The Journal of Physical Chemistry, Vol. 82, No. 4, 1978 495
Communications to the Editor
spatially coincident with the naphthalene when looked down the axis perpendicular to the molecular plane.4 Crystallographic data do not show such large overlap of the guest and host molecules if the guest molecule were substitutionally r e p l a ~ e d .Additionally, ~ ~ ~ ~ ~ if such overlap did exist, the decay rate constants for the quinoline in TCB should be comparable to that for 2-chloroquinoline in durene. Also, the decay rate for naphthalene in TCB should compare with those for 1-chloronaphthalene in durene. The data in Table I show that although the effect of the chlorinated host is similar to that of the chlorinated guest, the internal heavy atom effect is more pronounced than the external effect as reflected in the increase in k , and h, (also, see Table IV). From Table I, the ID1 and IEl values for a particular guest molecule decreased in durene compared to the same guest molecule in 1,2,4,5-tetrachlorobenzenehost. The only exception was the increase in the IEl value for naphthalene in durene relative to the chlorinated benzene host. The difference in ID1 value could be attributed to either a distortion in the excited state of the guest in one or both of the hosts due to the crystallographic differences in the two crystals, or an effect on the zero field splitting due to spin-orbit contribution from external heavy atom effect. Our study here does not preclude either interpretation. Considering the low symmetry of chloronaphthalene, quinoline, and chloroquinoline, the decrease in IEl may be attributed to the less anisotropic crystal fields arising from the external heavy atom of the chlorinated host. Conversely, the increase in IEl for a very symmetric molecule like naphthalene may be due to a more anisotropic environment in the tetrachlorobenzene host.
Acknowledgment. We thank the National Institutes of Health (GM 21770) for support in this research. Acknowledgment is made to the donors of the Petroleum
Research Fund, administered by the American Chemical Society, for partial support of this work. The liquid helium used in this study was kindly provided by the Kansas Refined Helium Co.
References and Notes S. P. McGlynn, T. Azumi, and M. Kinoshita in “Molecular Spectroscopy of the Triplet State”, Prentice Hall, Englewood Cliffs, N.J., 1969. J. B. Birks in “Photophysics of Aromatic Molecules”, Wiley, New York, N.Y., 1970. R. H. Hofeldt, R. Sahai, and S. H. Lin, J. Chem. Phys., 53, 4512 (1970); Trans Faraday SOC.,67, 1690 (1971). S. Yamaguchi, K. Matsuzaki, and T. Azumi, J . Lumin., 12/13, 369 (1976). C. T. Lin, J . Lumin., 12/13, 375 (1976). K. A. Martin, G. Moller, and A. M. Nishimura, J . Phys. Chem., 80, 2788 (1976). G. Molier and A. M. Nishimura, J. Phys. Chem., 81, 147 (1977). J. Schmidt, W. S. Veeman, and J. H. van der Waals, Chern. Phys. Lett., 4, 341 (1969). J. Schmidt, D. A. Antheunis, and J. H. van der Waals, Mol. Phys., 22, 1 (1971). R. K. Power, “An Active Data Analysis Program”, to be submitted to VOICE, Varian Minicomputer Users’ Software Library. M. J. D. Powell, Comp. J., 8, 303 (1965). C. B. Harris and R. J. Hoover, J . Chem. Phys., 56, 2199 (1972). R. S. Becker in “Theory and Interpretation of Fluorescence and Phosphorescence”, Wiley, New York, N.Y., 1969. We are grateful for the assistance in decay data analysis by P. C. Saint-Erne, A. Habash, and G. Cherry. C. A. Hutchison, Jr., and W. Mangum, J. Chem. phys., 34,908 (1961). J. S. Vincent and A. H. Maki, J. Chem. Phys., 42, 865 (1965). J. S. Vincent and A. H. Maki, J. Chem. Phys., 39, 3088 (1963). T. S. Kuan, D. S. Tinti, and M. A. El-Sayed, Chem. Phys. Lett., 4, 507 (1970). D. S. McClure, J . Chem. Phys., 20, 682 (1952). T. Pavlopoulos and M. A. El-Sayed, J. Chem. Phys., 41, 1082 (1964). M. S. de Groot, I. A. M. Hesselmann, J. Schmidt, and J. H. van der Waals, Mol. Phys., 15, 17 (1968). W. S. Veeman and J. H. van der Waals, Mol. Phys., 18, 63 (1970). G. Gafner and I. H. Herbstein, Acta Ciystallogr., 13, 702, 706 (1960). F. H. Herbstein, Acta Crystallogr., 18, 997 (1965). V. Ermoiaev and I. Koltyar, Opt. Spectrosc., 9, 183 (1960). M. Schwerer and H. Sixl, Chem. Phys. Lett., 2, 14 (1968); Z. Naturforsch. A , 24, 952 (1969).
