A reinvestigation of the crystal and molecular structure of hexachloroborazine

Gopinathan, M. S.; Whitehead, M. A.; Coulson, C. A.; Carruthers, J. R.; Rollett, J. S.

doi:10.1107/s0567740874003608

Cited by 9 publications

(1 citation statement)

References 22 publications

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“…The method proceeds from an initial model to a refined model by successive adjustments. There is no way of guaranteeing that the model will improve or even that the process is convergent, nor of knowing that, having found a good model, there does not exist an even better one (Gopinathan, Whitehead, Coulson, Carruthers & Rollett, 1974). Equations (1) and (2) (observational equations, see later) show one of the sources of the difficulty:…”

Section: Principlesmentioning

confidence: 99%

The control of difficult refinements

Watkin¹

1994

Acta Cryst Sect A

234

185

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Each year, many single-crystal structure analyses are reported that show evidence of over-or under-refinement.Often, the refinement strategies have been naive or overcomplex and alternative strategies might have been more effective. The many descriptions of crystallographic and numerical techniques suitable for assisting with the control of difficult refinements are distributed widely in the literature and so are not always easily accessible. Without being a review of these procedures (which would require a substantial book), this article attempts to list readily available procedures, together with a brief outline of their backgrounds and examples of their applications to organic and organometallic compounds. The analysis of extended-lattice materials (usually inorganic materials) often raises problems in addition to those covered here. The particular aim of this article is to remind the reader that X-ray structure analysis is a modelling process and that, while standard models may be adequate for most analyses, more care and imagination must be applied to the treatment of difficult cases. Principles of methods are described without detailed mathematical derivations, although sufficient references to the literature are provided to permit careful study. Future requirements for refinement processes are outlined, including the use of new machine architectures, applications of sparse-matrix methods and the development of expert systems.

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Section: Principlesmentioning

confidence: 99%

The control of difficult refinements

Watkin¹

1994

Acta Cryst Sect A

234

185

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The molecular and solid‐state structure of borazine has been determined at 115 and 160 K by single‐crystal X‐ray crystallography. The molecular structure has C₂ symmetry with small deviations from the expected D_3h symmetry. The mean value of the BN bond length is 1.429(1) A and the interbond angles of the six‐membered ring are 117.1(1)° at the boron atoms and 122.9(1)° at the nitrogen atoms (mean values). Similar to benzene an unambiguous assignment of the molecul

Boese¹,

Maulitz²,

Stellberg³

1994

Chem. Ber.

101

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~ ~~ ~~The molecular and solid-state structure of borazine has been determined at 115 and 160 K by single-crystal X-ray crystallography. The Borazine is generally introduced in textbooks as "inorganic benzene" under aspects of the isoelectronic relationship. This holds only for some physical and chemical properties. The prominent discrepancies in the reactivity are attributed to the significant difference between the electronegativities of boron and nitrogen atoms. Among the physical properties the density and boiling point are quite similar. However, the melting points differ by approximately 60°C. This suggests that the solid state of borazine must be quite different from that of benzene.The crystal structures under various conditions of benzene are known for a long time"], all of them exhibit the well-known herringbone pattern in the space groups Pnma and P 2 , k A crystal structure determination of borazine has never been performed, obviously due to crystallization problems.Moreover, even the molecular structures of both compounds are still debated. The vast amount of experimental data of benzene does not allow an unambiguous assignment of the symmetry of the equilibrium However, the results from quantum-mechanical calculations favor DhhThe molecular structure of borazine has been determined by gas-phase electron diffractionL4I. In the latest electron diffraction study by Harshbarger et al.[4c] a planar D3h and two nonplanar (C2 and C3,) models were discussed. The C3, model was excluded because of inconsistency with a microwave investigation. A decision between a D3h model with large vibrational amplitudes perpendicular to the ring plane and a nonplanar C, model was not possible. Thus, distinct assignments of the molecular geometry and symmetry by diffraction methods have not yet been made.After many attempts we succeeded in growing a cylindrical single crystal by means of a miniature zone-melting procedure in a capillary directly on the diffractometer applying an infrared heatThe structure refinements in the tetragonal space group P43212 from the diffraction data at 160 and 115 K give no evidence for solid-state phase transitions; the molecules behave like rigid bodies in the given temperature interval. Results and DiscussionBorazine has crystallographic C2 symmetry, the molecule deviates slightly from the expected D3h symmetry ( The bond angles in the six-membered ring are significantly different, 117.1(1)' at the boron atoms and 122.9(1)" at the nitrogen atoms (mean values). Similar findings have been mentioned for borazine derivati~es[6"~~1, but possible substituent effects or the accuracy of the results did not allow an unambiguous statement.The phenomenon of the alternating angles in the sixmembered ring can be understood by Bent's rule considering a rehybridization of the ring The more electronegative nitrogen atoms prefer hybrid orbitals with lower s character. Therefore, the increased p character at the boron atoms results in decreasing bond angles, whereas the opposite behavior is observed at the ...

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Dictionary of Inorganic Compounds

Macintyre¹,

Daniel²,

Stirling³

1992

Dictionary of Inorganic Compounds

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A reinvestigation of the crystal and molecular structure of hexachloroborazine

Cited by 9 publications

References 22 publications

The control of difficult refinements

The control of difficult refinements

Dictionary of Inorganic Compounds

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