Volume 213, Issue 9

Dipolar but achiral organic molecules may undergo orientational disorder in the bulk but particularly at the surface of a seed crystal. In cases where the activation energy for reversal of the dipoles in the bulk is much larger than that for reversal at the growing crystal faces, thermodynamic arguments predict that orientational disorder occurring at surface attachment sites can produce metastable but macroscopically large growth sectors, featuring different polar properties than the seed. If the process of nucleation produces a non-pyroelectric symmetry group, then orientational disorder at the surface may give rise to pyroelectric properties during subsequent growth. Similarly, for seeds belonging to a pyroelectric group, orientational disorder can reduce the initial pyroelectric effect. According to this new surface-related mechanism of intrinsic defect formation, acentric and achiral molecules are likely to form real-structures which show a pyroelectric effect, although from a crystallographic point of view, such crystals may be described topologically by e.g. a centric structure. These present views are applied to the space group P 2 1 / c , supplemented by comments on techniques available for an experimental investigation of the predicted real-structurs.

Using an x-ray fiber data set from poly(ethylene terephthalate) originally published in 1954, the crystal structure of the polymer was determined by direct methods, via symbolic addition. Of the 68 measured amplitudes, 62 were assigned phase values. Although eight of these terms were incorrectly determined, the ensuing electron density map was a sufficiently well-defined density envelope for fitting the polymer backbone, therefore directly establishing the validity of the earlier determined packing model. On the other hand, visualization of individual atomic positions in the benzene ring was not possible from the electron density map generated from this phase set, even though they would have been found if all phases had been correct.

A new geometric structure constant for crystal surfaces is introduced in this paper. We call it “ S urface M adelung F actor (SMF)” because it has some analogies to the Madelung factor of crystal structures: it is independent of the lattice constants and it has the same value for all members of a structure type for a given surface. The SMF can be used to calculate the Madelung Part of Surface Energy (MAPSE) and to estimate the crystal morphology. All calculations can be done with a program called MAPSECAL.

The crystal structure of dipotassium hexaaquacobalt(II) sulfate, K 2 [Co(H 2 O) 6 ](SO 4 ) 2 , has been determined at room temperature by single crystal X-ray diffraction. The deep purple crystals grown from aqueous solution are monoclinic, space group P 2 1 / a , with unit cell dimensions a = 9.057(1) Å, b = 12.211(2) Å, c = 6.155(1) Å, β = 104.82(1)°, Z = 2, D c = 2.207 Mg/m 3 . The hydrogen atom positions were obtained from a Rietveld refinement on neutron powder data measured on a deuterated sample. Fixing these positions in the X-ray structure refinement on 1162 unique reflections ( I > 2 σ ) resulted in R ( F ) = 0.0188, wR ( F 2 ) = 0.0504, GOF = 1.14. All structural details compare well with corresponding results reported for other members of the Tutton's salts series.

The distribution of In 3+ between tetrahedral and octahedral sites and coordinate of oxygen in In x Fe 3− x O 4 ( x = 0, 0.1, 0.2, and 0.3) spinels have been determined on powder samples by the Rietveld method. The weight function with a form wYi = l/ Y oi 2 ( Y oi = the observed profile intensity at the i th step) was incorporated in the present Rietveld refinement. Structures thus refined were satisfactory not only in their internal consistency but also in their conformity with single crystal data. These results corroborate earlier study, which indicated a preference of In 3+ for the tetrahedral site. iv In–O distances in the present specimens [1.97(3) Å–2.00(5) Å] were close to the value calculated from a bond-valence scheme. In In x Fe 3− x O 4 , calculated vi Fe 2+ –O distances increased with increasing x , and they were in between two vi Fe 2+ –O distances in Fe 3 O 4 and γ -Fe 2 SiO 4 (spinel structure). A change of semimetallic nature to semiconductive one by In-doping in Fe 3 O 4 would be responsible for the elongation of vi Fe 2+ –O distance.

X-ray structural investigations of 9-amino-1 H phenalen-1-one (C 13 H 9 NO), N-(1-oxo-1- H -phenalen-9-yl)-acetamide (C 15 H 11 NO 2 ) and 9-(methylamino)-1 H -phenalen-1-one-1,4-dioxan-2-yl hydroperoxide solvate (C 18 H 19 NO 5 ) ( R F = 0.039, 0.054 and 0.042, respectively) together with semiempirical calculations (AMI and INDO/S) of electronic excitations have been carried out. The changes in molecular geometries under the influence of intermolecular interactions are used to interpret spectral properties of these compounds, in particular the decrease of the Stockes shift of 6-aminophenalenone with the increase in the solvent polarity.

The title compound, C 20 H 24 B 2 F 8 N 4 , crystallises monoclinic (space group C 2, Z = 4, a = 23.797(5) Å, b = 8.330(2) Å, c = 11.471(2) Å, β = 90.17(2)°. R = 0.084 for 1922 [ I ≥ 2 σ ( I )]. The most notable features are the values of the C–C bond length [1.47(1) Å], of the adjacent endocyclic N–C bond length [1.363(1) Å], the torsion angle about the central C–C bond [83°] and the nearly trigonal planar environment of the nitrogen atom.

