of the NO stretching band. The spectra can be interpreted as arising from energetically favoured cis conformational isomers, exhibiting the higher of the observed NO stretching frequencies. In each case, assignment of the lower NO stretching frequency is uncertain on the basis of experimental evidence. The results of semi-empirical molecular orbital calculations, however, support the assignment of this feature to the gauche conformational isomers. Temperature dependent IR spectroscopy has been employed in the determination of conformational enthalpy differences in both the gas phase and non-polar solution, which are then compared to the results of semi-empirical molecular orbital calculations.     

Low-tempereture photolysis of diazoketones: the conformational isomerism of benzoylphenylmethylene and the IR spectrum of anthronylidene

Low-temperature photolysis of benzoylphenyldiazomethane, 1, and diazoanthrone, 2, in the polycrystal phase resulted in the high-intensity ESR spectra of benxoylphenymethylene 1a and anthronylidene 2a, respectively. Photolyses of argon matrix-isolated 1 and 2, coupled with IR spectroscopy, have now been carried out. Photolysis of 1 yielded the IR spectrum of diphenylketene as the only IR-observable product and indicated the presence in the matrix of the two s-E, s-Z, isomers of 1. On the other hand, photolysis of 2 yielded the IR spectrum of carbene 2a as the only observable product.     

具有极性大侧基乙烯基聚合物的分子尺寸和均方二极矩

基于旋转异构态模型和生成矩阵代数方法,重新推导了大侧基聚合物均方回转半径和均方二极矩等基础量公式,应用于对聚乙烯咔唑（PVK）、聚对氯苯乙烯（PPCS）、聚乙烯砒啶（P2VP）和聚乙烯砒咯烷酮（PVP）链构象-构型依赖性质的研究.计算得到其无规链的均方二极矩特征比分别为0.55、0.57、0.48和0.37,均方回转半径特征比分别为2.07、1.74、1.11和1.52,全同链温度系数在？6.1×10？3～2.1×10？3K？1之间,范围比间同和无规链大.比较各特征比变化曲线规律,发现PPCS、PVP和P2VP的基础量分别受聚合物聚合度、规整程度和构象能的影响相对比较明显,而大侧基使PVK的均方二极矩和均方回转半径特征差异显著;对照改进的计算结果表明,在计算均方二极矩和回转半径时考虑极性侧基的方向和尺寸是十分重要的,在聚合物极性和柔性分析中不能忽略极性大侧基的影响.     

A dipole moment study of the conformational properties of diaryl diselenides and related compounds

The electric dipole moments of the diaryl diselenides (RC₆H₄)₂Se₂ (R  H, 4-F, 4-Br, 4-CH₃, 3-F) were measured in benzene solution at 25 and 45°C. The conformations of these compounds were deduced by matching experimental moments with values calculated for a variety of possible conformations. In the dissolved state the diselenides exist at 25°C in fixed “skew” conformations characterized by dihedral angles of 75–106° between the CSeSe planes, corresponding to the conformational energy minima. At 45°C oscillations about the SeSe bonds are excited in the diphenyl and bis(4-methylphenyl) diselenides, whereas the 4-bromophenyl derivative exhibits free rotation. The fluoro compounds have temperature-independent dipole moments, suggesting “rigid conformations” with dihedral angles of 106° (4-F) and 74.4° (3-F). An analysis of the dipole moments at 25 and 45°C obtained for the compounds (RC₆H₄)₂X₂ (R  H, 3-F, 4-F, 4-Br, 4-CH₃; X  S, Se, Te) showed that the conformational properties of these derivatives change on passing from X  S to X  Te. The observed variations are explicable in terms of a decreasing repulsion between the lone electron pairs of the chalcogen atoms on going from the disulfides to the ditellurides and a concomitant reduction of the energy barrier to rotations about the XX bonds.     

Theoretical and infrared studies on the conformational isomerism of trans-2-bromo-alkoxycyclohexanes

The infrared spectra of trans-2-bromo-alkoxycyclohexanes (alcoxy = OMe, OEt, O(i)Pr and O(t)Bu) were obtained for the neat liquid, and the C-Br stretching mode was quantitatively analyzed to give insight about the conformational isomerism of these compounds. Frequency calculations supported the band assignments, and the relative band intensities suggest that the diaxial conformer is prevalent for the methoxy and tert-butoxy derivatives (51 and 56%, respectively), while the diequatorial form is preponderant for the ethoxy and isopropoxy derivatives (76 and 77%, respectively). Therefore, the size of the alkoxy group plays a determinant role in determining the conformational preferences of the title compounds.     

Investigation of analogs of conformational inhibitors of pyridoxal enzymes by the dipole moment method

Conclusions The dipole moments of isoxazolidone-3 and the methyl ether of acethydroxamic acid have been determined. The hydroxamide group of the latter exists in the trans-conformation in solution.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 12, pp. 2754–2757, December, 1967.     

Conformational analysis of organophosphorus compounds by means of IR spectroscopy and dipole moment methods

Some aspects of rotational isomerism about an ordinary P—X bond in compounds

have been discussed on the basis of spectroscopic data. Changes in spectra in the process of phase transition with variation of dielectric permeability of the environment and temperature have been analyzed. A scheme of structure identification by the dipole moment method with determination of conformer concentration from IR-spectra has been proposed. Data from IR-spectra, dipole moments and the Kerr effect have been applied to complicated systems.Gauche- and trans-orientated O—C and PO bonds are characteristic of compounds for which X = O. It has been shown that in the case of rotation of a chloromethyl group at the phosphorus atom trans and gauche arrangements of C—Cl and PO (S) are stable. The position of the conformational equilibrium depends on the nature of the substituents. The important role of electrostatic interaction has been established.     

