Chemistry Reference
In-Depth Information
Table 8 Geometrical data and O···H bcp density in 2-acetyl-1,8-dihydroxy-3,6-dimethylnaphta-
lene,
1
, its diphenolic tautomer
2
and their interconnecting TS,
3
a
Bond 1,
R
e
(
r
b
) 2,
R
e
(
r
b
) 3,TS;
R
e
(
r
b
)
O3···H1 1.419 (0.098) 1.065 (0.264) 1.153 (0.205)
H1···O2 1.040 (0.282) 1.366 (0.112) 1.229 (0.165)
O2···H2 1.749 (0.040) 1.689 (0.048) 1.715 (0.044)
H2···O1 0.977 (0.349) 0.988 (0.337) 0.984 (0.341)
O3···O2 2.393 2.372 2.333
O2···O1 2.598 2.573 2.588
a
Data from [
49
]. See Fig.
6
for molecular structure drawings. Distances in
˚
, bcp densities in
atomic units (au). Ab initio theoretical data
distances and bcp density values of the four O-H, or O
H interactions shown in the
Fig.
6
, which also displays the corresponding percentage SF atomic contributions
from atoms contributing more than 1%. The SF analysis clearly distinguishes
O2
H or O-H interactions, in all structures
1
-
3
. The sum of SF percentage contributions from O2, H2, and O1 atoms to the
O2
H2 bond from the remaining O
H2 bcp density is only 59%, 65% and 61% in
1
,
2
, 3, while the corresponding
contributions for the other three bonds in all the three structures always exceed 80%
of their bcp density. These results classify O1-H2
O2 as a short normal electro-
static HB and the others H
O bonds as short HBs with large covalent character
(O1-H2 in
1
-
3
is a normally covalent O-H bond). Hence, despite the similarities of
the six-membered rings to which they belong, H1 and H2 exhibit completely
different S%(H) values. The former atom contributes largely to the bcp density of
both bonds with its linked O atoms, while H2 shows a high and negative SF
percentage contribution to the bcp density of the O2
H2 bond, typical of an
electrostatic HB. The positive and large S%(H) values for H1 in
1
(17% and 34%
for O3
H1 and H1-O2, respectively) denotes a partially symmetric
L
(r) distribu-
tion for such an atom associable with a low-barrier or single-well HB. Indeed,
high-level DFT calculations on structures 1-
3
, including zero-point vibrational
corrections, suggested the O3
H1 hydrogen bond to be a double-well potential,
with a very low barrier between the two minima, so that in practice the H1 sits in a
single-well potential. Sorensen et al
.
[
49
] also noticed the large differences in the
contributions from O2, the only O atom common to the two rings, to the bcp
densities of the distant O3-H1 and O1-H2 bonds. In none of the three structures
does O2 contribute to the normal O1-H2 interaction, while it contributes signifi-
cantly to the O3-H1 interaction in all three structures, and in particular in
1
,in
which it provides 22%. These striking differences in S%(O2) denote electron
delocalization only in the left keto-enol fragment and may be easily rationalized
in terms of the HB classification based on the SF tool given by Gatti et al
.
(see
earlier). The
-delocalization is, however, only partial since this fragment is far
from being symmetric, as stated earlier. In order to retain the fully delocalized
naphthalene structure, C11-O3 has to be mostly of double-bond character and
C1-O2 must be a single bond, which is closely realized in
1
. Comparison of the S%
contributions from the C atoms linked by a formal double bond to the H-acceptor
p
