'NOTES free OH should be decidedly more acidic than normal protons in water, but apparently it is not acidic enough to react with pyridine. Water and pyridine added in low concentrations to the dehydrated zeolites adsorb on separate cations as indicated by the simultaneous presence of the sharp OH band (3695 cm-l for NaX) of adsorbed water and the pseudo-Lewis band of pyridine adsorbed on the cation (1445 cm-l on NaX). Alternatively, the Brgnsted acidity might be due to hydrogen sites formed by partial hydrolysis of the cations and hence be a measure of the cation deficiency. A 1% cation deficiency corresponds to around 110 mequiv/equiv of large cavities in X zeolite and to around 70 mequiv/equiv of large cavities in Y zeolite. With the possible exception of the high-purity N a y zeolite, the observed cation deficiencies are all more than sufficient to account for the Brgnsted acidity, and even for the high-purity NaY the 3y0 cation deficiency of the second analysis would correspond to 210 mequiv/ equiv of large cavities, as compared with 190 mequiv/ equiv of large cavities observed. On the other hand, the X zeolites which were already cation deficient as received did not show Brgnsted acidity until they had undergone some further hydrolysis. Also, although the Brgnsted sites disappeared upon heating to 500°, they were not replaced by Lewis sites (presumably exposed aluminum ions) that with adsorbed pyridine give a band at 1452 cm-' (easily distinguishable from the 1441-1446-cm-' band) as is the case for HY.4 The possible role of cation deficiences could be clarified by experiments with a carefully prepared zeolite sample having the stoichiometric concentration of cations. The development of Brgnsted acidity by group I a zeolites in the presence of small amounts of water had been indicated by earlier kinetic studies of cyclopropane i s ~ m e r i z a t i o n , ~although ~J~ the results of Habgood and George1' suggested that concentrations of active centers were probably much lower than the concentrations of Br@nstedsites given in Table 111. The results, taken as a whole, indicate that the Br@nsted acidity present on these group I a zeolites is associated with some sort of metastable structural element in the zeolite crystal-one which disappears on intense dehydration but reforms after complete rehydration. While these sites may in some way be associated with cation deficiency, it is tempting to look for an association of these effects with the reported shifts in cation positions16or with the apparent change from octahedral to tetrahedral coordination around some aluminum ions16accompanying dehydration. It is also interesting that the observed concentrations of acid sites given in Table I11 (0.2-0.5 site/cavity) are similar to the concentrations of various other cation-associated strong adsorptions found in the group I a zeo1ites.l' Acknowledgment. We thank John W. Ward of Union Oil Co., Brea, Calif., for several helpful discussions and
3069 for making available some of his results prior to publication. (14) D. W. Bassett and H. W. Habgood, J. Phys. Chem., 64, 769 (1960). (16) K. Seff and D . P. Shoemaker, Acta Crystallogr., 22, 162 (1967). (16) J. Turkevich, Catal. Rev., 1, 1 (1967). (17) H. W. Habgood, Chem. Eng. Progr. Symp. Ser., 63, No. 73, 46 (1967).
Observation of a Minimum in the Kerr Constants of Light and Heavy Water Near 30"'
by Yeong-jgi Chen and William H. Orttung Department of Chemistry, University of California, Riverside, California 8.9601 (Received April 3,1968)
Interpretation of the Kerr effect of water has become a problem of considerable interest in recent years. A general theory of the Kerr constant of liquids was developed and applied to water by Buckingham and RaabS2 Experiments to test this theory yielded data a t visible wavelengths and 25°.3 Waibe14has recently reported dispersion data from the visible wavelength region to 2000 A, Data on the temperature dependence of the Kerr constants of H2O and D2O from 5 to 55" a t 365 mp (and also a t 436 mp for D2O) are reported in this note. Studies of the wavelength dependence should yield information about the electronic and vibrational transitions that,contribute to the Kerr effect. . The temperature dependence should help to decide the relative importance of effects such as hyperpolarizability (change of polarizability with field strength), induced-dipole orientation, and permanent-dipole orientation. The temperature dependence may also reflect the nature of the orientational correlations of neighboring molecules.
