Chemistry Reference
In-Depth Information
H
H
17 34 4 0
34 20 0 0
33 29 1 0
-1 0 -11 35
0 -1 -4 35
0 -1 -8 36
OOO
1
H1
H2
1 1 45 60
0 1 40 60
0 1 42 60
41 2 1 0
55 19 2 0
51 11 2 0
O3
O2
O1
22 57
3 44
7 48
25
0
H
H
OOO
29
0
27
0
C 11
C1
C8
2
5 1 2 0
2 1 1 0
2 1 1 0
1 1 3 0
1 4 7 0
1 2 6 0
0 0 2 1
0 0 2 1
0 0 2 1
H
H
O
O
O
3
TS
Fig. 6 Source function percentage contributions from the eight labeled atoms to the four O···H
bonds of structure
1
, its quinone-like tautomer
2
, and their interconnecting TS,
3
. Sources are listed
in matrices with rows corresponding to structures
1
,
2
, and
3
and columns 1-4 corresponding to the
four O···H bonds,
from left to right
(O3···H1, H1···O2, O2···H2, H2···O1). O···H bond distances are
listed for each structure in Table
8
. Sources from atoms contributing less than 1% are not shown
(adapted from Chart 1, with permission from [
49
], Copyright 2006, American Chemical Society)
8-dihydroxy-3,6-dimethylnaphtalene,
1
(Fig.
6
), studied by low-temperature synchro-
tron X-ray diffraction and DFT calculations. Two short intramolecular O-H
O
interactions are found in
1
,w thoneO
O distance being extremely short,
2.393
˚
, similar to that found in nma, and the other being 2.598
˚
short. Due to
system's neutrality, both interactions should be termed as RAHBs, though the very
short one lies outside the range typically associated with this class of HBs. However,
at variance with nma, which has a perfectly symmetric keto-enol fragment despite a
highly nonsymmetric HB, the keto-enol fragment in
1
is
far from being symmetric
and represents a severe outlier relative to published RAHB correlations [
48
,
52
].
Furthermore, the two OH
O HBs were found structurally quite different, though
being part of rings formed by the same kind and sequence of atoms. Investigation of
system
1
thus served two very interesting purposes. On one hand, to shed light on
why a very short O
-localization may realize,
in contrast to the widely acknowledged belief that very strong HBs in neutral systems
require to be assisted by resonance. On the other hand, to challenge the question of
which chemical environment forces the neat distinction between the two HBs in the
molecule. The SF tool was adopted, along with other investigative tools, to address
these two issues.
To fully account for the experimentally observed structure and to also model the
effect on HBs features of alternative
O distance combined with apparent
p
-bonding structures, DFT calculations were
performed on molecule
1
, its quinone-like tautomer
2
, lying about 1 kcal/mole
higher in energy, and their interconnecting TS
3
. (Fig.
6
). Table
8
lists the bond
p