COMMUNICATIONS TO THE EDITOR Comment on “A High-pressure Laser Raman Spectroscopic Investigation of Aqueous Magnesium Sulfate Solutions” Publication costs assisted by the University of California, San Diego
Mgz++ SO,z- + MgO m1
H H 0 SO, H H m,
MgO
H SO, + MgSO, (1) H m3
m4
In this model three ion pairs forms exist and the contact ion pair concentration is m4. The molal dissociation constant is
Sir: Chatterjee, Adams, and Davis1 report a AV” = -20 cm3/mol for the volume change of dissociation of MgSO, contact ion pairs. They conclude that their results in 2 M MgSO, solutions do not agree with the values of AV” obtained by other^.^-^ From pressure-conductance data a value of A P = -7.3 cm3/mo13and A V O = -7.7 cm3/mo14 was obtained from two independent investigations. The purpose of this letter is to show that their results are consistent with the other work with which they claim disagreement. Their work is in rough agreement with pressure-acoustic work of F i ~ h e r . ~ The reaction of interest is the three-step multistate dissociation model developed by Eigen and Tamm2 to explain ultrasonic absorption relaxation spectra at atmospheric pressure: 0022-3654/78/2082-0495$01 .OO/O
where the K , are the equilibrium constants for the various steps. The conventional AVO is obtained from the equation
a In K ,
lap
=
-A T I R T
(3) The relationship between the various AVLland KLlEigen and Tamm obtained from acoustic data and K,,, and AVO obtained from pressure-conductance work has been discussed by F i ~ h e r . The ~ , ~ Eigen and Tamm model does in fact a large decrease with pressure of the con0 1978 American Chemical Society
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The Journal of Physical Chemistry, Vol. 82, No. 4, 1978
centration of m4. The acoustic work of Fisher5 for 0.5 M MgS04 solutions demonstrated this. The reason for the apparent discrepancy between the A V O derived from the Raman work and that of others arises in the definition of an equilibrium constant Chatterjee, Adams, and Davis used:
Their A V was obtained from the equation
a In &lap = -Av”/RT
(5) The Eigen and Tamm2 model Fisher5 chose to explain both conductance and sound absorption as a function of pressure says that m4 is only 5% of the total ion pair concentration. Therefore, the pressure dependence of Q is determined principally by the change of m4 with pressure. For a pressure change of 1000 atm the Eigen and Tamm model predicts a decrease of a factor of about 2 in m4. The acoustic data show a larger decrease, a factor of 2.8. Therefore by using eq 4 and 5, the acoustic pressure data would yield a AV” of -29.4 cm3/mol and the Eigen and Tamm model Fisher chose would yield a AP” of -17 cm3/mol which bracket the value obtained by Chatterjee, Adams, and Davis. The Raman work indicated a higher concentration of m4, about 14% of the total concentration of the 2 M MgS04 in aqueous solutions. For this reason the quantity (ml m2 m3)would be more sensitive to changes in m4 than the Eigen and Tamm model would predict. Their work and that of Eigen and Tamm are consistent in that m4decreases by a factor of 2 for a pressure change of 1000 atm. In conclusion the work of Chatterjee, Adams and Davis is in general agreement with other work. The use of the laser Raman technique provides a valuable independent method of studying ion pairing and multistate dissociation at elevated pressures.