Experimental Section The optical part of the apparatus consisted of a light source, monochromator, beam splitter, polarizer, cell, analyzer, and photomultipliers. A 200-W mercury arc Iamp was used, followed by an Engis f/lO monochromator. The polarizer and analyzer were glan prisms from the Crystal Optics Co. The cell had a 10mm path length and used microscope slide windows. The dipping electrodes had a separation of 1.47 mm. (1) This investigation was supported in part by Public Health Service Grant GM11683 from the Division of General Medical Sciences. (2) A. D. Buckingham and R. E. Raab, J . Chem. SOC.,2341 (1957). (3) W. H. Orttung and J. A. Meyers, J. Phys. Chem., 67, 1906 (1963). (4) J. Waibel, 2. Naturforsch., 21a, 186 (1966). Volume 7.9,Number 8
August 1968
NOTES
3070 6
Table I: Parameters of Least-Squares Fit and Contributions to B a t 25"
6
H10
D10
DaO
(366 m d
(365 m r )
(436 m r )
10.122 1 2 . 5 7.3982 1 . 9 4.6605 1 1 . 5 -5.8558 & 1 . 5 -4.1780 f 1 . 1 -2.5941 & 0 . 9 8.8858 f 2 . 3 6.2346 f l . 6 3.92333~1~4 101.22 f 25 73.98 f 19 107~~ 46.61 f 15 -87.00 f 32 10'A1/2' -196.40 f 52 -140.13 f 37 70.13 f 18 44.13 f 16 1 0 7 ~ ~ / ~ 2 99.95 & 26 4.77 3.98 3.74 107~ lO6Ao
10'Ai lOAz
4
Discussion
t 3
f
c 0
10
30
20
40
50
60
70
T,O C .
Figure 1. The Kerr constant vs. temperature: (a) HzO at 365 mp; (b) DzO a t 365 mp; (c) D20 at 436 mp. Each point was determined from eight measurements covering a range of field strengths. The uncertainties shown are the corresponding standard deviations. The curves were fitted by least squares, weighting all points equally.
The electronics was similar to that used previously,s except that a Tektronix Type 551 oscilloscope and type D difference amplifier were incorporated. The method of analyzing the data has also been described.* Strain in the cell windows caused residual birefringence effects a t only a few temperatures. The water and DzO (99.8% for nmr use, from Mallinckrodt Co.) were purified by distillation. Temperature was controlled to better than f O . l " , as determined by a thermocouple a t the cell.
Results The Kerr constant is defined as
B = (nil
- ni)/XEz
(1)
where nl I and n1 are the refractive indices of the sample parallel and perpendicular to the applied field, X is the wavelength in air of the light, and E is the field strength in electrostatic units. The results are shown in Figure 1. The data were fitted by the least-squares method to equations of the form
B
=
AD
+ ( A i l T ) + (Az/T2)
(2)
The A t parameters, their standard deviations, and the values of the three terms at 298°K are shown in Table I.
The Journal of Physical Chemistry
I n polar gases and typical nonassociated polar liquids, the A. term of eq 2 arises from hyperpolarizability effects, the AI term from induced-moment orientation and hyperpolarizability, and the A2 term from dipole orientation. I n associated polar liquids, orientational correlations of neighboring molecules may have a significant temperature dependence, and the simple interpretation of the three terms may no longer be valid. If the simple interpretation is valid for water, one would expect A0 to be small, AI to be positive (if only induced-moment orientation is involved) and small, and A2 to be the dominant term. The results in Table I do not agree with the simple interpretation. Since little is known about hyperpolarizabilities, it is possible that the results are explained by large values of these parameters. It is more likely, however, that the temperature dependence of the orientational correlations is involved. At low temperature, strong correlations may cause cancellations in the optical anisotropies of neighboring molecules. As the temperature rises, the reduction of orientation by thermal agitation would account for the initial decrease, but this effect would soon be offset by the weakening of the orientational correlations, so that a larger net anisotropy would occur. At high enough temperature, the Kerr constant of water should pass through a maximum and then decrease more or less normally a t higher temperatures. The framework of the available theory2 does not appear to be sufficiently general to describe the cancellation of optical anisotropies of neighboring molecules by orientational correlations. A very basic assumption (that the average of the sum is equal to the sum of the averages of the induced moments of the molecules in the sample) must be removed in the development of the theory. It will then be possible to interpret the data presented here.
