Y.-H. L. Shaw, S. M. Wang, and N. C. Li
236
gnetic Resonance Study of Water Dimer in Water-Chloroform Solutions1
Yueh-Mo
L. Shaw, Sung M. Wang, and Norman C. Li*
Department of Chemistry, Duquesne University, Pittsburgh, Pennsylvania 15219
(Received August 7,1972)
Water proton shifts as a function of water concentration, up to 0.05 M , in chloroform at 2.2, 17, and 33.6" are well interpreted in terms of an equilibrium between monomeric and dimeric forms of water. The equilibrium constants for the dimerization of water in chloroform are 0.56, 0.51, and 0.40 M-I for 2.2, 17, and 33.6", respectively. The enthalpy change of dimerization is -1.8 kcal/mol, which is much smaller than the energy of 6 kcal/mol obtained from theoretical calculations for the most stable configuration of water dimer in the gaseous phase. The water monomer is associated with chloroform, Cl&H.-.OHZ. The water dimer in chloroform medium is probably a mixture of linear and cyclic species, and the oxygen of one of the water molecules in the dimer may act as electron donor to the chloroform hydrogen.
Introduction Because of the predominance of water as a medium in living systems the hydrogen-bonding properties of water have been of particular interest to chemists and biologists. The nature of water species in water-chloroform solutions is inferred from nmr observations of the water proton shifts as a function of water concentration in chloroform, in the manner described by Alei and Florin2 for waterammonia solutions, at 60 MHz. Because of the limited solubility of water in chloroform (1.5 g/l.),3 the nmr data for water iio chloroform are interpreted in terms of an equilibrium between monomeric and dimeric forms of water. Since theoretical calculations fiave shown that the most stable c o n f ~ g u ~ a t ~ofo nwater dimer in the gaseous phase is linear with an energy of 6 k c a l / m 0 1 , ~ it , ~ would be of interest to compare this value with the enthalpy of dimerization of water in solution. In order to obtain the enthalpy, we 'have determined the equilibrium constants for the dimerization of water in chloroform at three different tempertitures in the temperature range 2-34', and the results are reported here.
ane was placed before and after the -ON resonance peak at a separation of 10-15 Hz. The sideband frequencies were measured by means of audio oscillators and a frequency counter. For each solution four spectra were recorded and then averaged. The reported shifts are accurate to about 0.2 Hz.
Results and Discussion Figure 1 shows the nmr peak for HzQ protons in a NzOCHCl3 solution which is 0.024 M in HzO a t three different temperatures. The shifts of the nmr peak for I 1 2 0 protons as a function of H 2 0 molarity in chloroform at 2.2, 17, and 33.6" are given in Figure 2 and Table I. If we assume that in the dilute solutions of HzO (up to G(H20) = 0.05 M ) , the dependence of shift on HzO concentration could be explained in terms of an equilibrium between water monomer (probably associated with a chloroform molecule through Cl&H...OHz hydrogen bond) and a dimer water species, then we may use the method of Saunders and Hyne6 and of Alei and Florin2 in treating the data of Table I. The pertinent equations would then be Y
iVaterials ~ ~ h l o i r o f ~ was r m purified by shaking with concentrated sulfuric acid and then water until the washing was neutral. It was dried over calcium chloride and distilled, and the fraction boiling at 60" was collected. Deionized water was distilled twice before use. Nmr Meusuremmts. All the nmr spectra were obtained with a Varian Associates Model A-60 nmr spectrometer. For studies in chloroform as medium, samples were prepared and transferred to 5-mm nmr tubes in an atmosphere of nitrogen, and the tubes were then immersed in liquid nitrogea, evacuated, and sealed. During the running of the spectra the temperature remained constant to within fl",an indicated by the separation in Hz between two peaks in ethylene glycol or methanol. The molarity of the solutions, at temperatures other than room temperature, was calr dated from the change in density with temperature of chloroform. Chemical shifts of the water proton signal were measured with respect to tetramethylsilane (TMS) used an internal standard. All lines are downfield from T . A side band of the tetramethylsilThe Journal of P h p i c a i Chemistry, Vo/. 77, No. 2, 1973
( V I M ,+ 2uzKMi2)/(M-j-t- 2KMI2)
(1)
and
C = Mi
+ 2KMi2
(2)
where Y = observed shift, = average shift for water protons in monomeric water species, v2 = average shift for water protons in dimeric water species, K equilibrium constant for formation of dimeric water species, Mi = concentration of monomeric water species, and C = total water concentration. If a monomer-dimer equilibrium is sufficient to interpret our observed shift data, then we must be able to find values of the parameters U T , v2, and K which, when substituted in the above equations, will give calculated shifts in agreement with the observed shifts. For this purpose, we used a non-linear least-squares program, with a Hew2=
Work supported by Public Health Service Grant No. GM10539-09. M. Alei, Jr., and A. E. Florin, J. Phys. Chem., 9 3 , 863 (1968). T. V. Healy, J. Inorg. Nucl. Chem,, 19, 328 (1961). J. D. Bene and J. A. Pople, J. Chem. Phys., 52,4858 (1970). K. Morokumaand J. R. Winick, J. Chem. Phys., 52, 1301 (1970). M. Saunders and J. 8.Hyne, J. Chem. Phys., 29, 1319 (1958).
