Triclinic crystallographic system

The translational direction of the lattice (c), which is the direction that the crystallite axis is not, because of the triclinic crystallographic system, is perpendicular to the plane of the unit cell base (ab). 

Despite the fact that the triclinic system is the only crystallographic system that unequivocally allows singlecrystal orientations to provide both enhanced in-plane and out-of-plane NRVS spectra, many other crystalline heme derivatives fortunately have all porphyrin planes close to being parallel to a common crystallographic plane. For example, [Fe(TPP)(l-MeIm)(NO)] has four independent porphyrin plane orientations, but all planes are within 13.8° of a common plane in the crystal. Data were collected using this common plane as the crystal-orienting plane to obtain an in-plane and an out-of-plane spectrum. As shown in Figure 10, a comparison of the in-plane , out-of-plane , and powder spectra show that useful intensity enhancements are still evident. Moreover, and fortunately, this situation of nearly coincident planes appears to be moderately frequently observed. 

Block-shaped black crystals with a distinct metallic luster are formed which upon crystallographic study are found to crystallize in the triclinic system with lattice constants as follows a = 7.786 (1) A, fe = 13.033 (3) A, c = 18.590 (4) A, a = 110.09 (2)°, 0 = 90.21 (2)°, y = 105.27 (2)°, and Vc = 1699.8 (6) A3. These lattice parameters indicate that the crystals contain 0.5 molecules of TCE in the formula unit as is the case for isostructural (BEDT-TTF)2ClO4(TCE)0 5 in which Vc = 1689.8 A3.10 The electrical properties of this material indicate that they are metallic from room temperature to 90 K at which point a metal-insulator transition occurs. 

An X-ray crystallographic analysis of DAD single crystals shows a triclinic system with the non-centrosymmetric space group P1 and that the direction of polarization of DAD in the molecular crystal is perfectly aligned in one dimension. 

Six types of spectra are theoretically possible in minerals of the orthorhombic, monoclinic and triclinic systems (McClure, 1959). However, for electric dipole transitions only three spectra are usually distinguished. These are the a, P and y spectra obtained when light is polarized along each of the three indicatrix axes, which in orthorhombic minerals such as olivine and orthopyroxene correspond to the three crystallographic axes. The majority of the spectra of minerals described in chapters 4 and 5 consist of polarized spectra measured in the three mutually perpendicular directions corresponding to a, P and y polarized light... 

The salt yields green crystals, readily soluble in water. They belong to the triclinic system, their crystallographic elements being as follow —4... 

The very rich structural chemistry of organometallic compounds also provides many examples of conformational polymorphism. For instance, cyclic ligands often have low barriers to rotation so that conformational variations between polymorphs might not be unexpected. Riley and Davis (1976) reported the structures of the dimorphic sandwich compound 5-VII, a system in which the molecular geometry is determined by the crystallographic site symmetry for both polymorphs (Fig. 5.8). In the triclinic form, the molecular site symmetry is C, requiring the trans orientation... 

II, A). Crystallographic data show that barbiturates generally have low-symmetric crystal systems, i.e., monoclinic or sometimes triclinic (Table V). 

The crystal structures of the two systems are also very alike. Both are triclinic with comparable unit-cell parameters and with the "herring-bone" structure known from TTF-TCNQ /8/. The most pronounced crystallographic difference between the two systems is a rather elongated /6.1 %/ b-axis in TMTSF-DMTCNQ. This is to be expected because the b-direc-tion is across the width of the molecules where the additional methyl-groups tend to push the chains apart thereb. y probably reducing the interchain coupling. 

The description of the symmetry elements of the space groups is similar to that of the point groups [9-19]. The main difference is that the order in which the symmetry elements of the space groups are listed may be of great importance, except for the triclinic system. The order of the symmetry elements expresses their relative orientation in space with respect to the three crystallographic axes. For the monoclinic system, the unique axis may be the c or the h axis. For the P2 space group, the complete symbol may be P112 or... 

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