ABSTRACT
Our prediction that phase II of dipotassium hydrogen chromatoarsenate, K(2)[HCr(2)AsO(10)], is ferroelectric, based on the analysis of the atomic coordinates by Averbuch-Pouchot, Durif & Guitel [Acta Cryst. (1978), B34, 3725-3727], led to an independent redetermination of the structure using two separate crystals. The resulting improved accuracy allows the inference that the H atom is located in the hydrogen bonds of length 2.555 (5) angstroms which form between the terminal O atoms of shared AsO(3)OH tetrahedra in adjacent HCr(2)AsO(10)(2-) ions. The largest atomic displacement of 0.586 angstroms between phase II and the predicted paraelectric phase I is by these two O atoms. The H atoms form helices of radius approximately 0.60 A about the 3(1) or 3(2) axes. Normal probability analysis reveals systematic error in seven or more of the earlier atomic coordinates.
ABSTRACT
Crystals of phase II K2Cr2O7, potassium dichromate, space group P1 , grown from aqueous solution undergo a first-order transition to phase I, space group reportedly P21/n, at a phase-transition temperature, TPT, of 544 (2) K on first heating; the corresponding transition on cooling is at 502 (2) K. The endotherm on subsequent heatings occurs reproducibly at TPT = 531 (2) K. Mass loss between ca 531 and 544 K, identified as included water, is rapid and continues more slowly to higher temperatures for a total loss of ca 0.20%. The higher TPT on first heating is associated with the increasing pressure of superheated water occupying inclusion defects. The latent diagonal glide plane in phase II allows the structure of phase I to be inferred. The triclinic structure at 296 K has been independently redetermined. Normal probability analysis shows high consistency between the resulting and previous atomic coordinates, but with uncertainties reduced by a factor of ca 2. The earlier uncertainties are systematically underestimated by a comparable factor. The structure of phase IIb, space group A2/a on transposing axes, was determined at ca 300 K by Krivovichev et al. [Acta Cryst. (2000), C56, 629-630]. The first-order transition between phases I and II arises from the ca 60 degrees relative rotation of terminal O atoms in each tetrahedron as the n glide plane is gained or lost. A transition between phases IIb and I, also of first order, is likely but not between phases II and IIb. An intermediate phase may exist between phases IIb and I.
ABSTRACT
This study explored the effect of radical size, chain length, and mass on the cage recombination efficiency of photochemically generated radical cage pairs. Radical cage pairs containing long-chain radicals of the type [(CpCH(2)CH(2)N(CH(3))C(O)(CH(2))(n)CH(3))(CO)(3)Mo*, *Mo(CO)(3)(CpCH(2)CH(2)(CH(3))NC(O)(CH(2))(n)CH(3))] were generated in hexanes/squalane solution by photolysis (lambda = 546 nm) of the Mo-Mo bonds in (CpCH(2)CH(2)N(CH(3))C(O)(CH(2))(n)CH(3))(2)Mo(2)(CO)(6) (n = 3, 8, 18). The cage recombination efficiencies (denoted as F(cP), where F(cP) = k(cP)/(k(cP) + k(dP)), k(dP) is the diffusion rate constant, and k(cP) is the radical recombination rate constant) for the radical cage pairs were obtained by extracting them from quantum yield measurements for the photoreactions with CCl(4) (a metal-radical trap) as a function of solvent system viscosity. The results show that F(cP) increases as the length of the chain on a radical center increases. This finding likely provides at least one of the reasons why the quantum yields for photolytic polymer degradation (and long-chain molecules, in general) decrease as the polymer chains get longer. In quantitative terms, plots of k(dP)/k(cP) were linearly proportional to mass(1/2)/radius(2), in agreement with the prediction of Noyes' cage effect theory. The "radius" of a long-chain radical, such as those studied herein, is rather vague, and for that reason a less ambiguous structural parameter was sought to replace the r(2) term in the Noyes expression. Plots of k(dP)/k(cP) vs mass(1/2)/surface area suggest that surface area can be used in place of the radius(2) term in the Noyes expression. The significance of being able to use a particle's surface area in the Noyes expression is that the expression becomes useful for nonspherical particles. The new expression allows the approximate prediction of F(cP) values for radicals of different sizes and masses.
