An ab initio SCF and CI study of ketene imine

The electronic structures of the ground and excited states of ketene imine (_H^HCCNH) have been studied by ab initio SCF and CI calculations. The nucleophilic nature of the β carbon with respect to nitrogen has been discussed using calculated electrostatic potentials and by calculated energy differences between the parent and protonated species. The electronically excited ¹A″ and ³A″ states are found to be almost degenerate.     

Bonding properties of bicyclopropylidene: gas phase electron diffraction study and X-ray crystal structure analysis

The C=C double bond length in bicyclopropylidene (1) was found to be 131.4(1) and 130.4(8) pm by electron diffraction at 20° and X-ray diffraction at −40°C respectively. This value is in remarkable agreement with that obtained by molecular mechanics calculations; it is significantly shorter than the double bond in normal olefins.     

Formation constant and stoichiometry of the acetonitrile:18-crown-6 complex by nuclear magnetic resonance, raman and infrared spectroscopy

Crown ethers are increasingly used in a variety of chemical applications. While crown ether complexes with alkali-metal cations have been extensively studied, relatively little is known about their complexes with neutral molecules to form so-called host: guest complexes. The use of NMR is reported for the determination of the formation constant (2.1 +/- 0.1) for the acetonitrile: 18-crown-6 complex. The suitability of Raman spectroscopy for studies of the solid-phase complex is demonstrated and limitations in the use of infrared spectroscopy are discussed.     

Electronic structure of N,N-dimethylnitramine and N,N-dimethylnitrosamine from X-ray and UV electron spectroscopy

The electronic structure of N,N-dimethylnitranine. (CH₃)₂NNO₂, and N,N,-dimethylnitrosamine, (CH₃)₂NNO, has been investigated through the use of electron spectroscopy and quantum chemical methods. Experimentally, the electron spectra of (CH₃)₂NNO₂ and (CH₃)₂NNO were measured in the valence region on the gas phase molecules using UV (HeI and He II) radiation and in the valence and core regions on the solid materials (frozen at 170 K) using X-radiation (AlK_α). Theoretically, the results of INDO and MWH Extended Hucket MO calculations are used for spectral interpretation. The semi-empirical potential model of Ellison is used for predicting is chemical shifts. The results of this investigation exemplify how the UV and X-ray electron spectroscopy techniques are symbiotic and how application of the two techniques along with the use of quantum chemical methods provide unique information about electronic structures.     

Complex formation of 1,4,7,10-tetraoxacyclododecane with alkali metal ions studied by 13C spectroscopy

Complex formation of 1,4,7,10-tetraoxacyclododecane (12-crown-4) with lithium, sodium and potassium salts in methanol solution was investigated. The strong influence of complexing on the chemical shift of the single ¹³C NMR line permitted titration of the ligand with alkali metal salts. Concentration stability constants of the complexes were obtained by a computerized iterative least squares method. Na⁺ and K⁺ form both 1: 1 and 1: 2 complexes, log K₁ = 2.1 and log K₂ = 3.8, log K₁ = 1.7 and log K₂ = 2.4 respectively. Li⁺ is complexed weakly. Assuming 1: 1 stoichiometry the complex stability constant is estimated to be < 1.     

Effects of mixing rules on phase equilibrium calculations

The density dependence in the mixing rule of the cohesion parameter of the cubic equation of state is found not to be necessary in vapour—liquid equilibrium (VLE) calculations. The effects of the functional form and the number of the adjustable parameters of the mixing rules on VLE calculations are examined. It is found that the functional form is less influential in VLE calculations than the number of adjustable parameters and that two adjustable parameters in the mixing rules are sufficient for practical applications.     

Synthesis and characterization by IR spectroscopy and x-ray diffraction of a quinazoline-complex of dibutyldichlorotin(IV)

The synthesis, the X-ray analysis and the IR spectrum of the complex SnBu₂Cl₂(AIP) [AIP = 2-(2′-pyridyl)-3-(N-2-picolylimino)-4-oxo-l,2,3,4 - are reported. The coordination polyhedron can be described as a distorted pentagonal bipyramid. The AIP molecule behaves as a terdentate ONN donor ligand, with the N atoms involved in two equational weaker interactions with respect to the O atom.     

