Deformation electron densities, X–X _HO Fourier synthesis vs ab initio calculations

Abstract

The electron density deformation in acetamide, C2HsNO, has been determined by X-XHo Fourier synthesis. The experimental results are compared with theoretical densities calculated by ab initio HartreeFock methods. In addition, theoretical densities for diformohydrazide, C2H4N202, and carbonohydrazide, CH6N40, have also been computed, and these are compared with previously obtained experimental results. The theoretical maps have been approximately adjusted for thermal smearing in order to be more readily comparable with experimental maps. The most striking feature of the maps is the very low electron density in the N-N bonds, showing little or no increase compared to the free-atom densities.

Crystallographic and spectroscopic studies of 1,2-sn-dipalmitoylglycerol with a comment on lipid phases and membranes

“…Only six atoms have £/«)> 0.02 A2. As expected, the largest values are associated with C atoms at the chain ends, with a maximum £/«, ~0.03 Á2 for methyl carbon C (36). Even the adp's for the H atoms are generally in consonance with a physically reasonable model.…”

“…27 Atomic displacement parameters for the chain atoms of DPG were analyzed in terms of rigid body motion,28 and the physical significance of the obtained parameters was examined by a Hirshfeld rigid bond test. 29 The differences in the mean-square atomic displacements, Ay, along the bonded atoms are in the range (0-16) X 10-4 A2 for the -chain C(2)-C( 16) and (2-23) X -4 A2 for the jS-chain C(22)-C (36), with rms values, (Ay2)1/2, 9 X lth4 and 13 X 1(H A2, respectively. In a truly rigid body, all interatomic distances should be invariant.30 When [w/y(A¿/,y)2]/EyH',y}1/2, are 10 X 10-4 and 12 X 10-4 Á2 for the «and 0-chain, respectively, compared to 8 X 1(H Á2, the value of (o2(i/y)obs)l/2 for both chains.…”