Triclinic packing

Figure C2.3.3. Molecular packing of SDS monohydrate viewed as projected on the ac plane. This polymoriDh crystallizes in a triclinic cell with unit cell constants a, b and c of 10.423 A, 5.662 A and 28.913 A, respectively, and with a = 86.70°, (3 = 93.44°, y = 89.55°. There are four molecules per unit cell. Adapted from figure 2 of [18].

Wess, T.J., et al. Type 1 collagen packing, conformation of the triclinic unit cell. J. Mol. Biol. 248 487-493, 1995. 

This is, for instance, the case of PTFE, which at atmospheric pressure presents two reversible first-order transitions at 19 °C and 30 °C [67], In the transition at 19 °C the molecular conformation changes slightly, from a 13/6 to a 15/7 helix and the molecular packing changes from an ordered structure with a triclinic unit cell (corresponding to a positioning of the chain axes nearly hexagonal) toward a partially disordered structure (partial intermolecular rotational disorder) with a... 

Ethyl a-cyano-4-[2-(2-pyridyl)ethenyl]cinnamate (7 OEt) also crystallizes with /3-centrosymmetric-type packing yielding photoreactive crystals and, upon photoirradiation (>410 nm), is converted into the /3-hetero-type dimer (7 OEt-dimer) nearly quantitatively. The 7 OEt-dimer (space group PT, triclinic) has the structure which is predicted from the crystal structure of 7 OEt (space group PT, triclinic). 

The structure of a second polymorph of 4,5-diphenyl- lH-imidazole has been discussed, with the new form exhibiting significantly different phenyl/imidazole dihedral angles and mode of crystal packing relative to the known form [53], A new triclinic polymorph of 1,4-dibenzoyl-butane was found, differing from the monoclinic form in the torsional angles of the central chain [54], Two polymorphs of diphenyl-(4-pyridyl)methyl methacrylate have been found, where the molecules in the two forms contain weak C—H— n and C—H O/N contacts that lead to the existence of different conformations [55]. 

The unit cell considered here is a primitive (P) unit cell that is, each unit cell has one lattice point. Nonprimitive cells contain two or more lattice points per unit cell. If the unit cell is centered in the (010) planes, this cell becomes a B unit cell for the (100) planes, an A cell for the (001) planes a C cell. Body-centered unit cells are designated I, and face-centered cells are called F. Regular packing of molecules into a crystal lattice often leads to symmetry relationships between the molecules. Common symmetry operations are two- or three-fold screw (rotation) axes, mirror planes, inversion centers (centers of symmetry), and rotation followed by inversion. There are 230 different ways to combine allowed symmetry operations in a crystal leading to 230 space groups.12 Not all of these are allowed for protein crystals because of amino acid asymmetry (only L-amino acids are found in proteins). Only those space groups without symmetry (triclinic) or with rotation or screw axes are allowed. However, mirror lines and inversion centers may occur in protein structures along an axis. 

Figure 2.23 Models of packing of chains in form IV of sPP according to space groups (a) P1148 and (b) C2149. Arrows indicate crystallographic twofold axes, present in (b) and lost in (a). In (a) chains are rotated by 6.5° according to direction indicated by arrows and continuous and dashed lines show triclinic and monoclinic unit cells, respectively.

Thus, pure samples of the complex formed crystals with varying luminescence. Most of the crystals exhibited the described yellow luminescence, but others displayed a bluish-white luminescence and a few of the crystals showed a pink luminescence. The two new forms of trinudear complex can be obtained by evaporation of dichloromethane solutions of the complex. They consisted of one triclinic form and one monoclinic form and both crystallized as colorless blocks, in contrast to the results with the original solvoluminescent hexagonal form that crystallized as colorless needles. These polymorphs differed in the packing of the nearly planar molecules and in the nature of the aurophilic interactions between the trinudear units (see Figure 6.16). 

Tals consists of iodine atoms in a hexagonal-close-packed arrangement in which 2/5 of the octahedral holes are randomly occupied by tantalum atoms.21 The Ta2Ii0 molecules are arranged in layers (Ta—Ib 2.932 A Ta—Iax 2.704 A Ta—I 2.619 A). Triclinic Nbl5 is isostructural with /3-UCl5.22... 

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