An assessment of an atmospheric pressure helium microwave plasma produced by a surfatron as an excitation source in atomic emission spectroscopy

A new microwave plasma at atmospheric pressure is described. The plasma is supported by a surface wave excitation structure called “surfatron”. Both argon and helium can be used to sustain the plasma. In this study, helium has been selected. The sample introduction system consists of an ultrasonic nebulizer associated with a dcsolvation system. The analytical performance in terms of detection limits, calibration curves and some interferences have been assessed. Because of the relatively low power and the short residence time the main problem seems to be the atomization process.     

Ab initio calculations on large molecules using molecular fragments. Preliminary investigation of ethyl chlorophyllide a and related molecules

Ab initio SCF calculations using the molecular fragment method are reported for four molecules related to chlorophyll a, i.e., free-base porphine, magnesium porphine, magnesium chlorin, and ethyl chlorophyllide a. Molecular orbital structure, the “four-orbital model”, reactivity sites, and Mg···N interactions are discussed.     

The formation and decay mechanisms of HCO in the photodissociation of gas phase acetaldehyde

The kinetic mechanism for the formation and decay of HCO(0,0,0) following flashlamp excitation (10 μs pulse width) into the ¹A″ → ¹A′ absorption transition of gas phase acetaldehyde (0.2 Torr) was examined by time-resolved intracavity laser detection (TRMD) and by phosphorescence lifetime measurements. The HCO radical was found to appear primarily in the vibrationless level reaching a maximum concentration about 250 μs after the excitation of acetaldehyde. The formation rate of HCO(0,0,0) was observed to be insensitive to an order of magnitude change in the number of collisions of excited-state acetaldehyde with either argon, cyclohexane, or the cell wall. Contrastingly, the decay rate of HCO exhibited a strong dependence on the collisional environment. The rate constants for HCO(0,0,0) decay by collisions with acetaldehyde, argon, and cyclohexane and by reaction with O₂ were measured by TRILD. The rate constant for O₂, quenching of ³A″ phosphorescence was also obtained.The potential for HCO(0,0,0) being either a primary or secondary dissociation product is considered in the formulation of a kinetic mechanism describing both the formation and decay behavior observed. Evidence is presented in support of a mechanism in which (1) HCO(0,0,0) is formed by the thermal reaction between acetyl radicals. CH₃CO, and ground-state acetaldehyde after excited-state acetaldehyde undergoes primary dissociation to CH₃CO, and (2) HCO(0,0,0) decays principally by collisionally-induced dissociation at the cell wall.     

A study of infrared absorption in the oxidation of zinc-substituted magnetites to defect phase γ and hematite

The effect of substitution extent on formation of superstructure-ordered vacancies in zinc-substituted lacunar spinels of type $\text{(}\text{Zn}{}^{\mathrm{2+}}{}_{\mathrm{x}}\text{Fe}{}^{\mathrm{3+}}{}_{\mathrm{1?x}}\text{)}{}_{\mathrm{A}}\text{(}\text{Fe}{}^{\mathrm{3+}}{}_{\mathrm{<mtext>(5+x)</mtext><mtext>3</mtext>}}\text{□}{}_{\mathrm{<mtext>(1?x)</mtext><mtext>3</mtext>}}\text{)}{}_{\mathrm{B}}\text{O}{}^{\mathrm{2?}}{}_4$ was investigated using ir spectrometry. Only those lacunar phases whose substitution extent x is less than about 0.3 show a vacancy ordering on octahedral sites. In addition, referring to the disappearance of the 635-cm^?1 absorption band, which is characteristic of these lacunar spinels, we show that the transformation temperature of the γ phases into αFe₂O₃ increases with zinc substitution extent. For the α phase obtained at 700°C we have found a linear variation between the intensity difference of the 390- and 450-cm^?1 absorption bands and the percentage of αFe₂O₃.     