Nmr Study of Water Dimer
237
TABLE I: Nmr Frequencies of Water Protons in Chloroform Solutions 2.2OC
17°C
33.6"C l i _ l _ _ _ -
Uobsd
Vcalcd ~
0.0021 94.8 0.0037 0.0082 0.0164 0.0246 0.0328 0.0398
I
I
40
EO
lil0
0 Hz
Figure 1. Nrrir spectra of water in chloroform: (1) 0.0246 M H20, 2.2"; (2) 0.0242 M 1-120,17': (3) 0.0236 M H20, 33.8".
1
95.1 95.5 96.5 96.7 97.0 97.8
95.0 95.5 96.1 96.7 97.3 97.8
-
C(H20)
uobsd
ucaled
0.0020 0.0081 0.0161 0.0242 0.0322 0.0393
92.2 92.6 93.4 93.8 93.9 94.8
92.5 93.1 93.7 94.2 95.7
C(n20)
VObsd
0.0020 0.0038 0.0079
89.5
0.0158 0.0236 0.0315 0.0503
89.6 90.0 90.5 90.8 91.0 91.6
___I
UC61Cd
89.7 89.9 90.3 90.6 90.9 91.8
shifts are in excellent agreement with the observed shifts for all the temperatures investigated. The variation of V I and v2 with temperature is in line with the expected decrease in association between chloroform and water molecules with temperature increase. The value of LQ at 33.6", 89.4 Hz, is far downfield of the proton shift in gaseous H20, 50 Hz,7,8 and gives strong support to the concept of chloroform-water association. Alei and Florin2 reported that H2 proton shifts as a function of H2O concentration in liquid NWs at 29.6" are well interpreted in terms of' an equilibrium between monomeric and dimeric forms of H28. They tested their nmr data against a model assuming an equilibrium between a monomeric and a trimeric water species, and found that the monomer-dimer model gives better agreement between experimental shifts and calculated shifts than a monomer-trimer model. We have tested our nmr data at 33.6" in similar fashion against a model assuming an equilibrium between a monomeric and trimeric water species in chloroform. The unweighted sum of the squares of the deviations of the six observed shifts from the six calculated shifts at 33.6" are 0.15 and 0.4% Hz2? resyectively, for models assuming monomer-dimer and monomer-trimer equilibria. At 33.6", therefore, the monomerdimer model gives better agreement between experirnental shifts and calculated shifts, in agreement with the finding of Alei and Florin2for HzO in liquid NH3 at 29.6" The enthalpy change, AH, a c c o m ~ ~ ~the y ~dimeriza~lg tion of water, is obtained from the slope of B log K us. I / T plot. A least-squares treatment gives AH = -1.8 kcall mol. This value is much smaller than the energy mol obtained from theoretical calculations for stable configuration of water dimer in the gaseous Water dimer in chloroform is probably a ~ ~ x t ofu linr ~ ear9Jo and cyclicll species, and both water monomer and dimer may act as electron donor to the c h ~ o ~ o f hydroor~ gen. It i s not surprising therefore that the enthalpy change is very much different from that calculated in the gaseous phase for the most stable configuration. Alei and Florin2 have measured I7O shifts in NH3 liquid mixtures a t room temperature and co these with shifts in ~ ~ l 7 ~ - and ~ ~H2170~ t o ~ e (CHl3)sN mixtures. At 5 mol % N 2 0 , l7 acetone and HzO-(CH&N is about 22 ppm upfield from that in H20-NH3 mixture. This suggests that in W 2 0 I
0
0.C1
concn. of H$,
0.03
0.05
M
Figure 2. Chemical shift of water protons vs. concentration of water in chloroform at ('I) 2.2, (2) 17, and (3) 33.6'.
lett-Packard cdculatm, to select parameter values which minimized the sum of the squares of the deviation of calculated from experimental shifts. On the basis of the data of Table I, best values for the parameters were found to be 111 = 94.8, 91.8, and 89.4 Hz; v2 = 166.8, 163.2, and 150.0 Hz;and K = 0.56, 0.51, and 0.40 M - l , for 2.2, 17, and 33.6", respectively. Table I shows that the calculated
(7) F. Takahashi and N. C. Li, J. Amer. Chem. SOC.,88, 1117 (1966). (8) S. F. Ting, S. M. Wang, and N. C. Li, Can. J. Chern., 45, 425 (1967). (9) W. L. Masterton and M. C. Gendrans, J. Phvs. Chem., 19. 2895 (1966). (IO) J. R. Holmes, D. Kivelson, and W. C. Drinkard, J . Amer Chern.
