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

Abstract

~ ~~ ~~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 ...

Molecular electric moments and electric field gradients from X-ray diffraction data: model studies

“…l.ti, #,j, Oij, V E,, Stewart & Jensen (1967) Kvick & Popelier (1992); (e) Jeffrey, Ruble, McMullan & Pople (1987); (f) Coppens (1967); (g) Jeffrey, Ruble, McMullan, DeFrees, Binkley & Pople (1980); (h) Savariault & Lehmann (1980); (i) Swaminathan, Craven & McMullan (1984); (j) Boese, Maulitz & Stellberg (1994); (k) Nijveldt & Vos (1988); (/) Kampermann, Ruble & Craven (1994).…”

Retrieval of Structure-Factor Phases in Non-centrosymmetric Space Groups. Model Studies Using Multipole Refinements

“…spherical atoms) typically leads to an underestimate in peak heights in Fourier deformation electron density maps, and by as much as 50% (Savariault & Lehmann, 1980), although this is clearly space-group-dependent because of the varying contribution from restricted phases (see below). Although most current charge density studies performed on non-centrosymmetric molecular cystals seek more than a Fourier deformation density, a number of recent analyses have presented electron density maps as the sole basis for any chemical discussion (see, for example, Belaj, 1992;Radaev et al, 1992;Boese, Maulitz & Stellberg, 1994;Antipin, Chernega, Lysenko, Struchkov & Nixon, 1995), sometimes without acknowledging the existence of a phase problem per se.…”

Aromaticity of benzenoid hydrocarbons with inserted –B=B– and –BH–BH– groups: a comparison

“…Borazine, B 3 N 3 H 6 , the most famous member of the azaborines group, is even named ''inorganic benzene'' [22]. Aromaticity of other boron compounds was also reported several times [23][24][25][26][27][28].…”