Polarization of Water Molecules Adsorbed by Smectites
source of chemically activated molecules which contain on the average -20 kcal/mol more energy than do chemically activated molecules formed by diazomethane photolysis at longer wavelengths. At 214 nm the energy distributions of the chemically activated molecules are quite broad which makes the unimolecular rate constants strongly pressure dependent. The pressure dependence of the unimolecular rate constants presents the possibility of studying energy distributions of the 214-nm reacting singlet methylenes via RRKM calculations. The intermolecular effect of various inert gases on the energy distributions could also be determined. However, before RRKM calculations can be used to make quantitative predictions about the reacting singlet methylene energy distributions, the heat of formation for CH2(IA1)must be established and various uncertainties and approximations in the RRKM calculations need to be overcome.
Acknowledgment. We thank Shigeo Okajima and Dr. William Wassam for many helpful discussions. References and Notes For a review see, W. Kirmske, "Carbene Chemistry", Academic Press, New York, N.Y., 1971. Alkanes: F. 8. Growcock, W. L. Hase, and J. W. Simons, Inf. J. Chem. Kinet., 5, 77 (1973); and references therein. Alkylsilanes: W. L. Hase, C, J. Mazac, and J. W. Simons, J . Am. Chem. SOC., 95, 3454 (1973); and references therein. Alkylgermanes: R. L. Scott, A. E. Richardson, J. W. Simons, and W. L. Hase, Int. J. Chem. Kinet., 7, 547 (1975). J. W. Rabalais, J. M. McDonakl, V. Scherer, and S. P. McGlynn, Chem. Rev., 71, 73 (1971). S. P. Waich and W. A. Goddard, 111, J. Am. Chem. Soc., 97, 5319 (1975). G. Herzberg, Proc. R. SOC. London, Ser. A , 262, 291 (1961); "Molecular Spectra and Molecular Structure. 111. Electronic Spectra and Electronic Structure of Polyatomic Molecules", Van Nostrand, Princeton, N.J., 1966, p 491. J. Lievin and G. Verhaegen, Theor. Chim. Acta, 42, 47 (1976). R. L. Russell and F. S. Rowland, J. Am. Chem. SOC.,90, 1671 (1968). A. H. Laufer and A. M. Bass, J. Phys. Chem., 78, 1344 (1974). Effective collision diameters were calculated by muitiplying the LennardJones collision diameters by the square root of the collision Integral @32)*(kTlt):J. 0. Hirschfelder, C. F. Curtis, and R. B. Bird,
The Journal of Physical Chemistry, Vol. 82, No. 16, 1978
1855
"Molecular Theory of Gases and Liquids", Wiley, New York, N.Y., 1954: S. C. Chan, B. S. Rabinovitch, J. T. Bryant, L. D. Spicer, T. Fujimto, Y. N. Lin, and S. P. Pavlou, J. Phys. Chem., 74, 3160 (1970); and the effective collision diameters are O,, 4.6 A: CH,N, 6.6 A; C,H, 5.2 A; C3Hs, 5.8 A; n-C,H,,, 6.6 A; and (CH,),Si, 8.6 A. W. L. Hase and J. W. Simons, J . Chem. Phys., 54, 1277 (1971). J. W. Simons, C. J. Mazac, and G. W. Taylor, J. Phys. Chem., 72, 749 (1968). K. A. Krohn, N. J. Parks, and J. W. Root, J. Chem. Phys., 55, 5785 (1971). A. Hosaka and F. S. Rowland, J . Phys. Chem., 75, 3781 (1971). P. J. Robinson and K. A. Holbrook, "Unimolecular Reactions", Wiley, New York, N.Y., 1972, p 315. T. H. Richardson and J. W. Simons, Chem. Phys. Lett., 41, 168 (1976). The RRKM calculations were performed using the general RRKM program of W. L. Hase and D. L. Bunker, Quantum Chemistry Program Exchange No. 234, 1975. F. B. Growcock, W. L. Hase, and J. W. Simons, J. Phys. Chem., 76, 607 (1972). W. L. Hase, R. L. Johnson, and J. W. Simons, Int. J. Chem. Kinet., 4, 1 (1972). American Petroleum Institute Research Project No. 44, Carnegie Institute of Technology, Pittsburgh, Pa., 1944-1952. W. A. Chupka and C. Lifshitz, J . Chem. Phys., 48, 1109 (1968). D. W. Setser and B. S. Rabinovitch, Can. J. Chem., 40, 1425 (1962). A. H. Laufer and H. Okabe, J. Am. Chem. SOC.,93, 4137 (1971). F. H. Dorer, J. Phys. Chem., 77, 954 (1973). (a) T. H. Richardson and J. W. Simons, J. Am. Chem. Soc., 100, 1062 (1978); (b) J. W. Simons, private communication. For a review of AH,'[CH,('A,)] values see, W. L. Hase and P. M. Kelley, J . Chem. Phys., 66, 5093 (1977). J. W. Simons, W. L. Hase, R. J. Phillips, E. J. Porter, and F. B. Growcock, Int. J . Chem. Kinet., 7, 879 (1975). J. W. Simons and R. Curry, Chem. Phys. Lett., 38, 171 (1976). D. C. Tardy and B. S. Rabinovitch, Chem. Rev., 77, 369 (1977). V. P. Zabransky and R. W. Carr, Jr., J , Am. Chem. Soc., 98, 1130 (19761. M. J. billing and J. A. Robertson, J. Chem. Soc., Faraday Trans. 