X-ray investigations

In 1921, a discontinuous index of refraction of vitreous Si02 near the a—P transition of quart2 (crystalline Si02) was noted (17). These data and subsequent x-ray investigations of vitreous siHca led to the suggestion (18) that crystaUites on the order of 1.5 nm were present. It was demonstrated, however, that the crystal size would be less than 0.8 nm, and it was suggested that the term crystal loses meaning for these dimensions (19,20). 

A large number of polymeric substances, (RAs) or (ArAs), are also known (113). They are usually prepared by the reduction of arsonic acids with hypophosphorous acid (100,114) or sodium dithionite (115). Most of these polymers have not been well characterized. An insoluble, purple material, poly(methylarsinidene) [26403-94-1], (CH As), prepared by the interaction of methylarsine and a dihalomethylarsine, however, has been shown by an x-ray investigation to have a ladderlike polymeric stmcture in which the inter-mng distances correspond to one-electron bonds (116) ... 

For x-ray investigations, the diffractometer method is generally used. The lattice constants indicate purity or composition of soHd solutions the rapid counting-tube goniometric method can be used at the manufacturing plant for quaUty control. The rotating-crystal and neutron diffraction methods are sometimes used for stmcture elucidation. 

The spectra of aldimines 74 and X-ray investigations on hydrazones of type 75 demonstrate the existence of these eompounds in the eorrespond-ing keto forms (74JPR971 92IZV917). 

The relative stereostructure of 9-acetyl-7-hydroxy-l,2-dimethyl-7-meth-oxycarbonyl-4-phenyl-6-oxo-l, 4,7,8-tetrahydro-6/7-pyrido[l, 2-u]pyri-midine-3-carboxylate 122 was justified by an X-ray diffraction analysis (97JOC3109). The stereochemistry and solid state structure of racemic trans-6,9-//-l, 6-dimethyl-9 z-ethoxy-9-hydroxy-4-oxo-l,6,7,8,9,9 z-hexahydro-4//-pyrido[l,2- z]pyrimidine-3-carboxylate (123), adopting a cw-fused conformation, were determined by X-ray investigations (97H(45)2175). 

X-ray investigations on 3,3-bis(tcrt-butoxycarbonyl)-4,5-difluoro-l-methyl-7-oxo-2,3-dihydro-l//,7//-pyrido[3,2,l-(/]cinnoline-8-carboxylate 57 revealed, that 1-methyl group is perpendicular to the plane of the 4-quinolone moiety (96BCJ1371). 

Structure of 4//-pyrido[l,2-a]pyrazines 348-350 was confirmed by X-ray investigations (99JPR332). The stereostructure of 1,3,4,6,1 l,lla-hexahydro-2/f-pyrazino[l,2-A]isoquinoline-l,4-dione 351 was determined by X-ray investigation (01TL543). 

Rontgen-strahlung, /. X radiation, -imter-suchimg, /. X-ray investigation or examination. 

T.A. Kaidalova X-Ray investigation of some complex oxyfluoroniobates - Abstract of dissertation, Moscow, 1974 (in Russian). 

As explained in the preceding section, we will discuss the structure of aromatic diazonium salts on the basis of evidence from X-ray investigations. We will supplement those results with data obtained by other physical methods, in particular NMR and IR spectroscopy. Earlier experience with the more stable arenediazonium salts enabled those scientists who first obtained alkanediazonium ions in solution to characterize them by NMR spectroscopy (see Zollinger, 1995, Sec. 2.1). 

This summary of X-ray investigations of aromatic diazonium salts will concentrate on those results that are essential to understanding the structure and reactivity of these compounds. Most important are, first of all ... 

The X-ray investigation of lH-3,5-dimethylpyrazole-4-diazonium chloride (4.7) demonstrates that heteroaromatic diazonium ions are very similar to those of the aromatic compounds (NN distance 111.3 pm, NC 136.9 pm, indicating a greater degree of NC double bond character (Brint et al., 1985). 

In (Z)-azobenzene the benzene rings are twisted by 34° from the C — NN — C plane because of the repulsion between the 2- and 2 -CH groups, as found by the X-ray investigations mentioned. 

The first diazonium-salt-crown-ether adduct was isolated and identified as a 1 1 complex by Haymore et al. (1975). Unfortunately Haymore never published the X-ray structural analysis of benzenediazonium hexafluorophosphate with 18-crown-6 which he performed in 1980. ORTEP drawings with measured bond angles and lengths from Haymore s investigation can be found in a review chapter by Bartsch (1983, p. 893). A few data from Haymore s work (e.g., R = 0.064) were also mentioned by Cram and Doxsee (1986, footnote 7). Groth (1981) published the results of his X-ray investigation of 4-methoxybenzenediazonium tetrafluoroborate and 21-crown-7 (R = 0.042) and Xu et al. (1986) those of 4-methoxybenzenediazonium tetrafluoroborate and dibenzo-24-crown-8 (R = 0.086). 

This interesting solvent effect was explained by Baro et al. on the basis of the formation of cyclic dimers (13.7) as suggested earlier by Hunter (1937) and by Le Fevre and Vine (1937). It was later confirmed in an X-ray investigation by Omel chenko and Kondrashev (1967). The X-ray analysis of 1,3-diphenyltriazene copper(i) (Brown... 

---