Solvation of lignin related phenols
Russ.Chem.Bull., Int.Ed., Vol. 63, No. 9, September, 2014
2047
before measurements, the appliance was calibrated using an inꢀ
corporated holmium light filter until the absolute accuracy of
wavelength measurements was no less than 0.05 nm. Each specꢀ
trum was recorded at least three times with subsequent Goꢀ
lay—Savitsky smoothing and determination of the positions of
absorption maxima.
Regression analysis of the dependences of positions of abꢀ
sorption bands on the solvent composition was carried out using
the CurveExpert version 1.4 program (Daniel Hyams, USA).
the solvent composition for the anionic forms of comꢀ
pounds under study, containing no proton donor groups.
The results of the regression analysis of the experimenꢀ
tal data using Eq. (6) are given in Tables 1—3. Attention
should be paid to the correlation coefficients r, in all the
cases exceeding 0.99 and indicating the adequacy of the
mathematical model used.
The coefficients f2/1 in all the media under study radiꢀ
cally differ for the phenols in molecular form and the
corresponding phenolate anions, with the second, as a rule,
having f2/1 < 1. This means that DMSO, MeCN, and DO
belong to the class of NDHBꢀsolvents (not donors of hyꢀ
drogen bonds), which virtually cannot solvate anions. It is
interesting that coefficients f2/1 for anions do not correlate
with acidity (proton donor ability) of the solvent, the minꢀ
imal parameter values are characteristic of DMSO, while
dioxane shows the best solvation ability. This can be apꢀ
parently explained by a considerable contribution in the
free energy of solvation, besides donorꢀacceptor interacꢀ
tions, of nonspecific hydrophobic interactions. Similarly,
for the nondissociated phenol molecules the preferential
solvation with an organic component of the mixed solvent
in the water—DMSO system is more pronounced for the
most polar components (vanillin and ferulic acid), while
in the waterꢀdioxane medium for all the guaiacol derivaꢀ
tive f2/1 >> 1.
It is obvious that the complex water—aprotic solvent,
possessing a high solvation ability toward both the phenols
and the corresponding phenolate anions, demonstrated
a pronounced synergetic effect upon interaction with
dissolved compounds. It should be noted that parameꢀ
ters f12/1 for the neutral and the ionic forms are comꢀ
parable virtually for all the compounds under study.
The solvents differ in the solvating abilities of complexes
water—aprotic solvent toward phenols, which increase in
the order: dimethyl sulfoxide < acetonitrile < 1,4ꢀdioxane.
In the case of phenolate anions, this sequence of solꢀ
vents is characteristic of only -guaiacylpropanol and
ferulic acid, for the remaining compounds the maximal
f12/1 values are observed in the water—acetonitrile solꢀ
vent system.
Table 4 summarizes the coordinates of the maxima for
the dependences of the "mixed solvent" content in the
solvation shell on the composition of a bulk solution calꢀ
culated using Eq. (4). From the data obtained, it follows
that the complexes of water with DMSO, MeCN, and DO
within a certain range of compositions of the binary solꢀ
vent play the role of the main solvating species. Their
fraction in the closest solvation surrounding of phenols
under study reaches 29—77%, whereas for the phenolate
anions this index lie within 66—96%. For the most polar
compound in the anionic form containing carbonyl and
carboxy groups (vanillin, ferulic acid), a virtually comꢀ
plete substitution of the solvent components with their
equimolecular complex is observed, this is especially true
Results and Discussion
The experimental data on the dependence of the posiꢀ
tion of the long wavelength absorption band of phenols
under study, which corresponds to the transition of aroꢀ
matic ꢀelectrons to the excited state (*), on the solꢀ
vent composition are shown in Fig. 1. In all the cases, one
can observe considerable deviations from the ideality (deꢀ
viations from a straight line), indicating the presence of
the preferential solvation effects exhibited by one of the
components of the mixed solvent. This is most characterꢀ
istic of the mixtures of water with 1,4ꢀdioxane, disꢀ
tinguished by the extremely low dielectric permittivity
( = 2.21 at 298 K), as well as of the water—acetonitrile
~
solvent mixture. In these mixtures, the dependence of max
for the most polar phenols on the molar fraction of organꢀ
ic cosolvent is of a profound extreme character. Generalꢀ
ly, the shapes of the curves in Fig. 1 correspond to the
results obtained for the water—N,Nꢀdimethylformamide
solvent system,7 which confirms the necessity to use the
model of preferential solvation for their description, since
it takes into account the presence in the solvation shell of
the dissolved compound of not only the molecules of water
and an aprotic solvent, but also of the "mixed solvent"
molecule in accordance with the scheme of equilibria sugꢀ
gested by us. From these positions, the presence of exꢀ
trema on the diagrams for ferulic acid and vanillin is exꢀ
plained by the fact that in the hypothetical binary solvent
consisting of only equimolecular complexes with DMSO,
DO, or MeCN, the absorption bands of these phenols are
considerably shifted toward the region of long wavelengths
as compared to the corresponding pure solvents. The reaꢀ
son for this phenomenon can be a bifunctional nature of
chromophores in compounds in question, which contain
a phenol hydroxy group and a carbonyl group conjugated
with the aromatic ring, which serve as a donor and an
acceptor of hydrogen bonds, respectively. The solvation of
such structures with clusters containing molecules of water
and highly basic aprotic solvents, the structure and propꢀ
erties of which strongly differ from the components of
which they are formed,14—17 leads to the relative stabilizaꢀ
tion of the electronic excited state of the molecule as comꢀ
pared to the ground state and, as a consequence, to the
bathochromic shift of the absorption band in the electronꢀ
ic spectrum. This suggestion is additionally confirmed by
~
the absence of the extrema in the dependences of max on
Kosyakov
