It is widely believed that the continuous random network (CRN) model represents the structural topology of amorphous silicon. The key evidence is that the model can reproduce well experimental reduced density functions (RDFs) obtained by diffraction. By using a combination of electron diffraction and fluctuation electron microscopy (FEM) variance data as experimental constraints in a structural relaxation procedure, we show that the CRN is not unique in matching the experimental RDF. We find that inhomogeneous paracrystalline structures containing local cubic ordering at the 10 to 20 angstrom length scale are also fully consistent with the RDF data. Crucially, they also matched the FEM variance data, unlike the CRN model. The paracrystalline model has implications for understanding phase transformation processes in various materials that extend beyond amorphous silicon.

The molecular structure of tetra-tert-butyldiphosphine has been determined in the gas phase by electron diffraction using the new DYNAMITE method and in the crystalline phase by X-ray diffraction. Ab initio methods were employed to gain a greater understanding of the structural preferences of this molecule in the gas phase, and to determine the intrinsic P-P bond energy, using recently described methods. Although the P-P bond is relatively long [GED 226.4(8) pm; X-ray 223.4(1) pm] and the dissociation energy is computed to be correspondingly small (150.6 kJ mol(-1)), the intrinsic energy of this bond (258.2 kJ mol(-1)) is normal for a diphosphine. The gaseous data were refined using the new Edinburgh structure refinement program ed@ed, which is described in detail. The molecular structure of gaseous P(2)Bu(t)(4) is compared to that of the isoelectronic 1,1,2,2-tetra-tert-butyldisilane. The molecules adopt a conformation with C(2) symmetry. The P-P-C angles returned from the gas electron diffraction refinement are 118.8(6) and 98.9(6) degrees, a difference of 20 degrees, whilst the C-P-C angle is 110.3(8) degrees. The corresponding parameters in the crystal are 120.9(1), 99.5(1) and 109.5(1) degrees. There are also large deformations within the tert-butyl groups, making the DYNAMITE analysis for this molecule extremely important.

The molecular geometries of benzaldehyde and salicylaldehyde have been determined by gas-phase electron diffraction and ab initio molecular orbital calculations at the MP2(FC)/6-31G* level. Several parameter differences from the molecular orbital calculations were incorporated as constraints in the electron diffraction analysis of salicylaldehyde. Some selected bond lengths (r g) and angles obtained in the electron diffraction analyses are as follows:  benzaldehyde (C−H)mean 1.095 ± 0.005 Å; (C−C)mean (benzene) 1.397 ± 0.003 Å; C2−C7 1.479 ± 0.004 Å; CO 1.212 ± 0.003 Å; C2−C7O 123.6 ± 0.4°; the benzene ring is undistorted within experimental error; salicylaldehyde (C−H)mean 1.090 ± 0.011 Å; (C−C)mean (benzene) 1.404 ± 0.003 Å; C1−C2 1.418 ± 0.014 Å; C−O 1.362 ± 0.010 Å; O−H 0.985 ± 0.014 Å; C2−C13 1.462 ± 0.011 Å; CO 1.225 ± 0.004 Å; C2−C13O 123.8 ± 1.2°; C2−C1−O 120.9 ± 1.1°. All the data are consistent with planar equilibrium structures for both molecules. The barrier to formyl group torsion is estimated to be appreciably higher for salicylaldehyde (at least 30 kJ/mol) than for benzaldehyde (at least 20 kJ/mol). There is intramolecular hydrogen bonding in the salicylaldehyde molecule of comparable strength with that in o-nitrophenol. The hydrogen bond is characterized by the following observed/calculated distances:  O···H(−O) 1.74(2)/1.80 Å and O···O 2.65(1)/2.68 Å. The structural changes in the rest of the molecule, as compared with the parent benzaldehyde and phenol molecules, are consistent with resonance-assisted hydrogen bonding similar to the o-nitrophenols. These changes include a lengthening of the CO bond (0.013 Å), a shortening of the exocyclic C−C bond (0.020 Å), a lengthening of the ring C−C bond between the substituents (0.017 Å), and a shortening of the hydroxy C−O bond (0.022 Å).