Electron diffraction measurements, cyclic

The five-membered methylarsenic ring has been characterized by various methods. A single-crystal X-ray study has shown that the solid consists of discrete cyclic pentamers with equal As-As distances. The ring is puckered with mean As—As—As bond angles of 102°. Methyl groups are arranged in such a way that a staggered conformation at the As—As bonds results. Electron diffraction measurements of the vapour of the yellow form are consistent with the cyclic pentameric structure. ... 

In Table 8-1 we have listed the lengths of the triple bonds and the angles at the sp centers for cyclic monoalkynes as derived by X-ray and electron diffraction measurements [6, 33, 36-41], It is seen that bending the triple bond even to 150° does not have a significant effect on the bond lengths. The deviation from 1.20 A - the value of the C-C triple bond in unstrained systems - is small. 

Benzene itself is the simplest such neutral molecule. It was isolated by Faraday in 1825, and the accepted structure for benzene is usually attributed to a proposal by Kekule in 1865. ° Most organic chemists are familiar with the (perhaps apocryphal) snake tale of Kekule s vision of cyclic benzene molecules. In the late 1800s there was great debate about e molecular structure of benzene, and a number of possible structures were proposed for it. The electron diffraction results shown in Figure 4.27 indicate a planar structure with hexagonal symmetry. With other physical measurements, benzene was determined to have the structure shown in Figure 4.28. ... 

The purity of the terminal Au-oxo complexes, 3 and 4, was established by several methods including P NMR (3 and 4 have only one phosphorus peak at —8.55 and —13.15 ppm, respectively), cyclic voltammetry, electronic absorption spectroscopy, vibrational spectroscopy, detailed magnetic measurements and elemental analysis on all elements (triplicate analyses for Au) (44). The single peak in the PNMR spectra is consistent with the C2V symmetry of 3 and 4 established by multiple X-ray crystallographic structure determinations and a neutron diffraction study on 3 at liquid He... 

A very few highly diluted gases have, however, been the objects of coherent electron scattering experiments. In more dense gases, absorption of electrons rapidly becomes predominant and hinders measuring scattering intensities. Diffraction of electrons thus allowed determining with precision the structure of H-bonded cyclic dimers of carboxylic acids (26-28) that constitute model systems of H-bonds we have already seen in Ch. 2 and shall see later in this book. They nevertheless remain scarcely used methods. 

The solution of a protein crystal structure can still be a lengthy process, even when crystals are available, because of the phase problem. In contrast, small molecule (< 100 atoms) structures can be solved routinely by direct methods. In the early fifties it was shown that certain mathematical relationships exist between the phases and the amplitudes of the structure factors if it is assumed that the electron density is positive and atoms are resolved [255]. These mathematical methods have been developed [256,257] so that it is possible to solve a small molecule structure directly from the intensity data [258]. For example, the crystal structure of gramicidin S [259] (a cyclic polypeptide of 10 amino acids, 92 atoms) has been solved using the computer programme MULTAN. Traditional direct methods are not applicable to protein structures, partly because the diffraction data seldom extend to atomic resolution. Recently, a new method derived from information theory and based on the maximum entropy (minimum information) principle has been developed. In the immediate future the application will require an approximate starting phase set. However, the method has the potential for an ab initio structure determination from the measured intensities and a very small sub-set of starting phases, once the formidable problems in providing numerical methods for the solution of the fundamental equations have been solved. 

The reactions of [RuCl2(HMB)]2 with Tm gave [Ru(Tm)(HMB)]Cl, characterized by diffraction analysis and cyclic voltammetric measurements.23 [RuCl(Tm)(DMSO)2h [Ru(Tm)(p-cymene)]Cl], [RuCp(Tm)], and [Mn(Tm)(CO)3] have been synthesized, and [Ru(Tm)(p-cymene)]Cl and [Mn(Tm)(CO)3] structurally characterized. The Tm ligand acts as a tripod through its sulfur donors in all complexes except in [RuCp(Tm)], where bidentate coordination and formation of a 16-electron species are found.24... 

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