Trigonal form

A specific polymorph may be absolutely essential for a crystalline product, for example, one polymorph may have a more desirable color or greater hardness or disperse in water more easily than another polymorph. Often, one polymorphic form is more stable than another (for example, at 80°C the orthorhombic I form of ammonium nitrate is more stable than the trigonal form) at conditions to which a product is exposed. An interesting approach to... 

A priori, a vinyl cation can have two possible geometries a linear structure, 172a, with a sp-hybridized carbon and an empty p orbital or a trigonal form, 172b, with a sp -hybridized carbon and an empty sp orbital. 

Sb (c). The metallic trigonal form is taken as the standard state. There is apparently a transition between 100° and 150°, but no thermal data on it are recorded. Another form of antimony is the so-called... 

There is a trigonal form of T12O, Dgd, R 3m, a = 3.516, and c = 37.84 A. The large c results from a complex sequence of positions of T1 atoms. It is ABAB CACA BCBC. It is a POP sandwich structure with the notation 12 3/2POP(h). 

Fig. 5. Solid-state absorption spectra of BkF ) in its orthorhombic and trigonal forms at room temperature. Reprinted with permission from the J. Inorg. Nucl. Chem. 43, D. D. Ensor, J. R. Peterson, R. G. Haire, and J. P. Young, 1981, Pergamon Press, Ltd. (13).

Their crystal structures have been mentioned briefly in connection with intercalation in Section 14.2. All five compounds can be obtained in the layered hexagonal crystal form, and most are also found in rhombohedral or trigonal form. The compounds of the Group 6 metals, molybdenum and tungsten, as well as niobium diselenide, have a hexagonal form similar to that of molybdenum disulphide, in which the metal atoms in one layer are displaced sideways from those in the layers immediately above and below. This structure results in the widest interlamellar spacing, the easiest interlamellar shear, and the lowest friction. 

The turning point in the structural studies came after the work by Hiraoka et al. (1980) revealed that a-lactalbumin is a metalloprotein in which calcium is strongly bound (see also Sections VI and VII). Soon afterward three new crystal forms of bovine a-lactalbumin were isolated by Fenna (1982a), particularly trigonal Form II. Fenna (1982b) also isolated calcium-containing crystals of human a-lactalbumin suitable for X-ray structural analyses. The various crystalline forms of a-lactalbumin are summarized in Table III. 

Most sets of hybrid orbitals are equivalent and symmetric, that is, four sp orbitals directed to the comers of a regular tetrahedron, six tPsp orbitals to the corners of an octahedron, etc. (n the case of hybrids the resulting orbitals are not equivalent. In the trigonal bipyramidal arrangement three orbitals directed trigonally form one set of equivalent orbitals (these may be considered sp hybrids) and two orbitals directed linearly (and perpendicular to the plane of the first three) form a second set of two (these may be considered dp hybrids). The former set is known as the equatorial orbitals and the latter as the axial orbitals. Because of the nature of the different orbitals involved, bonds formed from the two are intrinsically different and will have different properties even when bonded to irJenlica] atoms. For example, in molecules like PFj bond lengths differ for axial and equatorial bonds (see Chapter 6). 

Cubic Cs2PdFe (a = 9.03(1) A) and hexagonal KiPdFe (a = 5.777(2) and c = 9.367(2) A) are the usual products of these compositions from aHF, but the trigonal form of the latter (see ref 32) is also occasionally observed, indicating that, in this material, the two forms have very similar energies. 

KF 15 h. K PtF, XRDP showed only the trigonal form of known... 

We turn now to compounds that provide the most intimate views of the catalytic process itself Linus Pauling proposed in 1948 that compounds resembling the transition state of a catalyzed reaction should be very effective inhibitors of enzymes. These mimics are called transition-state analogs. The inhibition of proline racemase is an instructive example. The racemization of proline proceeds through a transition state in which the tetrahedral a- carbon atom has become trigonal by loss of a proton (Figure 8.24). In the trigonal form, all three bonds are in the same plane C also carries a... 

Fig. 5 Packing diagrams of carbamazepine anhydrous polymorphs (A) monoclinic, form III and (B) trigonal, form II. (From Ref i°l)...

Some phosphates of the AM2(P04)3 family form other modifications as well neptunium phosphate NaNp2(P04)3 with monazite type structure, Pl ln (this type was obtained by heating the trigonal form in Ar + 5% H2 at 1100 °C [5,35]) neptunium and plutonium phosphates AgNp2(P04)3 and AgPu2(P04)3 with scheelite type structure, 74i/a [46], thorium phosphate NaTh2(P04)3 with... 

There has been some confusion in the literature on the nomenclature of carbamazepine anhydrous polymorphs. For this chapter, we will use the following nomenclature Form I (Triclinic), Form II (Trigonal), Form III (P-monoclinic), and Form IV (C-monoclinic). 

The crystal structures of Mca FIX derivatives have been determined for X = CN, N3, NCS, or NCO " X = acetate, tropolonate, acetylacetonate, or dibenzoyl-methide X == tricyanomethide or dicyanamide. The NCO species is dimorphic, the orthorhombic form containing infinite layers of Me2XINCO molecules. The trigonal form, however, is built up from dimeric units. 

---