Single-Crystal X-Ray Methods

The determination of crystal structures is one of the outstanding triumphs of 20th century science and dates back to the discovery of X-ray diffraction in crystals by von Laue in 1912. Several Nobel prizes have been awarded for the X-ray determination of protein and vitamin structures, testifying to the power of the method in determining complex atomic arrangements. Less well publicized are the numerous crystallographic studies of smaller molecules and solid-state materials. Thousands of crystal structures have been worked out, and the results are of inestimable value in understanding the properties of materials. 

Two quantities are measured for each reflection on the X-ray diffraction pattern, the angular position 6 and the integrated intensity I. The size and shape of the unit cell are determined from the angles, and the intensities yield the locations of atoms within the unit cell. In addition to the atomic coordinates, less precise information concerning thermal vibration amplitudes and electron density are also derived from the intensity data. 

The powder method is difficult to apply in the determination of crystal structures except in simple cases because the overlapping reflections prevent unequivocal indexing and measurement of intensities. Although this is a fundamental limitation that cannot be overcome by better instrumentation, the powder method is nevertheless useful for a variety of structure studies. 

The number of molecules per unit cell Z is obtained from the cell volume V, the molecular weight M, the measured density p, and Avogadro s number N -.Z = pVNJM. From Z, we know how many atomic coordinates (x,, y,, z,) must be determined from the intensities 

The intensities are first converted to structure factor amplitudes by correcting for absorption, Lorentz-polarization factors, and multiplicity. In general the structure factors are complex, but only the amplitude can be measured experimentally, and not the phase angle. The so-called phase problem has prevented X-ray crystallography from becoming a routine procedure, although the development of powerful analytic techniques makes this less true now than in the past. 

---

Some mixed valence polynuclear thiolate complexes such as [Ni4(SC3H7)8Br] and [Ni4(SC3H7)8I] have been prepared and characterized by single-crystal X-ray methods.203... 

At the outset of the structure investigation it was hoped that single-crystal X-ray methods might be successful, particularly with the large alkaloids however, as a number of trials were not promising, the approaches to structures have relied on spectroscopic and chemical methods. Spectroscopic examination involved electron impact, chemical ionization, and field desorption mass spectrometry, H- and C-nuclear magnetic resonance study (sometimes NOE and correlation spectroscopy), and this was followed by isolation of the polyhydroxylated core by alcoholysis or hydrolysis. Core 30 was novel and its structure established spectroscopically 48), whereas euonyminol was characterized as its octaacetate and compared with an authentic sample. 

Hincksinojlustra dentlculata, which occurs in Tasmania has been found to contain hinckdentine A, a novel pentacyclic tribromo alkaloid [41]. The structure and absolute configuration was determined by single crystal X-ray methods. 

In order to determine the manner in which sulfuric acid interacts with PPTA we are investigating the structure of PPTA oligomers omplexed with sulfuric acid. We have recently determined the structure of a N,N -(p-phenylene)dibenzamide (PPDB)/sulfuric acid complex using single crystal x-ray methods and similar studies with longer PPTA oligomers are underway now. We expect that the general principles for the interaction of sulfuric acid and PPTA will become apparent from these studies. 

Because of the nascent state of technetium chemistry, considerable emphasis is currently being given to the characterization of technetium complexes by single crystal x-ray structure analysis. These analyses provide a firm foundation upon which subsequent development and elaboration of technetium chemistry may be based. Approximately twenty technetium complexes have been characterized by single crystal x-ray methods (p, and several of the resulting structures have considerable relevance to radiopharmaceutical development. 

It is worth mentioning tetrameric complexes synthesized by Lahoz et al. based on a pyridine imine derivative and characterized by single-crystal X-ray methods. Unfortunately, none of these materials, despite their analogy to the Ru saw-horse compounds seen above (185), were mesomorphic. 

The structure of YNiSn has been determined by using a single-crystal X-ray method (Fran9ois et al. 1985). This structure can also be described as an AIB2 type derivative and it may be compared with the LaPtSi-type structure, which is a derivative of the ThSi2 type (FrauQois et al. 1985). 

The corresponding potassium compound K2[Ce(N03)g] has not yet been studied by single crystal X-ray methods although it is reported to form well-formed crystals. IR data and preparative details are available (Gatehouse et al., 1957,1958). Also the hydrated hexanitratocerium(IV) complex M Ce (N03)g 8H2O (M = Mg, Zn, etc.) has been r orted (Pascal, 1959). The structure of the possibly isomorphous rh(IV) complex has been determined (Scavincar and Prodic, 1964). 

Reactions of azadienes and dienamides, carrying chiral substituents, are repotted295 to react with [Fe2(CO)9] to afford (T -diene) iron tricarbonyl complexes with good to excellent diasteroselectivity, depending on the nature of the chiral auxiliary employed. Complete selectivity was observed for the dienamide prepared from (5)-2-(diphenylhydroxymethyl)pyrrolidine and sorbic acid, leading to a chiral non-racemic diene complex, the stereochemistry of which was determined by single crystal X-ray methods. 

Reactions of cationic [(2,4-dimethylpentadienyl)Fe(CO)3]+ complexes with sodium methoxide and with phosphines have been reported. Use of (5)-neomenthyldiphenylphosphine under appropriate conditions afforded a single optically pure diastereomer, the stereochemistry of which was determined by single crystal X-ray methods. Treatment of [(Ti4-2-alkoxy-4-vinyl-2,5-dihydrofuran)Fe(CO)3] complexes with nucleophiles in the presence of Lewis acids afforded297 exo-addition products with high diastereoselectivity. Hydrolysis of the products afforded hydroxyaldehyde complexes. 

It is generally faster and more accurate to do structure analysis by single-crystal X-ray methods than by electron diffraction. While... 

The structure of TbNi4P2 was determined by the single-crystal X-ray method the atomic parameters are refined to 7i = 0.133 (Chykhrij et al. 1986). Phosphorus atoms are situated in trigonal prisms formed by metal atoms (fig. 37). The shortest interatomic distances are Tb-Ni ""0.303 5rb-p=0.282 5Ni p = 0.230. Isotypic compounds are ScFe4P2 and RNi4P2 where R = Y, Gd, Dy-Lu. 

---