Experimental and theoretical molecular and electronic structures of the N-oxides of pyridazine,pyrimidine and pyrazine

The structures of pyridazine N-oxide, pyrimidine N-oxide and pyrazine N-oxide have been determined by X-ray diffraction for the first time. Comparison with theoretical predictions of the equilibrium structures using the B3LYP method together with a cc-pVTZ basis set, show close agreement with the structural parameters observed, and experimental dipole moments, which suggests that the charge distribution is realistic. An ‘atoms in molecules’ (AIM) analysis of the computed wave-functions shows total electron densities rather different from the classical picture of a dative bond, whereas the same wave-functions subjected to Mulliken analysis show a more conventional view of the electron distribution. This latter procedure allows a bond dipole analysis of the N-oxide charge distribution.     

Nuclear quantum effects in calculated NMR shieldings of benzene; a Feynman path integral study

The Feynman path integral Monte Carlo approach has been coupled to the gauge including atomic orbital formalism in order to analyse the absolute magnetic shieldings of the benzene nuclei under the conditions of thermal equilibrium. The Hamiltonian employed in the derivation of ensemble averaged NMR quantities is of the Hartree-Fock type. The basis set used is of 6–31G quality. The spatial delocalization of the atoms leads to a deshielding of both types of benzene nuclei relative to the shieldings experienced at the minimum of the potential energy surface. This deshielding has to be traced back to bond length elongations in thermal equilibrium. The influence of the nuclear fluctuations on the NMR parameters of benzene is quantum driven up to temperatures of 400 K; classical fluctuations are of minor importance in this low-temperature window.     

High-resolution infrared and theoretical study of four fundamental bands of gaseous 1,3,4-oxadiazole between 800 and 1600 cm

The Fourier transform infrared spectrum of gaseous 1,3,4-oxadiazole, C₂H₂N₂O, has been recorded in the 800–1600 cm⁻¹ wavenumber region with a resolution around 0.0030 cm⁻¹. The four fundamental bands ν₉(B₁; 852.5 cm⁻¹), ν₁₄(B₂; 1078.5 cm⁻¹), ν₄(A₁; 1092.6 cm⁻¹), and ν₂(A₁; 1534.9 cm⁻¹) are analyzed by the standard Watson model. Ground state rotational and quartic centrifugal distortion constants are obtained from a simultaneous fit of ground state combination differences from three of these bands and previous microwave transitions. Upper state spectroscopic constants are obtained for all four bands from single band fits using the Watson model. The ν₄ and ν₁₄ bands form a c-Coriolis interacting dyad, and the two bands are analyzed simultaneously by a model including first and second order Coriolis resonance using the ab initio predicted Coriolis coupling constant . An extended local resonance in ν₂ is explained as higher order b-Coriolis type resonance with ν₆ + ν₁₀, which is further perturbed globally by the ν₁₅ + ν₁₀ level. A fit of selected low-J transitions to a triad model including ν₂(A₁), ν₆ + ν₁₀(B₁), and ν₁₅ + ν₁₀(A₂) using an ab initio calculated Coriolis coupling constant is performed.The rotational constants, ground state quartic centrifugal distortion constants, anharmonic frequencies, and vibration–rotational constants (α-constants) predicted by quantum chemical calculations using a cc-pVTZ and TZ2P basis with B3LYP methodology, are compared with the present experimental data, where there is generally good agreement. A complete set of anharmonic frequencies and α-constants for all fundamental levels of the molecule is given.     

A neutron scattering and nuclear magnetic resonance study of the structure of GeO2-P2O5 glasses

Germanophosphate (GeO2-P2O5) glasses were studied with neutron diffraction, phosphorus, and oxygen nuclear magnetic resonance, calorimetry, viscosity measurements, and first-principles calculations. These data sets were combined to propose a structural model of GeO2-P2O5 glasses, which includes tetrahedrally coordinated phosphorus, formation of octahedrally coordinated germanium as P2O5 content increases, an absence of trigonally coordinated oxygen, and hence an absence of rutile-like GeO2 domains. The structural model was then used to propose explanations for both the observed composition dependence of the glass transition temperature and the fragility of the GeO2-P2O5 liquids.     

