Low-temperature x-ray structure techniques for the characterization of thermolabile molecules

Abstract

Contents I. Introduction 81 A. General Aspects 81 B. Brief Comparison of the Methods for me 82 Characterization of Unstable Intermediates 11. Methods and Devices 82 A. General Remarks 82 B. Main Elements of Gas Stream X-ray 83 Diffraction Cooling Devices 1. Basic Principles 83 2. Gas Supply UnR 83 3. Delivery Nozzle 84 4. Transfer Line 84 5. Refill System 84 8. Automatic Temperature Control 85 85 Crystals 1. Crystal Growth in a Capillary 85 2. Crystal Growth in One Place and 86 Transfer to the Diffraction Device under Protective Conditions Elsewhere 111. Use of Three-Dimensional Structures of 87 Unstable Inorganic Molecules and Solids in the Evaluation of Their Reactivity C. Growih and Handling of Thermolabile Single A. Correlation of Instability and Molecular 87 B. Reaction Intermediates 89 C. Unstable Phases in Solid-state Chemistry 90 IV. Conclusion and Outlook 90 V. Acknowledgments 90 VI. References 90 Structure I. Introductlon A. General Aspects The chemist is currently confronted with compounds that are unstable for either kinetic or thermodynamic reasons. I f these compounds are produced during t h e course of a reaction sequence, t h e y are designated as reaction intermediates. As well as chemical procedures, a variety of physicochemical methods has been used (see n e x t chapter) to elucidate t h e composition and three-dimensional structure of t h e intermediate. T h e most complete and exact results are obtained by diffraction methods, followed by three-dimensional structure determinations. With X-ray diffraction on single crystals, the sample has to be cooled because of its instability, and the crystal growth and transfer must b e performed under low-temperature conditions. In this review the special techniques for the synthesis and handling of single crystals of unstable compounds are described. As far as we know, there is only one

Crystal handling at low temperatures

“…The advantages of crystal structure determination at low temperature, compared to conventional room-temperature measurements, are well known (Rudman, 1976;Veith & Frank, 1988;Luger, 1989). Crystals that are unstable under ambient conditions, as well as liquids or gases, can be studied in the solid state.…”

Investigations of the Structure and Reactivity of a Stannaketenimine

“…The spectra were measured at temperature intervals between -80 to + 70 "C in a 10 mm NMR tube with 0.3 g 5 in 0.5 mL C6D6 and 2 mL toluene. The Ii9Sn NMR chemical shift of 3 is known (6 = 678)[6], that of 5 was extrapolated to6 = -150. The experimental "'Sn NMRdatacollected at a number oftemperatures was used in the calculation of the dissociation enthalpy and entropy.…”

Si-O-Bindungsspaltung in einem Siloxamin durch Aluminiumhydrid

“…Mit Hilfe von Ku È hltechniken lassen sich die Kristalle auf ein Diffraktometer montieren [22]. Die Sammlung der Ro È ntgenstrukturdaten erfolgte bei 200 K auf einem Fla Èchenza È hler; weitere Daten zu der Strukturbestimmung sind in Tabelle 1 enthalten.…”