摘要:

Using electron paramagnetic resonance spectroscopy we have measured the temperature dependence of ionic rearrangements and domain nucleation around the antiferroelectric transition temperatureTcof NH4H2AsO4, ND4D2AsO4, NH4H2PO4, and ND4D2PO4. CrO43−radicals, produced by γ-irradiation of K2CrO4-doped samples, were used as microscopic probes because their charge, size, and symmetry match those of the host anions. The use of high deuteration level and "monodomain" samples prepared by temperature cycling was found to be crucial in the signal assignment and line-shape analysis. The results show that as a sample approachesTcfromT > Tc, the hydrogen-bonded protons slow down anomalously, much more slowly than expected for Arrhenius behaviour. Simultaneously, the lattices develop "clusters" of antiferroelectrically ordered molecules. These clusters eventuallyexpand to occupy the whole lattice and thus complete the lattice ordering. The probe selection and sample preparation methodology discussed and the microscopic details of the ionic rearrangements reported appear to be highly significant for further studies of these phase transitions.

ISSN:0008-4042    

DOI:10.1139/v88-329

出版商:NRC Research Press    

年代:1988

数据来源: NRC

摘要:

Intensity analyses with low-energy electron diffraction (LEED) are reported for surface structures obtained by adsorbing submonolayer amounts of oxygen on the (100) surface of copper. It is found for chemisorption on the four-coordinate hollow sites that the correspondence between experimental and calculated intensity-versus-energy curves, for 10 diffracted beams, is slightly better for coplanar adsorption than for adsorption about 0.75 Å above the surface (the latter model is essentially as proposed previously by both SEXAFS and photoelectron diffraction). Nevertheless, in neither of these contrasting situations is the agreement sufficient to conclude that the structural analysis is complete. Evidence is presented that nearly coplanar chemisorption, in combination with a missing row model, can accommodate a number of structural features for this system, including the existence of the (2√2 × √2)45°−O phase, beam splittings resulting from antiphase domain boundaries, and the structural analogy noted in other studies between the (2√2 × √2)45°−O phase and the stepped nature of the (410) copper surface which can form by faceting in oxygen. Considerations of O–Cu surface bond lengths, guided by those in bulk Cu2O, suggest that additional lateral relaxations are likely in the topmost copper layer. This study provides no support for the O chemisorption occurring in tetrahedral-type sites.

ISSN:0008-4042    

DOI:10.1139/v88-330

出版商:NRC Research Press    

年代:1988

数据来源: NRC

摘要:

The synthesis of porphyrins having a fully hydrophobic cavity capped by a durene moiety is described. The size of the cavity is adjusted by varying the number of methylene -(CH2)n- groups linking the tetramethylphenylene cap with the diametrically opposite β-positions of the porphyrin. Bis(chloromethyl)durene (1) was first converted to durene-bisalkanoic acids11(n = 4), 13 (n = 5), and18(n = 7) using standard chain extension methods. The diacid chlorides were then used to acylate two equivalents of a β-unsubstituted pyrrole to give a chain-linked bispyrrole. Each pyrrole, following appropriate manipulation of the α substituents, was condensed with an α-unsubstituted pyrrole and the resulting chain-linked dipyrromethane dimer was cyclizedintramolecularlyunder high dilution to give the capped porphyrin34(n = 4, 5, or 7).

ISSN:0008-4042    

DOI:10.1139/v88-331

出版商:NRC Research Press    

年代:1988

数据来源: NRC

摘要:

The complex (5, 10, 15, 20-tetraphenylporphinato)bis(tri-n-butylphosphine)iron(II),1, has been isolated in a triclinic crystal form,a = 12.499(3),b = 12.528(2),c = 12.039(2) Å, α = 116.39(1), β = 109.79(1), γ = 98.13(1)°,Z = 1, space group. The structure was solved by conventional heavy atom methods and was refined by full-matrix least-squares procedures toR = 0.060 andRw = 0.070 for 3551 reflections withI ≥ 3σ(I). The molecule, which has crystallographic, symmetry, displays a relatively undistorted pseudo-octahedral coordination geometry with Fe—P = 2.346(1) and Fe—N = 1.998(3) and 1.993(3) Å. In CH2Cl2solution,1reacts with CO and aldehydes to generate the carbonyl(phosphine) derivative, and decarbonylation of phenylacetaldehyde to toluene becomes catalytic under an Ar stream. The aldehyde decarbonylations involve radical pathways via carbonyl loss from. The hydrogen abstraction/initiation reaction probably utilizes trace O2(and possibly trace (water); speculative mechanisms are discussed.

