X-ray structural studies

X-ray structural studies have played a major role in transforming chemistry from a descriptive science at the beginning of the twentieth century to one in which the properties of novel compounds can be predicted on theoretical grounds. When W.L. Bragg solved the very first crystal structure, that of rock salt, NaCl, the results completely changed prevalent concepts of bonding forces in ionic compounds. 

The serine proteinases have been very extensively studied, both by kinetic methods in solution and by x-ray structural studies to high resolution. From all these studies the following reaction mechanism has emerged. 

The isomerization of isopentenyl diphosphate to dimethylally diphos phate is catalyzed by JPP isomerase and occurs through a carbocation pathway Protonation of the IPP double bond by a hydrogen-bonded cysteine residue ir the enzyme gives a tertiary carbocation intermediate, which is deprotonated b a glutamate residue as base to yield DMAPP. X-ray structural studies on the enzyme show that it holds the substrate in an unusually deep, well-protectec pocket to shield the highly reactive carbocation from reaction with solvent 01 other external substances. 

As has been shown by the X-ray diffraction method the parent metals (i.e. Pd or Ni), the a-phase, and /3-phase all have the same type of crystal lattice, namely face centered cubic of the NaCl type. However, the /9-phase exhibits a significant expansion of the lattice in comparison with the metal itself. Extensive X-ray structural studies of the Pd-H system have been carried out by Owen and Williams (14), and on the Ni-H system by Janko (8), Majchrzak (15), and Janko and Pielaszek (16). The relevant details arc to be found in the references cited. It should be emphasized here, however, that at moderate temperatures palladium and nickel hydrides have lattices of the NaCl type with parameters respectively 3.6% and 6% larger than those of the parent metals. Within the limits of the solid solution the metal lattice expands also with increased hydrogen concentration, but the lattice parameter does not depart significantly from that of the pure metal (for palladium at least up to about 100°C). 

The correlation of structural and vibrational parameters, and a detailed analysis of structural trends based on the available wealth of electron diffraction, microwave and X-ray structural studies on sulphoxides and sulphones, have been reported by Hargittai in a comprehensive study20 which will stand for a long time as a milestone in the structural... 

The monomeric Ir1 C/P-donor-atom compounds [Ir(troppph)2]+, troppFh = dibenzotropyl-idenyl phosphine, are synthesized via reaction Scheme 43. The X-ray structural study of (404) shows a... 

X-ray structural studies of the diamagnetic anion (406) confirm that the Ir(-I) center is in a distorted coordination geometry intermediate between square planar and tetrahedral, with the P donor atoms in a cis position. The metal-ligand bond distances do not show significant changes among (404), (405), and (406). The Ir1/0 and Ir0/(-1) redox couples are measured at easily accessible potentials and are solvent dependent. 

When 8 a is used instead of 2a for complexation with 50a, two kinds of 1 1 inclusion complexes 57 were obtained one (57a) is photoreactive and the other (57b) is photostable. Thus when a solution of 8a and an equimolar amount of 50a in benzene was kept at room temperature for 10 h, a 1 1 inclusion complex between both (57a) was obtained as colorless fine needles of mp 110 to 123 °C which upon irradiation in the solid state for 49 h gave (1 R,5S)-(+)-52a of 3.6% ee in 30% yield 13). However, recrystallization of 57a from benzene gave a different kind of 1 1 inclusion complex (57b) as colorless needles of mp 123-125 °C which is photostable. Photocyclization of 50a in 57b is probably prevented by a steric hindrance. The different behavior of 57a and 57b may be clarified in future by X-ray structural studies. 

The photocyclization reaction is in contrast to the photodimerization of 58 in 59. In the case of 61, the photodimerization of 58 is probably prevented by steric hindrance. It is almost certain that one optical conformer of 58 is included in 61, but a real proof of this fact requires an X-ray structural study in the future. However, the formation of the optically active 62 is valuable, because photoreaction of 58 in solution does not give any intramolecular photocyclization product34). 

W(n5-C5Me4But)Cl4]2, each in 50% yield (36). The identity of W(q5-C5Me4But)(MeCECMe)Cl2 was proven by an x-ray structural study which showed it to be similar to Ta(ri5-C5Me5)(PhC Ph)Cl2 (37) and related species. The W(V) dimer and W(III) acetylene complex probably form by disproportionation of some intermediate W(IV) complex. What we were most interested in was whether any intermediates not containing a n5-C5Me4Bu ligand could be isolated. 

We are all well aware of the tremendous advances over the past decade in both the quality and number of X-ray structural studies on molecules and ions, aimed at the accurate determination of the ground-state geometries. However, our knowledge of excited-state geometries has not developed at a comparable rate, and few techniques can be brought to bear upon this problem. 

X-ray structural studies. A considerable number of such complexes have been investigated by X-ray diffraction. Collectively, the structures are characterized by very wide diversity, and range from simple coordination geometries through to quite complex ones (Truter, 1973 Hilgenfeld Saenger, 1982). 

Armbruster T., Geiger C. A., and Lager G. A. (1992). Single-crystal X-ray structure study of synthetic pyrope ahnandine garnets at 100 and 293 K. Amer. Mineral, 77 512-521. Atkins P. W. (1978). Physical Chemistry. Oxford Oxford University Press. 

Several X-ray structural studies have been made on hydrido-ruthenium clusters (see Table... 

A further interesting X-ray structural study has been that of (AsPh4)2-[Pd2(N3)6] (66). This reveals similar asymmetry in the two N—N bond lengths for the bridging (1.239 and 1.142 A) and terminal (1.205 and 1.139 A) azide groups. Such structural equivalence indicates that there is no appreciable change in electronic character when a terminal azide co-ordinates to a second Pd via a normally unshared electron pair on the trigonal N atom (see Table... 

In the absence of suitable crystals for an X-ray structural study, combinations of FTIR, RR, XAS, EPR, ENDOR, XANES, and ESEEM spectroscopies were used to characterize the manganese site in the OEC. Vibrational spectroscopy, particularly low-frequency FT infrared and... 

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