Clustering of rare earths in GeAs sulfide glass

Clustering of rare-earth dopants in GeAs sulfide glasses was studied by fluorescence spectroscopy of Pr-doped glasses and by EPR measurements of Gd-doped samples. The linewidth of the g  2 resonance of Gd³⁺, as well as the relative intensity of emission from the ¹D₂ level of Pr³⁺, was used as a relative measure of rare-earth clustering. Rare earths were found to have low solubility in uncodoped GeAs sulfide glasses, which also displayed poor fluorescence efficiency due to severe clustering. Codoping such glasses with Ga greatly enhanced rare-earth solubility and dispersal, particularly for Ga:rare earth ratios ≥ 10:1, as evidenced by the narrower EPR resonances and more intense luminescence of Gd- and Pr-doped glasses, respectively. In, P and Sn were also observed to ‘decluster’ rare earths, although less efficiently than Ga, whereas codoping with I was found to have no effect on clustering. These phenomena are explained by a structural model in which (1) rare-earth dopants and codopants are spatially associated and (2) rare-earth dispersal is accomplished by a statistical distribution of codopants in tetrahedral network sites.     

7-Selenabicyclo[2.2.1]heptane

Thermolysis of a benzene solution of N-[4-(p-(methoxybenzyl)seleno)cyclohexanoyl]-N,S-dimethyldithiocarbonate affords the hitherto unknown 7-selenabicyclo[2.2.1]heptane in 48% conversion and in 20% yield after chromatography. G3(MP2)-RAD calculations predict a rate constant of 5 × 10(4) s(-1) at 80 °C (3.8 × 10(6) s(-1) at 200 °C) for the intramolecular homolytic substitution process involved in this cyclization.     

Synthesis of Amino Acid Derived Cyclic Phosphorus Ylides

Abstract

We recently reported the thermal elimination of Ph₃PO from suitably protected aminoacyl ylides 1 as a route to acetylenic amino acid analogues 2. Pyrolysis of ylides such as 3 with a free amino group takes a different course. Ethanol is eliminated to give the c h i d cyclic ylides 4 which can be viewed as 3-triphenylphosphoranylidene tetramic acids. Specific examples prepared include 4 (R = Me, Pri), the parent compound 5 (from glycine) and the six-membered ring compound 6 (from p-alanine). Using a similar approach, bicyclic ylides such as 7 (from proline) can be prepared. In the case of the glutamate derived ylide 8, thermolysis initially gives a mixturc of 9 and 10 but these both cyclise to the bicyclic product 11 with time. The structure and reactivity of these interesting cyclic ylides are now being examined.     

Preparation and spectroscopic studies of some cyclic urea adducts of triphenyl-tin and -lead halides

1,3-Dimethyl-2-imidazolidinone (dimethylethylene urea, DMEU) and 1,3-di- methyl-3,4,5,6-tetrahydro-2(IH)-pyrimidinone (dimethylpropylene urea, DMPU) adducts of the type Ph₃SnX·L (X = Cl, Br and I), Ph₃PbX·L (X = Br, I), 3Ph₃PbCl·2DMEU and 2Ph₃PbCl · DMPU have been prepared and characterized. Assignments are made for ν(CO) and ν(CN) frequencies in the IR, and for skeletal frequencies observed in both the IR and Raman spectra in the range 400 to 100 cm^?1 Infrared measurements show that the adducts are bound through the carbonyl oxygen, and are highly dissociated in dichloromethane solution. ¹H and ¹¹⁹Sn or ²⁰⁷Pb NMR measurements reveal that ligand exchange, fast on the NMR time scale, occurs in solution. Coordination of the ligand causes a large upfield shift in the ¹¹⁹Sn or ²⁰⁷Pb resonances, but Ph₃MI · L have shifts similar to those for the parent iodides, indicating almost complete dissociation. Thermodynamic parameters are reported for the dissociation of Ph₃SnX · DMPU (X = Cl, Br) in CH₂Cl₂ solution.