Conformational aspects of polymorphs and phases of 2-propyl-1H-benzimidazole.

This paper reports on the polymorphism of 2-propyl-1H-benzimidazole (2PrBzIm) induced by temperature change. Upon heating, an irreversible reconstructive-type phase transition at T = 384 K from the ordered form I (P212121) to a new polymorph, form II HT (Pcam), was observed. The structural transformation between forms I and II involves significant changes in the crystal packing, as well as a key conformational variation around the propyl chain of the molecule. After the first irreversible phase transition, the II HT form undergoes two further (reversible) phase transitions upon cooling at 361 K (II RT) and 181 K (II LT). All three phases (forms II HT, II RT and II LT) have almost identical crystal packing and, given the reversibility of the conversions as a function of temperature, they are referred to as form II temperature phases. They differ, however, with respect to conformational variations around the propyl chain of 2PrBzIm. Energy calculations of the gas-phase conformational energy landscape of this compound about its flexible bonds allowed us to classify the observed conformational variations of all forms into changes and adjustments of conformers. This reveals that forms I and II are related by conformational change, and that two of the form II phases (HT and RT) are related by conformational adjustment, whilst the other two (RT and LT) are related by conformational change. We introduce the term 'conformational phases' for different crystal phases with almost identical packing but showing changes in conformation.

Ordered vacancy distribution in 2/1 mullite: a superspace model.

A mullite single crystal with composition Al4.84Si1.16O9.58 (2) exhibiting sharp satellite reflections was investigated by means of X-ray diffraction. For the refinement of a superspace model in the superspace group Pbam(α0½)0ss different scale factors for main and satellite reflections were used in order to describe an ordered mullite structure embedded in a disordered polymorph. The ordered fraction of the mullite sample exhibits a completely ordered vacancy distribution and can be described as a block structure of vacancy blocks (VBs) that alternate with vacancy-free blocks (VFBs) along a and c. The incommensurate nature of mullite originates from a modulation of the block size, which depends on the composition. The displacive modulation is analyzed with respect to the vacancy distribution and a possible Al/Si ordering scheme is derived, although the measurement itself is not sensitive to the Al/Si distribution. An idealized, commensurate approximation for 2/1 mullite is also presented. Comparison of the ordered superspace model with different preceding models reconciles many key investigations of the last decades with partly contradicting conclusions, where mullite was usually treated as either ordered or disordered instead of considering simultaneously different states of order.

Nanoporosity, Inclusion Chemistry, and Spin Crossover in Orthogonally Interlocked Two-Dimensional Metal-Organic Frameworks.

[Fe(tvp)2 (NCS)2 ] (1) (tvp=trans-(4,4'-vinylenedipyridine)) consists of two independent perpendicular stacks of mutually interpenetrated two-dimensional grids. This uncommon supramolecular conformation defines square-sectional nanochannels (diagonal≈2.2 nm) in which inclusion molecules are located. The guest-loaded framework 1@guest displays complete thermal spin-crossover (SCO) behavior with the characteristic temperature T1/2 dependent on the guest molecule, whereas the guest-free species 1 is paramagnetic whatever the temperature. For the benzene-guest derivatives, the characteristic SCO temperature T1/2 decreases as the Hammet σp parameter increases. In general, the 1@guest series shows large entropy variations associated with the SCO and conformational changes of the interpenetrated grids that leads to a crystallographic-phase transition when the guest is benzonitrile or acetonitrile/H2 O.

Hidden degrees of freedom in aperiodic materials.

Numerous crystalline materials, including those of bioorganic origin, comprise incommensurate sublattices whose mutual arrangement is described in a superspace framework exceeding three dimensions. We report direct observation by neutron diffraction of superspace symmetry breaking in a solid-solid phase transition of an incommensurate host-guest system: the channel inclusion compound of nonadecane/urea. Strikingly, this phase transition generates a unit cell doubling that concerns only the modulation of one substructure by the other-an internal variable available only in superspace. This unanticipated pathway for degrees of freedom to rearrange leads to a second phase transition, which again is controlled by the higher dimensionality of superspace. These results reveal nature's capacity to explore the increased number of phases allowed in aperiodic crystals.

Interactions in self-organized nanoporous organic crystals.

This Letter reports on the structural analysis of a self-assembly material, the prototype host-guest urea-alkane nanoporous crystal. Different spectroscopic techniques, under hydrostatic pressure, reveal a totally unexpected ordered phase where ordering does not require any apparent deformation of the host. This fundamental observation raises the question of the actual interactions in other similar supramolecular or biological tubular systems.