ABSTRACT
The Mn-Mn bonds in the two independent molecules of the unsolvated title compound, [Mn(2)(C(25)H(22)P(2))(CO)(8)], (I), are 2.9714 (7) and 2.9746 (7) A. This bond is distinctly shortened in the toluene hemisolvate, [Mn(2)(C(25)H(22)P(2))(CO)(8)] x 0.5C(7)H(8), (II), to 2.9338 (14) A and this shortening is accompanied by an increase in magnitude of the P-Mn-Mn-P torsion angle [26.93 (3) and 28.44 (3) degrees in (I), and 33.25 (7) degrees in (II)], while the P...P 'bite' is much less affected [3.092 (2) and 3.099 (2) A in (I), and 3.091 (3) A in (II)]. The toluene solvate molecule in (II) lies on a twofold axis.
ABSTRACT
Enhanced global delocalization throughout the "stepped" π-electron systems of the [2.2]paracyclophane/dehydrobenzoannulene (PC/DBA) hybrids 1 and 2 is strongly suggested by a comparison of their electronic absorption spectra with those of model compounds with complete and interrupted classical aromatic delocalization. A distinct bathochromic shift (for 1) and greater absorption intensity at higher wavelengths (for 1 and 2) is observed versus the corresponding model hydrocarbons.
ABSTRACT
The title compound can be formulated as [Cr(H(2)O)(6)][Na(2)(H(2)O)(10)][IMo(6)O(24)].8H(2)O. The anion has the I atom on an inversion centre and has close to $\overline 3$m symmetry, with I-O bond lengths in the range 1.881-1.890 (2) A and Mo-O bond lengths in the ranges 1.697 (3)-1.714 (3), 1.915 (2)-1.948 (2) and 2.317 (2)-2.357 (2) A.
ABSTRACT
C18H15F2P, M(r) = 300.29, orthorhombic, Pbcn, a = 6.105 (2), b = 16.634 (3), c = 14.356 (3) A, V = 1458 (1) A3, Z = 4, Dx = 1.368 Mg m-3, lambda(Mo K alpha) = 0.71069 A, mu = 0.194 mm-1, F(000) = 624, T = 294 K, R = 0.045 for 729 observed independent reflections. Difluorotriphenylphosphorane [(C6H5)3PF2] displays a trigonal bipyramidal geometry about the P atom, with the two F atoms occupying axial positions. The three phenyl groups in the equatorial positions are not 'geared' in a regular propeller arrangement; one of the three rings is twisted in the opposite direction to that of the other two.
Subject(s)
Organophosphorus Compounds/chemistry , Chemical Phenomena , Chemistry, Physical , Molecular Structure , Phosphoranes , X-Ray DiffractionABSTRACT
N-(2-Iodophenyl)-N'-(tricyclo[3.3.1.1(3,7)]-decan-1-yl) guanid inium chloride, C17H23ClIN3, Mr = 431.75, monoclinic, P2(1)/c, a = 11.696 (5), b = 20.120 (4), c = 7.754 (2) A, beta = 95.26 (3) degrees, V = 1817 (1) A3, Z = 4, Dx = 1.578 Mg m-3, lambda (Mo K alpha) = 0.71069 A, mu = 1.89 mm-1, F(000) = 864, T = 295 K, R = 0.071 for 1169 observed [I greater than or equal to 3 sigma(I)] reflections. Both the phenyl and the adamantyl groups lie syn to the unsubstituted nitrogen of the guanidinium ion. The plane of the phenyl ring is nearly perpendicular to that of the guanidinium group.
Subject(s)
Guanidines/chemistry , Molecular Structure , X-Ray DiffractionABSTRACT
Phosphorus-containing standards for use during quantitative electron probe X-rays microanalysis of ultrathin tissue sections can be quickly and simply prepared using organo-phosphorus compounds which are soluble in Spurr's epoxy resin mixture. Trials of five such compounds (one solid, the rest liquid) are described.