Dynamic Jahn–Teller effect and average structure of dicyclopentadienylcobalt, (C5H5)2Co,studied by gas phase electron diffraction

The molecular structure of (C₅H₅)₂Co has been determined by gas phase electron diffraction. The best agreement between calculated and experimental intensity curves is obtained with a model with eclipsed C₅H₅ rings (symmetry D_5h), but a model with staggered rings (symmetry D_5d) cannot be ruled out. The mean CoC and CC bond distances are 2.119(3) Å and 1.429(2) Å respectively. The average angle between the CH bonds and the C₅ ring is 2.1(0.8)°. The value obtained for the CC vibrational amplitude, l(CC) = 0.055(1) Å, is significantly larger than the amplitude calculated from a molecular force field and the corresponding amplitudes in (C₅H₅)₂Fe and (C₅H₅)₂Ni determined by electron diffraction, and confirms the presence of a dynamic Jahn—Teller effect of the magnitude calculated from ESR data. The average structure is compared with those of the metallocenes of the other first row transition elements.     

Studies on yellow and colourless molybdophosphate complexes in the aqueous solution by laser raman spectroscopy

Yellow and colourless complexes of molybdophosphate were investigated by the use of laser Raman spectroscopy. Two kinds of yellow molybdophosphates were identified in the weakly acidic solutions: 12-molybdophosphoric and 11-molybdophosphoric acid which are in equilibrium in solution at pH 1-2. When excess phosphate is present, the colourless molybdophosphate is formed. This complex exists in the solutions of pH 4-1 under the condition of excess phosphate ([P]/[Mo] > 2). This complex was confirmed to be P₂Mo₅O^6?₂₃ and is so stable that the yellow molybdophosphate is converted into the colourless by excess phosphate.     

Rotational tunnelling of methane on the surface of graphitized carbon blacks studied by neutron scattering spectroscopy

Rotational tunnelling transitions of methane adsorbed on the surface of graphitized carbon black have been observed by incoherent neutron inelastic scattering. Two transitions have been observed, at 58 μeV and 108 μeV, which arise from the barrier to rotation about axes parallel to the surface. The methane molecules undergo rotational diffusion about an axis perpendicular to the surface. The tunnelling transitions are quite sharp at 6 K but become broader as the temperature is raised. At 30 K the motion is best described as isotropic rotational diffusion. The tunnelling spectrum has been observed on two different graphitized carbon blacks, Vulcan III (71 m² g^?1) and Sterling FT (11 m² g^?1). the latter has a more homogeneous surface and gives a sharper spectrum. Tunnelling has also been observed for methane adsorbed on Sterling FT preplated with a monolayer of xenon. When a second layer of methane is added to the first the tunnelling spectrum becomes broad and an inelastic feature appears at higher energy corresponding to a slightly hindered rotation. The value of using tunnelling spectra of adsorbed species to test models of surface forces is discussed with examples of different atom—atom potential parameters.     

The study of molecular motions in substituted ferrocenes by means of NMR spectroscopy

The proton magnetic resonance spectra for ferrocene derivatives have been investigated in the solid state at a frequency of 27.5 MHz and at temperatures within the range 4.2–300 K. Evidence for re-orientation in various substituent groups and cyclopentadienyl rings has been deduced on the basis of the temperature dependence of the second moment of the PMR lines. The experimentally observed decrease in the second moment has been compared with the calculated contributions of different molecular groups to the second moment thus enabling an identification of these groups. In computing the second moment, account has been taken of both intra- and intermolecular dipole-dipole interactions. It is shown that the introduction of substituents into some ferrocene rings affects the magnitude of the re-orientation energy for both the substituted and unsubstituted rings. The change of the re-orientation energy of the substituted and unsubstituted rings in substituted ferrocenes with the structure and electronic properties of the substituents is discussed.     

