The Clathrate Hydrates

Publisher Summary This chapter describes the various aspects of the vibrational spectra of carbohydrates. Vibrational spectroscopy is one of the techniques most adapted techniques for the study of hydrogen bonding, which is the indicator of molecular interaction in the aqueous solutions of sugars. A polyatomic molecule, such as sugar, may be regarded as a system of masses joined by bonds having spring-like properties. The vibration of each of the masses (atoms) can be resolved into components parallel to the x, y, and z axis of a Cartesian system of coordinates. The harmonic approximation is applied in which any motion of the molecule is resolved in a sum of displacements parallel to the Cartesian coordinates and these are called “fundamental, normal modes of vibration.” The analysis of crystallographic results shows the important role played by anomeric center in the structure of sugars. Infrared dichroism in polysaccharides was applied to obtain information on the orientation of chemical groups in the crystalline structure. The computational calculation of the frequencies of carbohydrates' vibration is also elaborated in the chapter.

Vibrational spectra of carbohydrates.

Publisher Summary This chapter discusses the several possibilities regarding the specification of the energy associated with the anomeric effect, and the experimental data concerning the configurational and conformational equilibria of the anomeric center and the related variations of valence geometry. The free-energy aspect of the anomeric effect as displayed by the equilibria of isomers is the area wherein the very concept was incepted and developed, and which still dominates its investigation. A considerable amount of data has been accumulated during the past two decades on the anomeric effect in terms of structure, energy, reactivity, and other properties. The description and understanding of the nature of stereoelectronic effects is an appropriate field for the application of organic quantum chemistry. Although several molecular orbital (MO) methods successfully describe the stereochemical behavior of pyranose models, their application to the more complex carbohydrates and oligosaccharides is limited at present due to economical reasons. The various rationalizations of the anomeric effect have been offered over the past years; they may be roughly divided into two main groups—namely, the electrostatic and the delocalization rationalization of the anomeric effect. It may be expected that the intramolecular interactions involved in the anomeric effect also influence the other points on the generalized reaction hypersurface.

Anomeric and Exo-Anomeric Effects in Carbohydrate Chemistry

Publisher Summary This chapter describes the complexes (briefly designated as metal-sugar complexes) that are formed, in detectable proportions, in aqueous solution from cations and unionized carbohydrates. Complex formation between cations and carbohydrates can be readily detected by paper electrophoresis. Thin-layer ligand-exchange chromatography makes the use of commercially available, thin-layer plates coated with a cation-exchange resin in the sodium form; the sodium ions can readily be exchanged for other cations by immersing the plate in an appropriate salt solution. Complex-formation causes the changes of the chemical shifts in the nuclear magnetic resonance spectra of sugars and polyols. Complex-formation with a cation does not, in itself, affect the optical rotatory power of a carbohydrate. However, complex-formation is often accompanied by a change of conformation that causes a change in the optical rotation. The proposal that strong complexing occurs between cations and a contiguous a,e,a sequence of hydroxyl groups in carbohydrates was first based simply on the observation that many compounds having such an arrangement complex well and many others lacking such an arrangement complex poorly, as shown by electrophoretic mobilities. The nature of complex- formation between cations and monosaccharides is now well understood; this knowledge should form a firm basis for the study of cation-polysaccharide interactions.

Complexes of Metal Cations with Carbohydrates in Solution

There are three general classes of hydrate inclusion compounds: the ‘gas’ hydrates, the per-alkyl onium salt hydrates, and the alkylamine hydrates. The first are clathrates, the second are ionic inclusion compounds, the third are ‘semi-clathrates’. Crystallization occurs because the H2O molecules, like SiO2, can form three-dimensional four-connected nets. With water alone, these are the ices. In the inclusion hydrates, nets with larger voids are stabilized by including other ‘guest’ molecules. Anions and hydrogen-bonding functional groups can replace water molecules in these nets, in which case the ‘guest’ species are cations or hydrophobic moieties of organic molecules. The guest must satisfy two criteria. One is dimensional, to ensure a ‘comfortable’ fit within the voids. The other is functional. The guest molecules cannot have either a single strong hydrogen-bonding group, such as an amide or a carboxylate, or a number of moderately strong hydrogen-bonding groups, as in a polyol or a carbohydrate. The common topological feature of these nets is the pentagonal dodecahedra: i.e., 512-hedron. These are combined with 51262-hedra, 51263-hedra, 51264-hedra and combinations of these polyhedra, to from five known nets. Two of these are the well-known 12 and 17 A cubic gas hydrate structures,Pm3n, Fd3m; one is tetragonal,P4
2/mnm, and two are hexagonal,P6
3/mmc andP6/mmm. The clathrate hydrates provide examples of the two cubic and the tetragonal structures. The alkyl onium salt hydrates have distorted versions of thePm3n cubic, the tetragonal, and one of the hexagonal structures. The alkylamine hydrate structures hitherto determined provide examples of distorted versions of the two hexagonal structures. There are also three hydrate inclusion structures, represented by single examples, which do not involve the 512-hedra. These are 4(CH3)3CHNH2·39H2O which is a clathrate; HPF6·6H2O and (CH3)4NOH·5H2O which are ionic-water inclusion hydrates. In the monoclinic 6(CH3CH2CH2NH2)·105H2O and the orthorhombic 3(CH2CH2)2NH·26H2O, the water structure is more complex. The idealization of these nets in terms of the close-packing of semi-regular polyhedra becomes difficult and artificial. There is an approach towards the complexity of the water salt structures found in the crystals of proteins.