7, 73, 698 (1977). W. Braun, A. M. Bass, and M. Pillina, J. Chem. Phvs., 52. 5131 (1970). H. M. Frey, G. R. Jackson, M. T6ompson, and R. Walsh, J. Chem. SOC., Faraday Trans. 1, 69, 2054 (1973). G. B. Kistiakowsky and B. B. Saunders, J . Phys. Chem., 77, 427 (1973). W. L. Hase, J . Chem. Phys., 64, 2442 (1976). D. L. Bunker. J . Chem. Phvs.. 57. 332 (1972). D. G. Kein, K. P. Lynch, J. A. Coher, and J.'V. Michael, Int. J. Chem. Kinet., 8, 825 (1976). D. L. Bunker, Ber. Bunsenges. Phys. Chem., 81, 155 (1977); E. R. Grant and D. L. Bunker, J . Chem. Phys., 68, 628 (1978).
Electrical Polarization of Water Molecules Adsorbed by Smectites. An Infrared Study C. Poinsignon, J. M. Cases, Centre de Recherches sur la Valorisation des Minerais, E.N.S.G., B.P. 452, 54001 Nancy Cedex, L.A. 235 CNRS, France
and J. J. Fripiat" Centre de Recherches sur les Soiides 2 Organisation Cristaiiine Imparfaite CNRS, 45045 Orleans Cedex, France (Received February 2 1, 1978) Publication costs assisted by Centre de Recherches sur la Valorisation des Minerais
The high degree of dissociation of water molecules adsorbed in the interlamellar space of layer lattice silicates is presently well documented. The vibrational spectrum of H 2 0in the coordination shell is perturbed by the polarizing field of the cation. The aim of this contribution is to measure the frequencies and absorption coefficients of the OH stretching and HOH deformation bands of water molecules in the coordination shell and by introducing these experimental data in a simple theoretical treatment for an anharmonic oscillator to deduce the charge brought by the water proton. In the cases where the asymmetric and symmetric stretching OH modes were observed these calculations lead to an evaluation of the orientation of the polarizing field with respect to the symmetry elements of the molecule. It has been observed (1)that this field is oriented almost parallel with one of the two OH bonds; (2) that the more polarizing the cation the higher the charge brought by the water proton; (3) and that the higher the number of water molecules in the hydration shell, the lower is the charge per water proton. The high degree of dissociation of water molecules adsorbed in the interlammellar space of layer lattice sil0022-3654/78/2082- 1855$0 1.OO/O
icates of the smectites family is presently well documented.1,2 The polarizing electrical field of the ex@ 1978 American Chemical Society
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The Journal of Physical Chemistry, Vol. 82, NO. 16, 1978
changeable cations is a t the origin of an increase of the (H30+/H20)ratio by about six orders of magnitude. This means for instance that, a t the monolayer content, (H30+/HzO)is in the range of Yet the absolute H30+ content does not permit the observation of the vibrational spectrum of this species because of overlapping with bands due either to H 2 0 or the smectite lattice. However, the vibrational spectrum of H 2 0 itself may be deeply perturbed by the polarizing field produced by the cations, specially if these water molecules belong to their first hydration shell (Mortland and Raman3). It is therefore worthwhile to study the frequencies and the integrated intensities of the OH stretching and HOH deformation bands of water in this situation. In the past, several contributions dealing with this subject have been published but they were mainly devoted to the frequency shifts with respect to the nature of the exchangeable cations. More recently Prost4 has studied the molecular symmetry by using mixtures of HzO and D2O. The aim of this contribution is (a) to measure the frequencies and the absorption coefficients of the OH stretching and HOH deformation bands of water in the hydration shell of various exchangeable cations; (b) to introduce these experimental data in a simple theoretical treatment such as that proposed by Coggeshal15for the anharmonic oscillator perturbed by an electrical field; and (c) to deduce from this treatment the relative electrical charge brought by the water proton and to correlate this parameter with the polarizing field of the cation.