The molecular crystal structures of 3-amino-4-nitro-6-mefhyl-8-oxopyrazolo[1,5-a]pyrimidine (C 7 H 7 N 5 O 3 ; space group P 2 1 / n ; Z = 4; a = 18.920(4) Å, b = 8.441(2) Å, c = 5.210(1) Å, β = 90.82(2)°) and 3-amino-4-nitro-6,8-dimethylpyrazolo[1,5-a]pyrimidine (C 8 H 9 N 5 O 2 ; space group P [unk]; Z = 2; a = 7.643(2) Å, b = 9.142(3) Å, c = 7.492(1) Å, α = 111.12(2)°, β = 100.66(2)°, γ = 102.58(2)°) have been determined from X-ray and neutron powder diffraction data using grid search procedure. The hydrogen-bonded molecules of the former compound form chains directed along the diagonals of the bc plane, while the latter crystal structure adopts dimeric arrangement.

The crystal and molecular structure of trans -bis((4-(phenylethynyl)phenyl)ethynyl)bis(tributylphosphine) platinum(II), trans -[( n Bu 3 P) 2 Pt(C≡CC 6 H 4 C≡CC 6 H 5 ) 2 ], C 56 H 72 P 2 Pt, has been determined at room temperature. The pale-yellow crystals are triclinic, space group P [unk] with unit cell dimensions a = 10.884(2) Å, b = 12.979(3) Å, c = 10.083(2) Å, a = 109.75(2)°, β = 95.17(2)°, γ = 101.94(2)°, Z = 1 and D x = 1.288 Mg m −3 . The structure was refined by a full-matrix least-squares procedure on F to final R = 0.036 using 3828 reflections with I ≥ 1.0 σ ( I ). The Pt atom in the centrosymmetric structure forms Pt–P and Pt–C distances of 2.304(1) Å and 1.981 (6) Å, respectively. The optical properties of the title complex were also determined, i.e. α , β , and γ are 1.531, 1.615 and 1.947, respectively.

The crystal and molecular structures of tris(3,3-dimethylbutyldithiocarbonato)-arsenic(III), -antimony(III) and -bismuth(III), A(S 2 COCH 2 CH 2 CMe 3 ) 3 , have been determined at room temperature. The colorless crystals of A = As(III) are triclinic, space group P [unk] with unit cell dimensions a = 11.837(2) Å, b = 13.430(2) Å, c = 10.284(1) Å, α = 98.39(1)°, β = 102.472(9)°, γ = 101.73(1)°, Z = 2 and D x = 1.316 Mg m −3 . The colorless crystals of A = Sb(III) are monoclinic, space group P 2 1 / c with unit cell dimensions a = 16.110(3) Å, b = 6.077(4) Å, c = 31.101(3) Å, β = 97.65(1)°, Z = 4 and D x = 1.439 Mg m −3 . The yellow crystals of A = Bi(III) are also monoclinic, space group P 2 1 / c with unit cell dimensions a = 16.149(3) Å, b = 6.063(3) Å, c = 31.052(7) Å, β = 97.25(2)°, Z = 4 and D x = 1.632 Mg m −3 The structures were solved by direct methods and each refined by a full-matrix least-squares procedure to final R = 0.042 using 3118 reflections for A = As(III); to R = 0.051 using 3357 reflections for A = Sb(III); and to final R = 0.056 for 2606 reflections for A = Bi(III). The A = As(III) structure comprises isolated molecules with the central atom existing in a distorted capped octahedral geometry. A disorted pentagonal bipyramidal geometry is found in the A = Sb(III) structure where loose association, via Sb … S interactions, are found between centrosymmetrically related pairs. An analogous structure is found for A = Bi (III), however, relatively stronger Bi … S interactions are in existence. A discussion of the different motifs found for A(S 2 COR) 3 structures indicates the importance of crystal packing effects, i.e. the desire to maximise A … S intermolecular contacts, in these structures.

The complex [NiL 2 (NCS) 2 ] ( 1 ), L = N , N -dimethyl-l,3-propanediamine, exhibits thennochromic solid state phase transition (temperature range 383 K–397.5 K) producing a high temperature isomer [NiL 2 (NCS) 2 ] ( 2 ). The crystal structure of ( 2 ) has been solved by the X-ray powder as well as single crystal data sets and the precision of the structure determined with the X-ray single crystal diffraction data is higher than that obtained from the powder diffraction data. The triclinic unit cell of ( 2 ) having a volume approximately double to that of ( 1 ) consists of two symmetrically independent octahedral [NiL 2 (NCS) 2 ] complexes with the metal atoms lying at (0 0 0) and (0 1/2 1/2), respectively. The equatorial planes around the two metal centers in isomer ( 2 ) are inclined to each other by 155.8(2)°. With the decrease of the specific volume per molecule of [NiL 2 (NCS) 2 ] on isomerization ( 1 → 2 ) and significant changes in the molecular packing of the two isomers ( 1 ) and ( 2 ), the possible way of phase transition ( 1 → 2 ) is through a break up of the crystal body of isomer ( 1 ) and recrystallization into isomer ( 2 ).