+ +
Acknowledgment. The work is supported by the National Science Foundation Grant OCE 76-02253. References and Notes (1) R. M. Chatterjee, W. A. Adams, and A. R. Davis, J. Phys. Chem., 78, 246-250 (1974). (2) M. Eigen and K. Tamm, Z. Electrochem., Ber. Bunsenges. Phys. Chem., 66, 93-121 (1962). (3) F. H. Fisher, J. Phys. Chern., 66, 1607-1611 (1962). (4) E. Inada, K. Shimizu, and J. Osugi, Nippon Kagaku Zasshi, 92, 1096-1 101 (1971). (5) F. H. Fisher, J. Acoust. SOC.Am., 38, 805-812 (1965) (6) F. H. Fisher, J. Phys. Chem., 69, 695-696 (1965).
University of California, San Diego Marine Physical Laboratory of the Scripps Institution of Oceanography San Diego, California 9 2 152
F. H. Fisher
Received September 12, 1977
A Reply to the Comment on “A High-pressure Laser Raman Spectroscopic Investigation of Aqueous Magnesium Sulfate Solutions” Publication costs assisted by the Defense Research Establishment
Sir: In the preceding communication1 Fisher has commented on the results for AV of dissociation of MgS04 contact ion pairs reported by Chatterjee, Adams, and 0022-3654/78/2082-0496$01 .OO/O
Communications to the Editor
Davis2 measured by high-pressure laser Raman spectroscopy (LRS). We are in agreement with this analysis of the experimental dissociation volume changes measured by pressure-electrical conductivity methods, pressureacoustic methods, and the pressure-LRS method as interpreted quantitatively by Fisher1 in terms of the three-step multistate dissociation model of Eigen and Tamm.3 However, it was not our intention in ref 2 to leave the impression that the pressure-LRS A V values were in disagreement with those obtained by other methods. In our discussion, we pointed out that the Raman band at 995 cm-’ originates from the sulfate ion in the contact pair from state 4 while sulfate in any other state contributes to the 982-cm-’ band.4 Therefore, in calculating the volume change for the dissociation from the pressure dependence of the concentration quotient, Q, (eq 8 in ref 2), we would expect to obtain a A V that represented a composite of the volume changes from state 4 to the other three states. It is encouraging that the A V O values calculated by Fisher’ are close to the pressure-LRS AV, but it is not unexpected, since the dissociation of the contact ion pair is the step which results in the most charge production and hence the greatest electrostrictive volume effect as we pointed out earlier.2 There is a factor not discussed by Fisher1 which concerns the concentration dependence of AV. This is an important consideration since the pressure-LRS results were reported for a 2.0 M MgSO, solution whereas the other techniques were applied to much more dilute systems. The AP at a given concentration is defined by
I
A T = .ZV(product) - ZT(reactant)
(1)
In the MgSO, system, as seen by pressure-LRS, the reactant is the state 4, uncharged contact ion pair (MgS040),and the products are a sum of the ionized states 1-3. There are_ therefore two factors that govern the magnitude of AVas compared to AVO: (i) the dependence of V(products) and V(reactants) on concentration, and (ii) changes in the distribution of species with concentration. In some systems the former has been found to be very small, e.g., the A V associated with the formation of the sodium borate ion pair (by Ward and Millero5) is only about 5% larger in a sea water medium (0.725 m ionic strength) compared to water. This has not been found to be the case for all systems. The A V associated with the first ionization of phosphoric acid has a considerable increase (becomes less negative) as concentration is increased (Adams, Preston, and Chew?. The origin of factor (i) is in the ion-solvent interaction and the dependence of the partial molal volumes of charged species on the structural properties of the solvent in nondilute solutions. Laser Raman spectroscopy is well suited to investigate both factors (i) and (ii) since it provides identification of species present in solution as well as their molal concentrations. In addition, evidence for strong interactions of ionic species with the solvent can often be found, e.g., evidence for the species Mg(H20)2+.7Combined with a thermodynamic analysis of the pressure, temperature, and concentration dependence of the Raman spectra of SOlutions, LRS is helping to provide a better model for nondilute electrolyte solutions than was available from nonspecies specific techniques such as electrical conductivity. References and Notes (1) F. H. Fisher, J . Phys. Chem., preceding paper in this issue. (2) R. M. Chatterjee, W. A. Adams, and A. R. Davis, J. Phys. Chem., 78, 246-250 (1974). (3) M. Eigen and K. Tamm, Z. Nectrochern., Ber. Bunsenges. Phys. Chem., 66, 93-121 (1962).
0 1978 American Chemical Society
