Chemistry Reference
In-Depth Information
1
H NMR studies on solution of tetrameric [Al(OPr
i
)
3
]
4
by Shiner
et al
.
489
and
Mehrotra and Mehrotra
578
show three doublets for the CH
3
protons in a 1:1:2 ratio
(Fig. 2.7), as the methyl groups in the bridging isopropoxo ligands are nonequivalent.
These observations are consistent with structure (2-IX) which has been confirmed by
X-ray crystallography
579
,
580
(when R D Pr
i
).
In order to study the structure of aluminium isopropoxide in detail, Worrall
et al
.
490
,
492
,
581
measured the
1
H NMR spectra of aluminium isopropoxide under
different experimental conditions. The freshly distilled aluminium isopropoxide on
crystallization at 20
Ž
C was dissolved in benzene and the
1
H NMR spectrum of this
solution was almost identical to that obtained by Shiner
et al
.
489
consisting of three sets
of doublets, thus confirming yet again the proposed tetrameric structure for aluminium
isopropoxide. However, these workers assumed the nonequivalence of methyl protons
to be due to the asymmetric nature of the molecule which has D
3
symmetry and in
principle is optically active.
The results of Shiner
et al
.
489
and those of Mehrotra and Mehrotra
578
are presented in
Table 2.21. In the case of the 100 MHz spectrum, two distinct sets of septets with equal
intensity ratio were observed; these peaks could be assigned to the methine protons
of the terminal and bridging isopropoxo groups. Following Worrall
et al
.
490
,
492
the
explanation of the splitting of methyl protons in the spectra of tetrameric aluminium
isopropoxide could be sought in terms of magnetic nonequivalence of the two methyls
on the same isopropoxide group due to asymmetry, but then the corresponding methine
protons on the isopropoxide groups in similar environment should be identical. This
general conclusion was confirmed by the appearance of only two sets of methine proton
Figure 2.7
1
HNMR
spectrum of aluminium
isopropoxide in CDCl
3
at
60 MHz.
