Chemical Crystallography and Structure 1947-67

Structural studies by diffraction methods

The accurate study by neutron diffraction of hydrogen bonded crystals has led to a fuller understanding of the potential energy surfaces in these crystals. In particular, the isotope effects on structures with very short hydrogen bonds have been especially illuminating. Among such structures studied have been those of potassium acid salt of aspirin, oxalic acid dihydrate, crystals containing the bifluoride ion, and chromous acid.

Comparisons between x-ray and neutron diffraction data in simple organic crystals have led to the first convincing demonstration of bonding effects in the electron density maps for these crystals. Electron densities associated with chemical bonds and with lone pair electrons have been clearly seen.

Crystal structure investigations of transition-metal complex hydrides have provided evidence that most of these compounds have normal covalent bonds between metal and hydrogen. The nature of this bonding has been argued by inorganic chemists in recent years, and a neutron diffraction study of the crystal structure of manganese pentacarbonyl hydride has provided a definitive answer, in accord with that suggested by earlier x-ray diffraction studies at BNL.

A number of crystal structure studies of transition metal complexes of bidentate sulfur compounds (the first such studies) has provided evidence of the remarkable ability of these ligands to stabilize unusual valence states and unusual coordination geometries for the transition metals.

Crystallographic studies have provided the first quantitative evidence for the structures of pentacoordinated oxy-phosphorus compounds. These structures are of great interest to chemists attempting to understand the role of pentacoordinated transition states of phosphorus in biochemical processes.

In the study of the structure of an organic compound which shows great activity toward certain forms of leukemia it has been found that the spacing between the planes of the molecules in the crystal is shorter than any heretofore known.

In the structure of a reversible molecular-oxygen carrier, the oxygen is bound to a metal atom through the p orbitals of the molecule; and there is little distortion of the molecular geometry. The nature of this bonding is obviously related to the bonding of oxygen to hemoglobin.

The first structural confirmation has been obtained for the existence of pentacovalent silicon compounds. The silicon atom is attached to two nitrogen atoms and three hydrogen atoms; this result is of great interest to chemists studying reaction mechanisms involving silicon.

S. J. La Placa and J. A. Ibers, "Studies of the Metal-Hydrogen Bond: Structure of HRh(CO)(P(C6H5)3)3" J. Am. Chem. Soc. 85, 3501 (1963).
J. A. Ibers, "The Nature of the Hydrogen Bond in the Bifluoride Ion," J. de Physique 25, 474 (1964).
S. J. La Placa and J. A. Ibers, "Structure of IrO2Cl(CO) (P(C6H5)3)2, the Oxygen Adduct of a Synthetic Reversible Molecular Oxygen Carrier," J. Am. Chem. Soc. 87, 2581 (1965).
R. Eisenberg and J. A. Ibers, "Trigonal Prismatic Coordination: The Molecular Structure of Tris(cis-1,2-diphenlylethene-l,2- dithiolato)Rhenium," J. Am. Chem. Soc. 87, 3776 (1965).
W. C. Hamilton, S. J. La Placa, F. Ramirez, and C. P. Smith, "Crystal and Molecular Structures of Pentacoordinated Group V Compounds. I. 2,2,2-triisopropoxy-4,5-(2',2"-biphenyleno)- l,3,2-dioxaphospholene, Orthorhombic," J. Am. Chem. Soc. 89, 2268 (1967).
R. Rudman, W. C. Hamilton, S. Novick, and T. D. Goldfarb, "Pentacoordinate Silicon: The Crystal Structure of Dimethyl- silylamine Pentamer," J. Am. Chem. Soc. 89, 5157 (1967).
A. Sequeira, C. A. Berkebile, and W. C. Hamilton, "Structure and Dynamics in Hydrogen Bonding Systems: I. A Neutron Diffraction Study of Potassium Hydrogen Diaspirinate(Bisacetyl- salicylate), J. Mol. Structure 1, 283 (1967-68).
P. Coppens, "Comparative X-ray and Neutron Diffraction Study of Bonding Effects in s-triazine," Science 158, 1577 (1967).

Structure of rare earth ions from optical spectra

Studies of the polarization spectra of crystals of rare earth compounds at low temperatures have resulted in a detailed understanding of the low-lying electronic levels of the metal ions. This in turn has permitted the interpretation of the spectra of europium salts in various solvents at very low temperatures. From the fine splitting, the exact symmetry of the arrangement of solvent molecules about the metal ion was derived; in the case of solvent water an electrostatic compression into a high-pressure ice-like structure was indicated. In other cases anionic association could be observed directly.

E. V. Sayre and S. Freed, "Spectra and Quantum States of the Europic Ion in Crystals. I. Absorption Spectrum of Anhydrous Europic Chloride," J. Chem. Phys. 24, 1211 (1956).
E. V. Sayre, D. G. Miller, and S. Freed, "Symmetries of Electric Fields about Ions in Solutions. Absorption and Fluorescence Spectra of Europic Chloride in Water, Methanol and Ethanol." J. Chem. Phys. 109, 109 (1957).

Brookhaven has been a premier center for neutron diffraction studies since its inception. For the majority of this period, the Brookhaven Graphite Research Reactor was the workhorse for such studies. As this period grew to a close, the High Flux Beam Reactor began to take the place of the BGRR as a leading facility for neutron science.

Top of Page

Last Modified: June 28, 2012