Hydrate inclusion compounds

Evidence from determinations of crystal structure relating to the shortening of carbon-oxygen bonds in the anomeric position in pyranose sugars is presented. There is a high degree of probability that the C(1)-O(1) bond is shortened by about 0.04 A relative to the other C-O single-bond lengths, except in the case of an axially oriented glycosidic group where there is evidence of a distinction between the two ring C-O bonds.

Anomeric Bond-Character in the Pyranose Sugars

The crystal structure of 16(CH3)3CNH2·156H2O has been determined from three‐dimensional x‐ray‐diffraction data obtained at −30°C The crystals are cubic, space group I43d, with cell dimensions a=1881±002 A The host framework of hydrogen‐bonded water molecules consists of face‐sharing heptakaidecahedra which have three square, nine pentagonal, two hexagonal and three heptagonal faces The square and pentagonal faces also form octahedra which complete the space‐filling arrangement The amine molecules occupy the larger polyhedra as nonbonded guests undergoing marked oscillatory motion

Polyhedral Clathrate Hydrates. XIV. The Structure of (CH3)3CNH2·9¾H2O

The crystallographic data for nine alkylamine hydrates are reported. The hydrates of C2H5NH2 and (CH3)2NH are isostructural with the 12 A cubic gas hydrates and have unit cells with symmetries Pm3n and Pm3, and dimensions a=12.17 and 12.55 A, respectively. Those of (CH3)3N and n‐C3H7NH2 have symmetry P6/mmm and unit cell dimensions a=12.41 A, c=12.50 A and a=12.20 A, c=12.38 A, respectively, while that of iso‐C3H7NH2 has symmetry P63/mmc and cell dimensions a=12.42 A, c=25.22 A. These hexagonal hydrates are related in their water structure to (iso‐C5H11)4N+F−·38H2O. n‐C3H7NH2 forms a second hydrate with monoclinic symmetry P21/n and unit cell dimensions a=12.58 A, b=21.20 A, c=17.47 A, β=89.3°; (C2H5)2NH forms two hydrates, one of orthorhombic symmetry Pbcn and unit cell dimensions a=13.44 A, b=11.77 A, c=27.91 A, and one of monoclinic symmetry P21/c and cell dimensions a=13.86 A, b=8.44 A, c=10.93 A, β=97.5°; (CH3)3CNH2 forms a cubic hydrate with a unit cell of symmetry I43d and dimensions a=18.81 A. The total mass contents of the unit cells, calculated from the cell dimensions and experimental densities, are used to derive the probable stoichiometric formulas.

Polyhedral Clathrate Hydrates. XII. The Crystallographic Data on Hydrates of Ethylamine, Dimethylamine, Trimethylamine, n‐Propylamine (Two Forms), iso‐Propylamine, Diethylamine (Two Forms), and tert‐Butylamine

Hexamethylenetetramine hexahydrate has a three-dimensional framework structure of hydrogen-bonded water molecules. Each hexamethylenetetramine molecule is located in a cavity within this framework but is linked to the water structure by hydrogen-bonds through three of its four nitrogen atoms.

http://science.sciencemag.org/content/sci/149/3680/178.full.pdf

Hexamethylenetetramine Hexahydrate: A New Type of Clathrate Hydrate.

A crystallographic study of some alkylamine hydrates establishes that these are polyhedral clathrate hy drates. While the simpler alkylamines form hydrates with water structures re lated to those of the gas hydrates or the alkylonium salt hydrates, the di ethylamine and tert-butylamine hydrates have water frameworks forming cages which have not previously been ob served in the clathrate hydrates.

https://science.sciencemag.org/content/sci/155/3763/689.2.full.pdf

G. A. Jeffrey

Papers

Anomeric Bond-Character in the Pyranose Sugars

Polyhedral Clathrate Hydrates. XIV. The Structure of (CH3)3CNH2·9¾H2O

Polyhedral Clathrate Hydrates. XII. The Crystallographic Data on Hydrates of Ethylamine, Dimethylamine, Trimethylamine, n‐Propylamine (Two Forms), iso‐Propylamine, Diethylamine (Two Forms), and tert‐Butylamine

Hexamethylenetetramine Hexahydrate: A New Type of Clathrate Hydrate.

Clathrate hydrates of some amines.