Supplementary material:
The structure and vibrational features of proton disolvates in water-ethanol solutions of HCl.
The combined spectroscopic and theoretical study.
V.D. Maiorov, I. S. Kislina, A.A. Rykounov, M.V. Vener
The evaluation of the absorption coefficients at 1090 cm-1 of three types of alcohol molecules in
system 1.
According to Fig. 3, increase of acid concentration causes the decrease of the (l)1090 value in
system 1. This is due to the fact that the three types of alcohol molecules with different absorption
coefficients contribute to (l)1090, namely, free molecules EtOHfree, the molecules involved in the
formation of proton disolvates EtOHd, and the molecules solvating the proton disolvates EtOHsolv.
With an excess of solvent, OH terminal groups of proton disolvates form H-bonds with the solvent
molecules .[11, 69, 72, 73] The (Н2О⋅⋅⋅Н⋅⋅⋅ОН2)+ in dilute acidic solutions can be hydrated by four Н2О
molecules. Proton disolvate in solutions of alcohols (Et(H)OHO(H)Et)+ can be firmly solvated
only by two solvent molecules (Scheme 1). Therefore, when the acid: solvent ratio varies from 1:2 to
1:4 (4.0 < [HCl]0 < 7.4 mol/l), the contribution to D1090 comes from the two types of EtOH
molecules: the molecules forming the proton disolvates EtOHd and those firmly solvating the
EtOHsolv ions. Then, experimental data plotted in the coordinates of Eq. (S1), i.e., D1090/[EtOH]d
versus [EtOH]solv/[EtOH]d must give a linear dependence:
D1090 = dl1090[EtOH]d + solvl1090[EtOH]solv , and
D1090/[EtOH]d = dl1090 + solvl1090([EtOH]solv/[EtOH]d)
(S1),
here
[EtOH]d = 2[HCl]0 – [H2O]0 and
[EtOH]solv = [EtOH]0 – [EtOH]d .
Here, l1090 is the effective thickness of the absorbing layer at 1090 cm-1; d and solv are the
absorption coefficients of the corresponding types of EtOH molecules. Indeed, relationship (S1) is
linear (R = 0.999). Using the least squares method, the following values of the coefficients are
evaluated: dl1090 =(5.10.1)10-3 l/mol and solvl1090 = (10.80.2)10-3 l/mol.
1
All three types of EtOH molecules absorb in more dilute HCl solutions (the acid:solvent ration is
less than 1: 4; [HCl]0 < 4.0 mol/l) at 1090 cm-1. The d value is supposed to be independent of the
degree of salvation of the (Et(H)O···H···O(H)Et)+ ions. It enables one to define the optical density
D1090 , which include contributions from absorption by the EtOHsolv and EtOHfree molecules in the
following way:
D1090 = D1090 – dl1090[EtOH]d
(S2)
D1090 = freel1090[EtOH]free + solvl1090[EtOH]solv.
It is assumed that in solutions with more than fourfold excess of the alcohol compared with [HCl]0
each (Et(H)O···H···O(H)Et)+ is solvated by two EtOH molecules. Therefore, relationship (S3) must
be linear:
D1090/[EtOH]solv = solvl1090 + freel1090([EtOH]free/[EtOH]solv) ,
(S3)
here
[EtOH]solv = 2[HCl]0
[EtOH]free = [EtOH]0 – [EtOH]solv – [EtOH]d .
This is indeed true (R=0.999). The least squares method yields the following values of the
coefficients: freel1090 = (15.30.3)10-3 l/mol and solvl1090 = (10.50.7)10-3 l/mol.
Thus, the absorption coefficients of EtOHsolv obtained for different ranges of ratios of the
components are practically the same; i.e., the coefficient εsolv is independent of the presence and
amount of free EtOH molecules in the solution. The absorption coefficient εsolv of EtOH molecules
which form the first solvation shell of the (Н2О⋅⋅⋅Н⋅⋅⋅ОН2)+ and (Et(H)O···H···O(H)Et)+ ions is
substantially lower than that of molecules not involved in solvation.
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Table S1. The O…O and O…H distances, R(O…O) и R(O…H) respectively, harmonic frequencies
and IR-intensitiesa) of the vibrations of the gas-phase species with the (quasi)symmetrical O···H+···O
unit computed in the M06-2X/6-311++G** approximation. Experimental frequencies of the
asymmetric stretching vibrations of the O···H+···O unit are given in the last row.
Distance/
vibration
(Н5О2)+
(Et(H)O···H···O(H)Et)+ b)
(H2O···H···O(H)Et)+
Et(H)O···H+···Cl-
R(O…O), Å
R(O…H)d), Å
(OHO), cm-1
2.382
1.192
977
(2471)
921e)
2.395
1.139
729
(2201)
760[78]
2.451
1.060
2256
(2870)
1964[84]
3.064 c)
1.775
2371
(1041)
-
Exp. (OHO),
cm-1
a)
IR intensities (kM/mol) are given in parentheses;
b)
there exist the four low-lying conformers of the (Et(H)O···H···O(H)Et)+ ion in the gas phase and
polar solvent, see Fig. S1. Their energies, geometric parameters of the O···H+···O unit and harmonic
frequencies in ethanol are very close to each other;
c)
the O…Cl- distance;
d)
the shortest O…H distance;
e)
K.R. Asmis, N.L. Pivonka, G. Santambrogio, M. Brümmer, C. Kaposta, D.M. Neumark and L.
Wöste, Sci. 2003, 299, 1375-1377.
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Fig. S1. The four low-energy conformers of (Et(H)O···H···O(H)Et)+.
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