Naslov
At the limits of the method – X-ray diffraction determination of crystal structure of tetrabuthylammonium tetrabuthylborate

Autori
Stilinović, Vladimir ; Kaitner, Branko

Vrsta, podvrsta i kategorija rada
Sažeci sa skupova, sažetak, znanstveni

Izvornik
Eighteenth Croatian-Slovenian Crystallographic Meeting / Cetina, Mario ; Popović, Stanko ; Skoko, Željko ; Štefanić, Zoran ; Višnjevac Aleksandar - Zagreb : Hrvatska akademija znanosti i umjetnosti (HAZU) ; Hrvatska Kristalografska Zajednica, 2009, 27-27

Skup
Eighteenth Croatian-Slovenian Crystallographic Meeting

Mjesto i datum
Varaždin, Hrvatska, 18.06.2009. - 20.06.2009

Vrsta sudjelovanja
Predavanje

Vrsta recenzije
Nije recenziran

Ključne riječi
X-Ray diffraction analysis; isoelectronic species; tetraalkylammonium; tetraalkylborate

Sažetak
It is generally known that X-ray diffraction cannot distinguish between pairs of isoelectronic ions since they scatter the radiation identically. However, only examples of compounds comprised of pairs of isoelectronic ions are simple binary compounds such as alkali halogenides and alkaline-earth chalcogenides. The question that remains is whether it would be possible to distinguish polyatomic isoeletctronic species, which would probably slightly differ in bond lengths and angles. To the best of our knowledge, there were no X-ray diffraction studies of any such salts performed to date. An obvious choice of a model compound for such studies is a salt of the type [NR_4][BR_4] in which all R groups are identical. We have turned our attention to the title compound, [NBu_4][BBu_4], since both the cation and the anion are known and can be synthesized. [NBu_4][BBu_4] was synthesised, crystallised and a suitable crystal measured on an Oxford Diffraction Xcalibur Kappa CCD X-ray diffractometer with MoKα radiation. Data obtained from the diffraction experiment indicate that [NBu4][BBu4] crystallises in the space group P 4_2/n with a = 13.404 &Aring ; , c = 5.298 &Aring ; and Z = 1. This model obviously cannot be correct, for it supposes the cation and anion to be symmetrically equivalent. This could only be in the case of positional disorder, i. e. random distribution of the ions, which would be contrary both to chemical intuition and to structures of similar salts. Thus it is necessary to choose a larger unit cell volume in which a sensible periodical distribution of oppositely charged ions is possible. Such a cell is easily obtained from the original cell applying the transformation matrix [(1, -1, 0)(1, 1, 0)(0, 0, 2)]. Model obtained in this way belongs to the space group I 4_1/a with a = 18.958 &Aring ; , c = 10.396 &Aring ; and Z = 4. In this model cations and anions alternate with each cation surrounded by 6 anions and vice versa. Analogous packing can be seen in the structures of the [MPh_4][BPh_4] (M = P, As, Sb) series which supports the correctness of the model. However, if in this model instead of the ordered array of ions, a disordered one is presumed, the model can be refined as well as the ordered one. Switching the positions of the nitrogen and boron atoms does not significantly influence the final R factor, so that diffraction data cannot be used to determine which is which. However, since C— B bond is statistically somewhat longer than the C— N bond, it is possible to correctly assign the atom type of the central atoms.

Izvorni jezik
Engleski

Znanstvena područja
Kemija

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