Chemistry Reference
In-Depth Information
Fig. 7
ClF,
drawn to scale, are shown in comparison to the n-pair model of formaldehyde. The angle
φ
is again similar in both complexes but the non-linearity
θ
of the hydrogen bond is much
greater than that of the halogen bond. See Fig. 1 for key to the colour coding of atoms
The experimentally determined geometries of CH
2
O
···
HCl and CH
2
O
···
It is possible to test the n-pair model more precisely when O is the elec-
tron donor atom. In the series of cyclic ethers oxirane [(CH
2
)
2
O], oxetane
[(CH
2
)
3
O], and 2,5-dihydrofuran [(CHCH
2
)
2
O], the internal ring angle COC
increases from
109
◦
(the tetrahedral angle). It is
generally accepted that this increase is accompanied by a corresponding de-
crease in the angle between the n-pairs carried by O; if so the angle
60
◦
90
◦
∼
through
∼
to
∼
should
decrease correspondingly. The n-pair models of these three Lewis bases are
shown in Fig. 8 together with the observed geometries of their complexes
with HCl. Table 1 gives the angles
φ
φ
and
θ
for the two series B
···
HCl and
B
ClF, where B is oxirane [67, 120], oxetane [122] or 2,5-dihydrofuran [29,
77], all determined from their rotational spectra under assumptions identical
to those described earlier for the oxirane complexes [(CH
2
)
3
O
···
···
ClF has not
yet been investigated].
It is clear from Fig. 8 and Table 1 that the angle
does indeed decrease
as expected if the n-pair models and rule 1 were applicable. Moreover, the
hydrogen bond non-linearity
φ
decreases along the series B = oxirane, oxe-
tane, 2,5-dihydrofuran. On the other hand, the values of
θ
θ
for oxirane
···
ClF
and 2,5-dihydrofuran
ClF (included in Fig. 8) reveal that the halogen bond
shows little propensity to be non-linear.
OxiraneisanimportantLewisbaseinthepresentcontext.TheOatomcar-
ries two equivalent n-pairs of electrons, as it does in H
2
O, but oxirane has the
advantage over water in that it is possible to determine both angles
···
θ
for its complexes with HCl and ClF because the non-zero off-diagonal element
χ
ab
(Cl) of the Cl nuclear quadrupole coupling tensor is available. The cor-
responding Lewis base in which an S atom carries two equivalent n-pairs is
thiirane. Each of the pair of complexes (CH
2
)
2
S
φ
and
···
HCl and (CH
2
)
2
S
···
ClF has
C
S
symmetry and here it is the off-diagonal element
χ
ac
(Cl) that is non-zero
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