G. A. Jeffrey

The application of ab initio molecular orbital theory to the anomeric effect. A comparison of theoretical predictions and experimental data on conformations and bond lengths in some pyranoses and methyl pyranosides

Abstract Ab initio molecular orbital calculations on methanediol have been used to predict the favored orientations and interatomic distances of the C-O-C-O-R portion in pyranoses. The results found for methanediol suggest, for the sugars, favored conformations that are consistent with the observed anomeric and exo -anomeric effects. The calculations also show that shortenings of the C-O bond of the order of 0.01 to 0.04 A, relative to methanol, are to be expected, and that the bond lengths have a strong conformational dependence. A comparison with the experimental data from the X-ray crystal-structure determinations, both for conformational angles and bond lengths, of eighteen pyranoses and methyl pyranosides shows agreement with the theory that is surprisingly good when consideration is taken of the experimental errors, the limitations of the theoretical model, and the expected differences in the structures of the crystal and the isolated molecule.

Conformations of the alditols

The hypothesis that parallel C-nO (or C-nC) and C-(n+2)O [or C-(n+2)C] bonds in the alditols give rise to conformational instability accounts for the conformations observed in nine crystal-structure determinations. It relates conformation to configuration in the solid state, and enables predictions to be made concerning the most stable rotamers of the isolated molecules.

Anomeric Bond-Character in the Pyranose Sugars

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 � 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.

Crystal Structure of Tetramethylammonium Fluoride Tetrahydrate

Tetramethylammonium fluoride tetrahydrate is tetragonal, space group I41/a with a=10.853 and c=8.065 A at −26°C. The crystal structure was determined from the crystal data and the Patterson synthesis and refined by least squares. It has a hydrogen‐bounded ionic/water framework composed of four‐coordinated fluoride ions and three‐coordinated water molecules. The tetramethylammonium ions are in voids within this framework.

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

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=18.81±0.02 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. XII. The Crystallographic Data on Hydrates of Ethylamine, Dimethylamine, Trimethylamine, n‐Propylamine (Two Forms), iso‐Propylamine, Diethylamine (Two Forms), and tert‐Butylamine

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. Th...

Crystal Structure of Pinacol Hexahydrate

The crystal structure of pinacol hexahydrate, C6H14O2·6H2O, has been determined at room temperature from diffractometer data with CuKα radiation. The structure is tetragonal space group P42 / mnm, with two formula units per unit cell of dimensions a = b = 6.398(3) A, c = 15.926(8) A. The intensity data were measured with CuKα radiation on a Picker FACS I diffractometer. The parameters were refined anisotropically by least squares to a final R = 0.06 for 294 observed reflections. The hydrogen‐bonded water and pinacol molecules form a framework structure which is isostructural with that of piperazine hexahydrate. The water molecules form puckered layers consisting of edge‐sharing pentagons. These layers are linked into a three‐dimensional framework through the hydroxyls of the pinacol molecules. The pinacol molecules are in voids within this framework structure with twofold orientational disorder. Although this is not a clathrate hydrate, there is a marked structural similarity to that series of compounds.

The crystal structure of ribitol

Residual electron density During the refinements of this structure no attempt was made to pick out one particular oxygen atom as carrier of the negative charge of the anion. The form factor was always the one given in International Tables for X-ray Crystallography (1962) for the free oxygen atom. One extra electron is thus present in the anion in addition to those included in the calculations, and to conclude the analysis, a difference Fourier synthesis was evaluated. After omission of the uncertain 400 and 004 reflexions, the residual electron density in the region of the ascorbate anion is shown in Fig.4. These maxima are clearly the largest in the unit cell, and some of them fall in the space between the carbon and oxygen atoms. This result is quite similar to that for ascorbic acid, but whether the peaks adjacent to the O(2), 0(3) and 0(4) atoms outside the bond area have relevance to the extra electron is questionable because of the low significance of such peaks in the present case. The author would like to express his gratitude to Det Norske Videnskaps-Akademi for a grant, and also to J. Solbakk, P. Groth, B. Klewe, F. Gram and T. Dahl for the use of their computer programs. He is also indebted to Professor S. Furberg for illuminating discussions.

Conformation and intramolecular hydrogen-bonding in the crystal structure of potassium D-gluconate monohydrate

Abstract The D -gluconate ion is found to have the planar, extended carbon-chain conformation in the crystal structure of potassium D -gluconate monohydrate, with an intramolecular hydrogen-bond between 0-2 and 0-4. The D -gluconate ions and water molecules are linked in puckered sheets by a series of intermolecular hydrogen-bonds that involve the water molecules, the carboxylate groups, and pairs of hydroxyl groups. One hydroxyl group in the ion does not form a hydrogen bond. The potassium ions lie between the puckered sheets, with an eight-fold coordination of six D -gluconate groups and two water oxygen atoms. The crystal structure was determined from three-dimensional, CuKα, X-ray diffraction data taken on an automatic diffractometer.

Polyhedral Clathrate Hydrates. XVI. Structure of Isopropylamine Octahydrate

The crystal structure of isopropylamine octahydrate, 10(CH3)2CHNH2·80H2O, mp − 4°C, has been determined from three‐dimensional Weissenberg film data obtained at − 160°C with CuKα radiation. The crystals are hexagonal, P63 / mmc, with a = 12.30 A, c = 24.85 A. The structure was determined from the Patterson function and refined anisotropically to an R of 0.09 for 1041 observed reflections. The water structure is a clathrate‐type hydrogen‐bonded framework, which can be related to that of the cubic gas hydrates. It consists of layers of face‐sharing 14‐hedra (42 × 58 × 64), separated by pentagonal dodecahedra (512) and 16‐hedra (512 × 64). The amine molecules occupy positions with sixfold orientational disorder in the six 14 hedra and four 16 hedra per unit cell. They are hydrogen bonded to the water structure, singly within the 16‐hedra and doubly within the 14‐hedra. Because of the distortions caused by the inclusion of the amines in the 14‐hedra, there are additional voids in the arrangement of the 12‐, 1...