Experimental Technique and Results The smectites used in this study were either a Wyoming bentonite with the following chemical composition: (Si4+3.93A13+o.07)1V (A13+1~56Fe3+0.20Fe2f0.005Mg2'0,'2k)V' 010(0H)2M+0.312 or a hectorite, from Hector, Calif., with the composition: (Si4+4)(Mg2+2.71Li+0.29)~'(F, OH)2010M+0.29 where M+ is the exchangeable cation. The homoionic smectites were saturated by the following cations: Li+, Na+, K+, Mg2+,ea2+,Sr2+,and Ba2+. The method has been described elsewhereS6 The optical integrated density of the stretching (D,) or of the deformation (Dd) band was measured with respect to the OH content by recording, at 295 K, simultaneously the absorption bands at 3 and 6 pm, respectively. Each sample was made from a transparent wafer obtained by slow sedimentation of the smectite microcrystals from a slurry, The OH content was changed by increasing the temperature of the wafer by steps of 50 O C in vacuo Torr). The weight change of a powdered sample treated in an identical manner was measured separately. The lattice hydroxyls as well as the water hydroxyls groups contribute to the 3-pm stretching band region, whereas the 6-pm deformation band is due to water. The various contributions to the stretching band are separated in the following way. D, is plotted with respect to n, the total number of OH: n = n1 + n2 + ... ni D, = (l/S)(Aslnl + AS2n2... + Asins) (1) where nl is the number of lattice OH known from the chemical composition and where n2 ... ni represents the number of OH belonging to adsorbed molecules in various situations in the first coordination shell. Similarly Dd = (1/S)(Adzn2 + -.Ad&) (2) In (1)and (2) A , and Ad are the absorption coefficients
C.Poinsignon, J. M. Cases, and J. J. Fripiat TABLE I : Absorption Coefficient of the OH Stretching Bands of the Residual Water Molecules in the Cation Hydration Shell and of the H 2 0 Bending Band" (A)A, x lo6 mol (OH)'' cm sample Asz Li' B 9.74 Na' B 7.77 K' B 5.38 Be2+B 8.83 Mg2+B 8.58 Ca2+B 7.75 Srz+B 6.27 Baz+B 5.77 Li'H 5.68 Be2+H 10.4 (B) Ad X l o 6 mol sample Li* B Na' B K+ B Be2+B Mg2+B Ca2+B Sr2+B Li'H BeZ+H
Ad,
n,
t 0.99 1 1 1 0.98 1
1
0.7 0.5 3 1 1 1 1 3 3
AS3
n3
5.38
2
4.15 2.90
2 2
t
1
0.99 1 0.99
(OH)-' cm ( H 2 0Bending Band) n2 1
0.99 1.27 0.7 0.74 0.5 1.3 3 1.13 1 2.82 1 2.16 1 1.32 3 1.52 3
t
.
1 1 1
Ad,
n3
0.53
2
0.76 0.90
2 2
t 0.99
1 1 a B = bentonite; H = hectorite. The Student's t test is indicated for the each observable straight line segment 2 when the number of experimental data is large enough.
and S is the cross section of the wafer. By scanning the temperature range where the water molecules in the n, .,. ni-l ... n,..2 ... n2 populations are successively removed, the absorption coefficients Ad, A,+l) ... As2are calculated from the slopes of the linear segments observed by plotting D, with respect to n. The absorption coefficients of the deformation band are obtained in the same way. This is illustrated by two examples in Figure 1. For instance, when three straight lines segments are observed it is possible by this graphical technique to delineate three populations, namely, those corresponding to the lattice OH content (nl)and to two water populations n2 and n3. Under the dehydration conditions used in this study, the number of populations never exceeds three, The value obtained in this way for nl is in good agreement with that computed from the chemical composition, For each linear segment, the absorption coefficient and the Student's t test have been calculated. Table IA and B shows the absorption coefficients obtained by using eq 1and 2. The Student's t test indicates the degree of confidence of these measurements. It must be also pointed out that the population contents obtained from the stretching and bending bands are practically identical. It will be assumed that the n2 and n3molecules belong to the cation first hydration shell or, in other words, that they are directly bound to the cation. This assumption is basically founded on the reasonable hypothesis that the binding energy to the cation is higher than the adsorption energy by the surface oxygen of the lattice or that the hydrogen bond between these oxygens and the residual water molecules is weak with respect to their interaction with the cation. As3 as well as Ad3 are always lower than A,z and Ad2. It is only for strongly polarizing cations that the two populations n3and n2 are distinguishable, with the exception of Be2+which may be to a large extent buried in the lattice. The absolute intensity of the bending band Ad is however not significant because of the overlapping with
The Journal of Physical Chemistry, Vol. 82,No. 16, 1978
Polarization of Water Molecules Adsorbed by Smectites
TABLE 11: Vibration Frequencies (cm-' ) Observed in Homoionic Hectorites'
cation Li+ Na+ K' CaZ+ Sr2+ BaZ +
water content/ cation 3H,a < 1 H,O
