28
B. Tolnai et al. / Journal of Molecular Structure 924–926 (2009) 27–31
F
F
OCH3
F
CH3O
COOH
COOH
COOH
3
2
1
F
F
CH3O
F
F
CH3O
COOH
COOH
COOH
6
5
4
Fig. 1. The molecules of this study.
evaluated by the WIN-IR software package. The spectra were base-
line corrected but were not transformed in any other way.
3.2. IR spectroscopy of solutions and the solid acids
The comparison of the spectra of solutions with various concen-
trations of cinnamic acid derivatives to the relevant solid com-
pounds and identifying bands sensitive to concentration changes
proved to be a fruitful method in identifying hydrogen bonding
interactions [1–3,9,13]. This method was used here too.
2.3. Computational methods
Molecular modelling was performed with semiempirical and
DFT codes included in the HyperChem package [5]. Preoptimisation
was performed with the PM3 semiempirical method [6] and ab ini-
tio calculations were done with a hybrid DFT method applying the
B3LYP functional [7] and using the 6-31 G** and 3-21 G basis sets
for the dimers and the dimer of the dimers, respectively. Optimisa-
tion was considered to be over when the gradient became smaller
than 0.1. Stationary points obtained were checked with frequency
calculations and were found to be minima.
Since the aim was to identify OAH. . .O, (aromatic) CAH. . .F,
@CAH. . .F, and (aliphatic) CAH. . .F hydrogen bonds, whichever it
exists here, the possible shifts in the positions of
m
(OH),
m(@CAH),
m
(C@O), (C@C), (CAOH), (CAOAC) and, of course, m
m
m
m
(CAF) were
monitored. The frequency data together with some indications
concerning intensities are summarised in Table 1.
Most bands could be easily assigned, however, in the region
where the
m(CAF) vibration appears the
m
(CAO) band is quite in-
tense, thus, the
shoulders.
m(CAF) vibrations were mainly identified as
3. Results and discussion
On inspecting data in the table the following conclusions could
be drawn.
3.1. General considerations
The basic structural units for the acids were the dimers, in-
deed, kept together by two OAH. . .O hydrogen bonds. They are
the exclusive or major moieties in the solid state as well as in
the two most concentrated solutions. At the lowest concentra-
tions, however, considerable proportion of the acids exists in
the form of monomer. These statements by compounds are sup-
ported by the oppositely directed changes in the positions of the
bands (second and third columns of the table) for the monomer
In our earlier works we experienced that in the solid state
CAH. . .X (where X was O [8,9] or N [8,10]) hydrogen bonds had
crucial role in the formation of extended aggregates within the het-
erocycle-substituted propenoic acid and ester family of com-
pounds even if these interactions are considered to be weak [11].
The interactions were mainly identified with IR spectroscopy and
in many cases measurements were complemented with molecular
modelling computations. It was found that the carbon donor atom
was always part of the aromatic ring.
and the associated
m(OH) vibrations as well as by those in the
fifth and sixth columns for the monomer and the associated
In 2,3-diphenyl propenoic acids and their methyl esters where
the olefinic hydrogen was substituted to CF3 group [3], fluorine
was also engaged in hydrogen bonding, however, this time the ali-
phatic carbon of the methyl ester could also be the donor of the
hydrogen bond.
In the esters of cinnamic acids CAH. . .O hydrogen bonds where
the carbon atom was olefinic could be identified [12,13], but this
type of bond was not found in the acids.
m
(C@O) vibrations.
On searching for other types of hydrogen bonds the most dilute
solution (10ꢀ4 mol/dm3) is taken as the hydrogen bond-free sys-
tem (or at least the system, with very low abundance of hydrogen
bondings) and it is examined whether shifts in the positions of the
relevant vibrations occur or not on increasing the concentration of
the acids or in the solid state.
Shifts in the positions of the m(CAF) vibrations (column 9) indi-
Let us note here that for the acids the dimers (kept together by
two relatively strong OAH. . .O hydrogen bonds) were considered
to be the basic structural units, since even in the gas phase they
are the most abundant species, their proportion diminishes only
in dilute solutions.
To sum up, in the weak CAH. . .X (X = O, N, F) hydrogen bonding
interactions that keeps together extended networks of cinnamic
acid dimers and ester derivatives (the latter aggregates as mono-
mers) in the solid state, the carbon is typically aromatic, occasion-
ally, it is aliphatic and it can be olefinic only in limited cases.
cate that fluorine was engaged in some kind(s) of hydrogen bond-
ing in the solid state. For compounds 2 and 3, even two different
vibrations could be detected, both of which were shifted relative
to those observed in solution. This may mean that various kinds
of CAH. . .F hydrogen bonds coexisted in the crystalline form. For
the other compounds only one vibration could be observed as
shoulder, the other may be covered by stronger m(CAOAC) vibra-
tions. Somewhat surprisingly, for most compounds blue shift was
observed for this vibration instead of red shift, which is commonly
considered to be the sign of hydrogen bonding interaction. Never-
Tolnai
