Hydrogen bonding unit cell

Triphenylsilanol has 16 molecules in the unit cell, arranged as two sets of two independent tetrameric hydrogen-bonded units. These units each have a flattened tetrahedral arrangement of silicon atoms, Fig. 

To our knowledge this is the shortest distance known in a N—H—N bond. Unit cell data and infrared spectra of a series of other Hj M (CN)2J compounds indicate the presence of similarly short hydrogen bonds in all those crystalline acids (63, 64). There are many speculations and conjectures concerning the nature of these hydrogen bonds (62—64). Neutron diffraction studies of polycrystalline D3Co(CN)6 (29, 65) could not settle the question of whether a single-minimum or a double-minimum type potential is present. Based on the results of a model calculation, a double-minimum potential well has been proposed with a very low barrier that can easily be tunneled (66). 

The crystal stmcture of PPT is pseudo-orthorhombic (essentially monoclinic) with a = 0.785/nm b = 0.515/nm c (fiber axis) = 1.28/nm and d = 90°. The molecules are arranged in parallel hydrogen-bonded sheets. There are two chains in a unit cell and the theoretical crystal density is 1.48 g/cm. The observed fiber density is 1.45 g/cm. An interesting property of the dry jet-wet spun fibers is the lateral crystalline order. Based on electron microscopy studies of peeled sections of Kevlar-49, the supramolecular stmcture consists of radially oriented crystaUites. The fiber contains a pleated stmcture along the fiber axis, with a periodicity of 500—600 nm. 

Crystals of uranyl perchlorate, U02(C10[13093-00-0] have been obtained with six and seven hydration water molecules. The uranyl ion is coordinated with five water molecules (4) in the equatorial plane with a U—O(aquo) distance of 245 nm (2.45 E). The perchlorate anion does not complex the uranyl center. The unit cells contain two [0104] and one or two molecules of hydration water held together by hydrogen bonding (164). 

The long-chain alkanoic acids and their derivatives are polymorphic with the unit cell containing dimers formed by hydrogen bonding between carboxyl groups. 

Similar models for the crystal stmcture of Fortisan Cellulose II came from two separate studies despite quite different measured values of the diffraction intensities (66,70). Both studies concluded that the two chains in the unit cell were packed antiparallel. Hydrogen bonding between chains at the corners and the centers of the unit cells, not found in Cellulose I, was proposed to account for the increased stabiUty of Cellulose II. The same model, with... 

On the other hand, in the single crystals prepared from equivalent amounts of heterochiral 1 1 complexes, a pair of two heterochiral 1 1 complexes are incorporated in a unit cell to form a layered structure with alternate layer distances of 7.33 and 7.6 A. Two perchlorate ions stay in the narrower gap, and two additional acetone molecules as crystallization solvent occupy the wider gap. The perchlorate ions interact with two axial water ligands by hydrogen bonds (3.71 and 3.77 A) to construct a layered structure. The adjacent two molecules of heterochiral 1 1 com-... 

Fig. 3.—Parallel packing arrangement of the 2-fold helices of cellulose I (1). (a) Stereo view of two unit cells approximately normal to the ac-plane. The two comer chains (open bonds) in the back, separated by a, form a hydrogen-bonded sheet. The center chain is drawn in filled bonds. All hydrogen bonds are drawn in dashed lines in this and the remaining diagrams, (b) Projection of the unit cell along the c-axis, with a down and b across the page. No hydrogen bonds are present between the comer and center chains.

Fig. 8.—Packing arrangement of four symmetry-related 2-fold helices of mannan II (6). (a) Stereo view of two unit cells approximately normal to flic frc-plane. The two chains in the back (open bonds) and the two in the front (filled bonds) are linked successively by 6-0H-- 0-6 bonds. The front and back chains, both at left and right, are further connected by 0-2 -1V -0-2 bridges, (h) Projection of the unit cell along the c-axis the a-axis is down the page. This highlights the two sets of interchain hydrogen bonds between antiparallel chains, distinguished by filled and open bonds. The crossed circles are water molecules at special positions.

Fig. 9. — Antiparallel packing arrangement of the 3-fold helices of (1— 4)-(3-D-xylan (7). (a) Stereo view of two unit cells roughly normal to the helix axis and along the short diagonal of the ab-plane. The two helices, distinguished by filled and open bonds, are connected via water (crossed circles) bridges. Cellulose type 3-0H-0-5 hydrogen bonds stabilize each helix, (b) A view of the unit cell projected along the r-axis highlights that the closeness of the water molecules to the helix axis enables them to link adjacent helices.

Fig. 12.—Packing arrangement of shallow, 6-fold, V-amylose (10) helices, (a) Stereo view of two unit cells approximately normal to the fee-plane. The helix at the center (filled bonds) is antiparallel to the two helices at the comers in the back (open bonds). Intrachain hydrogen bonds (3-OH - - 0-2 and 6-OH 0-3) are shown in thin lines.

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