NOTES
1669
-
3.0
2.5
2.0 cr I .5
the relative contribution of the circulating ring currents to the chemical shifts of the aromatic ring protons and the aromatic methyl protons. The magnitude of this slope can be estimated by employing the model of Johnson and Bovey12to calculate the ring current contributions to the aromatic methyl shifts.1° A comparison of these computed methyl shifts with those calculated for the ring protons4 leads to a slope of 2.1 which is in good agreement with the slope of 2.5 for the experimental methyl shift data in Figure 1. Several polymethylated aromatics are also included in Table I. I n general, additional methyl substitution in the ring shifts the methyl proton resonance upfield. This result is expected in view of the electron-releasing nature of the methyl group and has been described elsewhere.4 (12) C. E. Johnson and F. A. Bovey, J . Chem. P h y s . , 29, 1012 (1958).
Changes in Dielectric Relaxation during Dehydration and Rehydration of Rochelle Salt Figrire 1. Relationship between computed aromatic proton shifts, uT. and methyl proton shifts, 6, in polycyclic hydrocarbons.
by P. G. Hall and F. C. Tompkins Department of Chemistry, Imperial College of Science and Technology, London, S . W.7, England (Received September 27, 1965)
was obtained in CSZ and measured relative to toluene in the same solvent. Spectra were recorded a t 60 Rlc/sec with a Varian 4-60 spectrometer.
Results Summarized in Table I are the chemical shifts, 6, in ppm for the protons of methyl groups substituted in various positions of six aromatic ring systems : benzene. naphthalene, phenanthrene, anthracene, pyrene, and coronene. Table I also lists the screening constants, uT, for the aromatic protons a t the corresponding positions in the ring as calculated by Jonathan, et aL4 Figure 1 illustrates a comparison of the 6 CH? values with the uT aromatic proton shifts. The linear agreement is quite good and provides further validation of the ring current model as well as of the conclusions of Maclean and 3Iackor pertaining to the relationship between aromatic methyl and aromatic proton chemical shifts. The ninr methyl data in Table I show a much poorer correlai,ion with the aromatic proton chemical shifts computed from the single benzene ring current model.2a The slope of the line in Figure 1 represents
Garner1 has reviewed early gravimetric studies of the rehydration of dehydrated salt hydrates. Discontinuities in the plots of water uptake against time did not correspond to any known hydrates. With common alurnl2multilayer adsorbed water controlled the rate of diffusion into the porous anhydride, but a phase transformation corresponding to A1,(SOS3.9.4H20 produced a “glassy” modification which was very stable to further rehydration after dehydration. JIore recent investigations, also using gravimetric methods, with lead styphnate3 and manganous oxalate4 showed that the conditions of dehydration can have a pronounced effect on the subsequent rehydration. This note concerns the use of a dielectric relaxation (1) W. E. Garner, E d , “Chemlstry of the Solid State,” Butterworth and Co. (Publishers) Ltd., London, 1955, Chapter 8 (2) A. Bielanski and F. C. Tompkins, T r a n s . Faraday SOC.,46, 1072 (1950). (3) T. B. Flanagan, $bid., 5 5 , 114 (1959). (4) T. B. Flanagan and M. K. Goldstein, J . P h y s . Chem., 68, 663 (1964).
Volume 70, Number 5
M a y 1966
NOTES
1670
method for investigating the dehydration and rehydration of potassium sodium tartrate tetrahydrate (Rochelle salt). The ferroelectric nature of Rochelle salt has attracted wide interest6 and is known to be bound up with the water of crystallization. Previous dielectric studies of a variety of other crystal hydrates, generally aimed a t distinguisjing between “free” and “bound” water, are reviewed by HastedS6
Experimental Section The apparatus and dielectric cell design have been described previously.7 Fixed pressures of water vapor were established by controlling the temperature of the water reservoir connected to the cell. AR grade Rochelle salt was crushed before use. Changes in dielectric relaxation a t constant frequency (0.90 RSc/sec) are reported as changes in Q-l, with AQ-l for the fully dehydrated sample taken as zero. The hydrate was dehydrated under vacuum a t 22”.
l5
t
X I
0
5
50
100 Time, min.
150
200
Figure 1. Dehydration and rehydration curves for Rochelle salt; variation of A&-1 with time: A, dehydration, B, rehydration, 22”, 20 mm of water vapor; C, rehydration, 0”, 4.58 mm of water vapor; X indicates isolation of the cell from the reservoir.
Results and Discussion Figure 1 shows the plots of AQ-l against time for vacuum dehydration at 22” and subsequent rehydration at 22 and 0” using 20 and 4.58 mm of water vapor, respectively. Initially, the Q value corresponding to the tetrahydrate was out of range of the instrument; after pumping for about 20 min, AQ-l decreased sharply to about 3 x and remained a t this value for several hours before decreasing to zero after continued pumping for over 48 hr. This indicates that part of the water of crystallization is firmly bound or structural water. AQ-l is expected to decrease with decreasing amounts of water adsorbed on or taken up by the solid but the shape of the dehydration curve suggests that a change in relaxation times is superimposed on this. Since the more firmly bound water is likely to have a longer relaxation time than the loosely bound water, the sharp decrease in AQ-l probably reflects a marked increase in relaxation time. The presence of loosely bound water confirms other observationsg made with Rochelle salt. Structural determinationlo shows that the water molecules are hydrogen bonded to oxygen atoms of the tartrate ion and that one of the water molecules differs from the other three in showing strong polarization along one direction of the crystal. Thus, the firmly bound water indicated by the present work may correspond to the monohydrate. The rehydration curves are both characterized inititlly by a sharp increase in A&-’. This compares close y with the sharp initial increase in weight observed w i t h the rehydration of dehydrated lead styphnate monohydrate, and is consistent with adsorption The Journal of Physical Chemistry
and rehydration at a surface rendered porous by the dehydration process. Further increases in AQ-’ are considerably more marked with curve B, which shows sigmoid-like steps, until a t the point marked X (where the cell was isolated from the reservoir) there is a sharp decrease which begins to be arrested a t about the value corresponding to the firmly bound water. With the rehydration experiments the solid is exposed to higher vapor pressures than the tetrahydrate equilibrium vapor pressures” of 7.5 mm a t 22” and