ISSN:0008-4042    

DOI:10.1139/v88-332

出版商:NRC Research Press    

年代:1988

数据来源: NRC

摘要:

Six new pyrazine-bridged complexes of iron(II) have been prepared and studied using magnetic susceptibility, differential scanning calorimetry, and vibrational, electronic, and Mössbauer spectroscopic methods. Fe(pyz)(CF3SO3)2exhibits a susceptibility maximum at 4.4 K and analysis of the magnetic data according to a two-dimensionalS = 2 system in the (isotropic) Heisenberg limit yields best-fit values ofJ = −0.26 cm−1and g = 2.20. Variable temperature Mössbauer spectra between 4.2 and 1.6 K are also reported for this compound and show the onset of magnetic hyperfine splitting below ~3.8 K. Analysis of magnetic data for Fe(pyz)2(CH3SO3)2and Fe(pyz)2(CF3SO3)2•CH3OH according to the two-dimensional Heisenberg model also indicates the presence of weak antiferromagnetic exchange in these complexes (J = −0.18 and −0.20 cm−1, respectively). No clear evidence for magnetic concentration was obtained from our studies on Fe(pyz)2(CIO4)2, Fe(pyz)(p-CH3C6H4SO3)2, and Fe(pyz)(p-CH3C6H4SO3)2•2CH3OH; any antiferromagnetic coupling in these complexes must be considerably weaker than in the other complexes studied.

ISSN:0008-4042    

DOI:10.1139/v88-333

出版商:NRC Research Press    

年代:1988

数据来源: NRC

摘要:

The infrared spectrum of gas phase aminodifluoroborane, NH2BF2, has been observed for the first time. A partial assignment to the fundamental vibrations has been made. Theband has been recorded at high resolution, and the rotational and centrifugal distortion constants of NH211F2have been obtained in both the ground and 21levels. A small rotational perturbation in the 21level has been attributed to a perpendicular Coriolis interaction. Using a newly written least-squares fitting program, spectroscopic constants of the unseen level have been evaluated, including its band origin and the Coriolis coupling coefficient. Possible identities of the unseen level are suggested.

ISSN:0008-4042    

DOI:10.1139/v88-334

出版商:NRC Research Press    

年代:1988

数据来源: NRC

摘要:

The compound (η5-C5Me5)W(NO)I2has been prepared by treating (η5-C5Me5)W(NO)(CO)2with I2. A single crystal X-ray crystallographic analysis establishes that the formally 16-electron complex possesses a monomeric, three-legged piano stool molecular structure in the solid state. Solution molecular weight measurements also indicate no significant degree of association of the compound.

ISSN:0008-4042    

DOI:10.1139/v88-335

出版商:NRC Research Press    

年代:1988

数据来源: NRC

摘要:

Absolute oscillator strength spectra in the C 1s(280–340 eV) and F 1s(680–740 eV) regions of the perfluoro-n-alkanes from C2to C6and perfluorocycloalkanes from C3to C6have been determined from inner-shell electron energy loss spectra recorded under electric-dipole scattering conditions. The spectral features are interpreted in terms of spatially localized transitions terminating at orbitals of predominantly σ*(C—F) and σ*(C—C) character. When compared to the spectra of the perfluoro-n-alkanes, both the C 1sand F 1sspectra of the perfluorocycloalkanes exhibit additional low-lying bands which are assigned to transitions terminating at σ*(C—C) orbitals which are shifted to low energy by the combination of the strain of cyclization and the inductive effect of the fluorination. The electron transmission spectra of selected perfluorocycloalkanes (which provide information on their anion states) show as well that the electron affinities of the cyclic systems are substantially lower than those of the corresponding perfluoro-n-alkanes, again as a result of a low-lying σ* orbital in the cyclic species. Quantum chemical calculations of the alkane and perfluoroalkane ground-state orbital structures support the experimental results. The localized character of the inner-shell excitations, indicated by the constancy of both term values and oscillator strengths with increasing chain length, contrasts with the more delocalized character of the states accessed in ultraviolet excitation or negative ion formation.

ISSN:0008-4042    

DOI:10.1139/v88-336

出版商:NRC Research Press    

年代:1988

数据来源: NRC