The molecular structure of p-bis(trimethylsilyl)-benzene from gas phase electron diffraction

Nearly regular tetrahedral silicon bond configuration and a considerably distorted ring characterize the p-bis(trimethylsilyl)benzene molecular geometry according to an electron diffraction study. The SiC_methyl bond is longer than the SiC_phenyl bond, in agreement with expectation but contrary to an X-ray diffraction determination. The extent of ring deformation is consistent with the electropositive character of the trimethylsilyl substituent and with the structural variations in other para-disubstituted benzene derivatives. The electron diffraction data are consistent with either free rotation around the SiC_phenyl bonds or with a rotamer deviating by about 15° from the eclipsed form. The following bond lengths (r_g, pm) and bond angles (°) have been determined with parenthesized estimated total errors: (CC)_mean 140.8(3), (C_ipso)(C_orthoC_meta) 1.6(7), (SiC)_mean 188.0(4), (SiC_methyl)(SiC_phenyl) 3.3(7), (CH)_methyl 111.3(3), CC_ipsoC 115.7(6), and C_phenylSiC_methyl 109.2(4).     

A new microwave plasma at atmospheric pressure

Plasma at atmospheric pressure can be obtained by surface wave propagation with a surfatron. If the plasma is produced within a quartz tube, it is constricted to a diameter of approx. 1 or 2 mm but its length can attain some tens of centimeters with microwave power as low as 100 W. This plasma is quite uniform along the axis, with a typical electron density of 3 × 10¹⁴ electrons/cm³. Since the excitation and gas temperatures are lower than 4000 K, the plasma is far from local thermodynamic equilibrium. High stability and repeatability is achieved with an argon flow of 0.2–17 l/min. Applications are foreseen in the field of optical spectroscopy and plasma chemistry.     

Absorption and emission spectroscopy and magnetic susceptibility of sodium β-alumina doped with Mn,Co, and Ni

The absorption spectrum and magnetic susceptibility vs temperature have been measured for β-alumina doped with Mn, Co, and Ni. In addition, the emission spectra of Mn-doped samples have been measured. Mn and Co are shown to occupy tetrahedral sites and to be in the +2 oxidation state in β-alumina, in agreement with the results of previous investigators. Nickel is shown to occupy both octahedral and tetrahedral sites in β-alumina. Evidence indicates that nickel on octahedral sites is in the +3 oxidation state while that occupying tetrahedral symmetry sites is in the +2 oxidation state. The bands in the absorption spectrum for each metal are assigned and Dq, B, and C are calculated where possible.     

Synthesis,X-ray structure and spectroscopy of Cu(II) complexes derived from diacetylazine dioxime

Complexes of Cu(II) with the polydentate diacetylazine dioxime ligand C₈H₁₄N₄O₂ have been synthesized by template and non-template reactions. The crystal structure of [Cu(C₈H₁₃N₄O₂)]₂[ClO₄]₂(C₂H₅OH)₂ has been determined to establish the conformation of the ligand and the coordination of the oxime function. The structural analysis shows that the nearly planar [C₈H₁₃N₄O₂]^? group behaves as a bridging tetradentate ligand. The distorted square-pyramidal coordination about each Cu atom includes three N donors from two oxime and azine and one O donor from the other oxime, the ethanol molecule occupying the apical position. A perchlorate anion is loosely bonded to Cu. The intramolecular Cu…Cu separation is 3.725(2) Å. The compound crystallizes in the centrosymmetric space group P$\text{1}$ triclinic lattice with refined lattice parameters of a = 11.963(8) Å, b = 9.784(7) Å, c = 7.501(6) Å, α = 90.69(4)°, β = 104.53(4)°, γ = 90.83(4)°, V = 849.7 ų and Z = 2. Full-matrix least-squares refinement of 1490 independent reflexions led to final discrepancy indices, R = 0.059, and R_w = 0.065. The Cu centres are highly coupled as indicated by the ESR spectrum. There is no evidence that the C₈H₁₄N₄O₂ ligand stabilizes unusual oxidation states of Cu.