1,16-Diiodohexadecane

The molecular structure of the title compound, C16H32I2, is centrosymmetric and the molecular skeleton, including both terminal I atoms, has an all-trans conformation. The molecules form layers of thickness a. These features are similar to those of the smectic C phase of liquid crystals.

The molecular structure of the title compound, C 16 H 32 I 2 , is centrosymmetric and the molecular skeleton, including both terminal I atoms, has an all-trans conformation. The molecules form layers of thickness a. These features are similar to those of the smectic C phase of liquid crystals.  1,16-Diiodohexadecane N. Nakamura and D. Ishizu

Comment
Normal long-chain aliphatic compounds, such are n-alkanes have been studied to elucidate the principles of a crystallization for long-chain organic compounds, because the molecular skeleton consists of a simple trans zigzag straight hydrocarbon chain. The molecular shape of these compounds can be regarded as a rod-like one, and the molecules in the crystalline state form a layered structure similar to those of the smectic liquid crystalline phase. Moreover, some of these long-chain compounds exhibited a high-temperature rotator phase just below their melting points, in which molecules have some degree of motional freedom, comparable with that in liquid crystals. Thus, these long-chain compounds have been studied as model compounds for smectic liquid crystals.
In order to perform the investigations of mechanism of phase transition, it is important to obtain detailed crystallographic data. Many researchers have been analyzed the crystal structure of many different kinds of normal long-chain aliphatic compounds. Recently we have systematically analyzed the crystal structures of the alkane-α,ω-diols containing 10-24 C atoms using single-crystal X-ray diffraction (Nakamura et al., 2001;Uno & Nakamura, 2003), and one of the present authors has studied the phase transition phenomena of the series of the alkane-α,ω-diols containing 13-24 C atoms (Ogawa & Nakamura, 1999). In the present paper, we report a result of the crystal structure analysis of the title compound, (I), in order to clarify an effect of the terminal groups in the normal long-chain compounds on a construction of the layered structure.
The molecular structure of (I) is shown in Fig. 1. The molecule is centrosymmetric and all torsion angles are close to ±180°, that is, the molecular structure including both terminal I atoms has an all-trans conformation. Figure 2 shows the projection of the crystal structure of (I) along the b axis. The molecules form layers with a thickness of a. In the layers, the long axes of all molecules are inclined to the bc plane. The layers are arranged in parallel manner between the neighboring layers, forming a bookshelf motif, as shown in Fig. 3. The molecular arrangement of (I) is similar to that of the smectic C phase of liquid crystals. In the crystal structure, the shortest I···I distance is 3.9095 (14) Å. In addition, it is attributed to the fact that the van der Waals radius of I atoms are longer than those of Cl and Br atoms, and I atoms cause strongest steric hindrance.
The results of structure analysis of 1,16-dichlorohexadecane (Nakamura & Shimizu, 2004) and 1,16-dibromohexadecane (Kobayashi et al., 1995) have been reported. These compounds are arranged in a zigzag manner between adjacent layers, forming a herring-bone motif. These molecular arrangement are similar to that of the tilt smectic C phase of liquid crystals. Therefore, it is elucidated that features of the structure of (I) is differ from those of 1,16-dichlorohexadecane and 1,16dibromohexadecane. It is considered that this difference in the crystal structure are caused by the difference of the steric hindrance of atoms located in both ends.

Experimental
The single-crystal used for analysis was obtained by slow evaporation of a solution in a mixture of heptane and 2-propanol (1:1).
supplementary materials sup-2 Refinement H atoms were positioned geometrically and treated as riding, with C-H = 0.97 Å and with U iso (H) = 1.2 U eq (C). Fig. 1. The molecular structure of (I), showing the atom numbering scheme. Displacement ellipsoids are drawn at the 50% probability level [symmetry code: (i) 1 − x, 1 − y, 2 − z].