Proton conductivity in benzenesulfonic acids
Russ.Chem.Bull., Int.Ed., Vol. 60, No. 6, June, 2011
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distances (2.499(1), 2.505(1), and 2.492(2) Å, respectively;
in 1, the O(2W)...O(3W) and O(4W)...O(3WA) distances
are 2.770(1) and 2.758(1) Å) (see Fig. 3). As opposed to
the dihydrate, strong hydrogen bonds in tetrahydrate 2 are
characterized by pronounced asymmetry, resulting in that
the H(1WA) and H(1WB) protons form stronger bonds
with the O(1W) atom (0.85—0.92 Å) than with the O(4W),
O(2W), and (O(2W´) atoms (1.58—1.60 Å).
The other hydrogen atoms of the cationic chains in 2
are involved in the formation of cationic hydrogen bonds
(O...O, 2.657(1)—3.148(1) Å) typical of sulfonic acids.
As in the case of 1, the shortest distances between the
proton donor and acceptor are observed for the onium
oxygen atom O(1W), whereas the longest distances
(2.816(1)—3.148(1) Å) are observed for the formally solꢀ
vent water molecule O(4W).
In the crystal structure of 2, the occupancies of two
components of the O(2W) atom are substantially different
and are 0.884(3) and 0.116(3) for O(2W) and O(2W´),
respectively, at 100 K (see Fig. 3). The Xꢀray diffraction
studies of 2 performed at higher temperatures showed that
the disorder has the dynamic character. Thus, at 173 and
250 K the occupancy of the O(2W´) position increases to
0.199(7) and 0.275(8), respectively. It should be noted
that, as in the crystal structure of 1, the isotropic temperaꢀ
ture factor of the H(1WA) proton (0.067(5) Å2) involved
in the hydrogen bonding with the disordered oxygen atom
is larger than that for the H(1WB) proton (0.046(3) Å2).
A similar relation is retained as the temperature increases
to 250 K (0.101(10) and 0.087(8) Å2). It should be noted
that the larger temperature factor cannot be attributed to the
stronger hydrogen bond and, consequently, to an increase
in the vibrational amplitude because the O(1W)...O(4W)
and O(1W)...O(2W) distances are slightly shorter regardꢀ
less of the temperature. It is interesting that, except for
variations in the occupancies of two O(2W) positions, the
other molecules/ions in 2 remain ordered.
This disorder, which has been observed earlier in other
sulfonic acids,13 may be attributed both to the inversion of
the oxygen atom14 and the aboveꢀmentioned superposiꢀ
tion of the "water" and "onium" components of the cation.
Earlier we have shown that the lone pair of the onium ion
is actually not revealed by topological analysis of the elecꢀ
tron density distribution function, as well as by the analyꢀ
sis of the electron pair localization function and is "inert"
with respect to the formation of specific interactions in
the crystals.15 However, taking into account very low barꢀ
rier of the inversion (about 1 kcal mol–1),14 it would be
expected that the occupancies of two oxygen positions are
equal, but this was not observed experimentally. Hence,
the aboveꢀdescribed disorder in the crystals of 1 and 2 is
most likely attributed to the proton transfer, and the
nonequivalence of the positions is, in turn, due to the
difference in the environment of the atoms in the crystals
resulting in the different strength of the hydrogen bonds
for two tautomers.16 It should be emphasized that the locaꢀ
lization of the position of the disordered O(2W) atom in
salt 2 at 100 K became possible due to the high resolution
of the Xꢀray diffraction data (2θmax = 100°), whereas it is
impossible to localize this weakly occupied position of the
oxygen atom (to be more precise, to suggest the presence
of this position) in the routine experiment. Hence, it canꢀ
not be ruled out that the observed disorder has a more
general character and may be observed in the formally
"ordered" (according to the literature data17) crystal strucꢀ
tures, resulting in systematic errors in the determination
of the O...O and O—H distances characterizing the
strength of the hydrogen bonding.
Apparently, the presence of an infinite hydrogenꢀ
bonded network formed by water molecules and the onium
ion in tetrahydrate 2 as opposed to the discrete system of
hydrogen bonds (SO3–...H5O2+...SO3–...H5O2+) in diꢀ
hydrate 1 (Fig. 4) leads to an increase in the specific proꢀ
ton conductivity by more than 1.5 orders of magnitude
(from 10–6 to 10–4 S cm–1 at room temperature). This fact
is in good agreement with the assumed Grotthussꢀtype
mechanism of the proton transfer in onium salts 1 and 2.
Taking into account that the disorder of the oxygen atom
may be in part considered as a structure defect, it can be
hypothesized that its presence is also favorable for an inꢀ
crease in the specific proton conductivity.
It should be noted that, in spite of the dramatically
different character of the cation associates, the arrangeꢀ
ment of the anions in salt 1 is almost identical to that in 2.
As it can be seen from the projections of the crystal packꢀ
ing (see Fig. 4), the anions are arranged in headꢀtoꢀtail
stacks in both structures. Based on the interplanar distances,
it may be concluded that the anionic columns in 1 and 2
are involved both in stacking interactions between the aroꢀ
matic systems and in quite unusual O2S—O–...O2Nꢀtype
interactions. Thus, pairs, which are linked together preꢀ
dominantly by short O...N interactions (2.991(1) Å) charꢀ
acterized by the distinct directionality (the angle between
the O(1)...N(1A) line and the plane of the nitro group is
86.5°), as well as dimers stabilized by stacking interactions
(the shortest C(3)...C(1A) distance is 3.480(1) Å), are obꢀ
served in the anionic columns in the crystal structure of 2
(Fig. 5). In the crystal structure of 1, there are apparently
no stacking interactions in the columns (the C...C disꢀ
tances (3.932(2) Å) are larger than 3.55 Å), and the O...N
anion—anion interactions are somewhat shorter (2.839(2) Å).
These anion—anion interactions are similar in nature to
the interactions between nitrate anions10 and, apparently,
correspond to the charge transfer from the lone pair of the
oxygen atom to the antibonding orbital of the nitrogen
atom of the nitro group.
Within the associates stabilized by both stacking interꢀ
actions and strong intermolecular interactions, the charge
transfer is always observed. It should be noted that the
conductivity determined by the presence of stacks of aroꢀ