Inductive effect of substituents on the dipole moment of α-substituted vinyl methyl ether

Dipole moments of several α-substituted vinyl methyl ethers R(OMe)C:CH₂; R = Me, Et, i-Pr, t-Bu, cyclopropyl, vinyl, Ph) have been determined by the Halverstadt-Kumler method in benzene solution at 293 K. The square of the total dipole moment μ_r was found to be a linear function of the Taft's inductive constant σ_r^*: μ_r²/D²=(0.619±0.033)+(1.092±0.10) σ_r^*. The inductive contribution of the substituent R on the total dipole moment may be expressed by the equation μ_j/D = ?0.52 σ^* + 0.25. This is in good agreement with the corresponding equation for the dipole moments of alkyl-substituted ethenes: μ_i/D = ?0.58 σ^* + 0.28 (based on dipole moments obtained by PCILO calculations).     

IR spectral and quantum-chemical studies of binary extractants

The composition and ionic character of formation of binary extractants based on quaternary ammonium bases and organic acids were confirmed using spectral and quantum-chemical methods, the structures of the binary extractants were obtained. The energy of bond between cation and anion in molecules of the binary extractants was determined, it is by an order of magnitude higher than the energy of H bond and close to the energy of chemical (ionic) bond.     

Ab initio studies on the vibrational spectra and conformational isomerism of 2,4-dimethyl-1,3-pentadiene

Ab initio was used to study the structure of various conformational isomers and their vibrational spectra of 2,4-dimethyl-1,3-pentadiene (2,4-PD) in detail. Two stable conformations, s-trans and s-cis, were found in which s-trans is more stable. The geometry of stable conformations and charge distributions were studied, and the effect of different basis sets on geometry optimization is discussed. The results of complete optimization indicate that molecular skeleton is nearly in a plane, its largest deviation is only 0.3 degrees. Therefore, it is reasonable and available to hypothesis that the molecule has Cs symmetry. The thermal dynamics conformations were calculated and compared with experimental values. DeltaH(o) between two conformations of 2,4-PD measured from experiment is 4.36 kJ/mol, deltaS(o) is 2.56 J/mol K., calculated results are slightly different from experimental ones. Vibrational frequencies of 2,4-PD conformers have been studied by ab initio molecular orbital calculations using different basis set. The calculated vibrational frequencies are analyzed and compared with the experimental spectra.     

Tautomeric and conformational isomerism of natural hydroxyanthraquinones

Natural 1,5-di-, 1,4,5-tri-, and 1,4,5,8-tetrahydroxyanthraquinones and their anions and metal complexes were shown to be equilibrium mixtures of tautomers and conformers using quantum-chemical and correlation analysis of elecronic absorption spectra. Solvent effects, ionization, complexation, and the introduction and substitution of substituents were accompanied by shifts of tautomeric and conformational equilibria that determine the color of the compounds. __________ Translated from Khimiya Prirodnykh Soedinenii, No. 3, pp. 224–229, May–June, 2006.     

Rotational isomerism of vinyl ethers of azines based on the data of IR spectroscopy and quantum-chemical AM1 calculations

The IR spectra of vinyl ethers of azines have two C=C stretching vibration bands of the vinyl group at 1620 and 1640 cm⁻¹, which correspond to thes-cis and nonplanars-trans conformers, respectively. According to the results of AM1 calculations, the hetaryl fragment and the vinyl group in thes-trans conformers of all the ethers under study are twisted out of the C−O−C plane by ∼30°. In thes-cis conformation, the angle of rotation of the heterocycle with respect to the vinyloxy group varies from 45 to 90°. The character of the change in the intensity of the v(C=C) absorption bands of the doublet under changes of the temperature indicates that thes-trans conformer is energetically favorable. The exception is 5-vinyloxyacridine for which thes-cis conformation is more favorable. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2439–2442, December, 1998.     

Gas-phase 21Ne NMR studies and the nuclear magnetic dipole moment of neon-21

Gas-phase ²¹Ne nuclear magnetic resonance spectra were measured at the natural abundance of ²¹Ne isotope for samples consisting of pressurized neon up to 60 bar at room temperature and applying the magnetic field of the strength B₀ = 11.7574 T. It showed that the nuclear magnetic resonance frequency is linearly dependent on the density of gaseous neon. The resonance frequency was extrapolated to the zero-density point, and it permitted the determination of the ²¹Ne nuclear magnetic moment, μ(²¹Ne) = 0.6617774(10) μ_N. The present value of μ(²¹Ne) is not influenced by the bulk magnetic susceptibility of neon and interactions between neon atoms; therefore, it is more precise and reliable than the previous result obtained for μ(²¹Ne).     

Copper-catalyzed synthesis of vinyl sulfides

[reaction: see text] We report a method for the synthesis of vinyl sulfides using the soluble copper(I) catalyst [Cu(phen)(PPh3)2]NO3. The desired vinyl sulfides are obtained in good to excellent yields, with retention of stereochemistry. This protocol tolerates a wide variety of functional groups or substrates, is palladium-free, and does not require the use of expensive or air-sensitive additives.