SOC.,a4,4677 (1 962). (11) L. 8.Magnusson, J. Phys. Chem., 74,4221 (1670). The Journal of Physical Chemistry, Voi. 77, No. 2, 1973
S.G. Frank, Y.-l-i. Shaw, and N. C. Li
238
NH3 solul,ion, the oxygen lone-pair electrons act as donors to solvent protons in hydrogen bond formation. Since it has been shown that solvent chloroform proton acts as hydrogen donor in several systems,12 it is not surprising that in our experiments both water monomer and dimer oxygen may act as eiec4,ron donor to the chloroform hydrogen. Holmes, et oL,lo Edudied dilute solutions of water in several organic solvents. They interpret their nmr shift data in terms of an equilibrium between a water monomer complexed to solvent S, through hydrogen bonding between water protons and basic solvent sites (I), and dimeric water species also complexed to basic solvent sites (11).Their equilnbrium constants are for the reaction
+
20H2*S2(I)=:Sz.HzO.**HOH*S(II) S
(3)
and cannot compare with our values, since eq 3 involves dimerization of water, the breaking of ordered S...H bonds, and the formation of ordered 0 . H bonds. s....H H,...S H**e*S / )o..*,H+-J H.*..S S....H I I1
-
Nishimura, Ke, and Li13 studied hydrogen bonding between water protons and tributyl phosphate (TBP) in the mixed solvent carbon tetrachloride-TBP. When the mole fraction of water is below 2.0 x self-association of water is negligible and the authors report the enthalpy change for the hydrogen bonding reaction (4)
to be AH = -4.1 kcal/mol. This is 2.3 kcal higher than the enthalpy change for the hydrogen bonding reaction between two water monomers in chloroform, and indicates stronger hydrogen bonding between HzO and phosphoryl oxygen than between water molecules. It may be noted that Alei and Florin,2 using data on downfield shift of I 7 0 resonance for HzO in liquid NH3, also report generally stronger hydrogen bonding between HzO and NHJ than between water molecules.
(12) S. Nishirnura. C. H. Ke, and N. C. Li, J. Pbys. Chem., 72, 1297 (1968), (13) S.Nishirnura, C. H. Ke, and N. C. Li, J. Amer. Chem. SOC., SO, 234
(1968).
Proton Magrietic esonance Study of Aerosol 07'- Water-Electrolyte-n-Octane Systems Sylvan (3. Frank,* Collegc?ot Pharmacy The Oh/o State University, Columbus, Oh!o 43210
Yueh-Ho Shaw, and Norman C. Li Department of Chemfstiy, Duquesne Un/vers/ty,Pittsburgh, Pennsylvania 15219
(Received July 12, 1972)
Proton magnetic resonance has been used to investigate the shift of water signals in a 2% w/w di(2-ethylhexy1)sodium sulfosuccinate (Aerosol 0T)-rz-octane system in the presence of various diamagnetic salts. In previous studies of this system in the absence of electrolyte, transitions of clear-turbid-clear-turbid were observed with increasing water content. Separation of the first turbid region gave a hydrocarbon layer containing surfactant-solubilized water and a water-surfactant lower layer. Two peaks are observed in this region at locations upfield from the standard water peak due to micellar-solubilized water in the hydrocarbon phase and the separated water phase, respectively. Shifts in the water peaks and changes in peak area indicate that the Aerosol OT micelle in the hydrocarbon phase undergoes reorganization with a concurrent separation of additional water. The magnitude of this effect is dependent on the electrolyte valency and concentration. The coexistence of the two signals indicates that the proton exchange rate between the solubilized water and the separated water is very slow.
Introduction Previous investigations have indicated that proton magnetic resona~nce(pmr) is a useful technique for following the behavior of water in the complex equilibria produced when water is added to solutions of di(2-ethylhexy1)sosulfosuccinate OTP in n-octane'l These systems were observed to undergo transitions of clear-turbidThe Journal of Pnysical Chemistry, Vol. 77, No. 2, 1973
clear-turbid with increasing water For a 2% solution of Aerosol OT (AOT) in Pt-octane, the first turbid region existed in the range of about 19 to 92 mol of Water Per 1 mol of AOT at 35". Separation of the first tur-
W/W
(1) S. G. Frank and G. Zografi, J. Colloid lnterface So., 28,66 (1968). (2) s. G.Frank and G . Zografi, J Colloid hterface Scr , 29,27 (1969).
