567
Communications to the Editor
A literature search was made to find values of the dipole moments of the compounds tested, and to look for a possible correlation with short-range order between solvent and solute molecules, but no such evidence was found. In many cases, an interesting correlation does appear to exist between values of k , and a molecule's sensitivity t o concentration quenching and/or excimer formation. Those compoundi, that have low values of h, because of steric factors, e.)!., 9,10-diphenylanthracene,are also immune to concentration quenching and/or excimer formation for the very same reason. Some compounds that are planar in both their g.tound and excited states, and have large values of k,, are also susceptible to concentration quenching, e.g., anthracene, or to excimer formation, e . g . , naphthalene. In art earlier experiment it was demonstrated that the quenching by oxygen was a diffusion-controlled process
and that its efficiency was directly related to the radiative lifetime of the fluorescence state.31 The values of h I in that work clustered around a value of 5 X lo7 sec-I. As noted above, fluoranthene and its substituted derivatives have remained the sole exception, with values of h~ of about 0.8 x 107 sec-1. Since 02 is a smaller molecule than BrB, it is much less sensitive to molecular topology. Moreover, the breakdown in the spin-selection rules for the case of oxygen is for reasons other than that it is a high Z material. Oxygen has a triplet ground state and is therefore paramagnetic. Acknowledgment. The author would like to thank Dr. Mitio Inokuti for several stimulating discussions.
(31) I. B. BerlmanandT. A. Walter, J. Chem. Phys.. 37, 1888 (1962).
TIQNS TO THE EDITOR
Comment on the apttr "Application of Density
Comments on the Paper "ionic Solvation Numbers
Matrix Methods ko %&eStudy of Spin Exchange," by
from Compressibilities and Ionic Vibration Potentials
Sir The above mentioned paper suffers from some err0rb.l First a careful study of the experimental curves inclieates that the transverse relaxation time, Tz, is not 0.65 sec, as stated, but nrtoxe nearly 4.0 sec. Also the parameters assigned to the curves in Figure 1 are not correct. Presumably the temperatures are correctly assigned but the values of T cannot be right. Each curve has been assigned a value of T about sixfold too low, e.g., the second curve in Figure I haas 7 = 0.125 see, this value should be about 0 8 sec. If the corrected value of 7 and Tz are used then the theoretical curves of Figure 1 may be obtained, however, they cannol be obtained if the incorrectly given values are used. One can only suggest that the authors have not converted angular frequencies to frequencies, or v x e versa at sonie stage in their calculations, this could lead to an error ot a factor of 2r which would account for the discrepancy. The errors in 7 mean that the Arrhenius plot displayed in Figure 3 will still be an approximately straight line with the same slope bui, with a different intercept. This ,n turn means that some of the values in Table TI are wrong; T, Ea, a d A H $ are unchanged, but A S becomes -4.2 2= 0.6 eu and A F becomes ..~ 13.4 kcal/mol. (1) K.-I Dahlqvist and S . Forsen, J. Phys. Chem., 73,4124 (1969)
Chemistry, Biochemistry, and Biophysics Depar.lrne47t Massey University Palmerston North, hlew Zealand Rt?csvvedApril 24, 7972
D. N. Pinder
Measurements," by J. O'M. Bockris and P. P. S. Saluja PuSlication costs assisted by the National Research Council o f Canada
Sir: Recently,l Bockris and Saluja have measured adiabatic compressibilities of electrolyte aqueous solutions and derived from them hydration numbers. I would like to comment on the interpretation of their data. The authors have obtained hydration numbers from compressibilities using Passynski's equation.2 An equivalent but more rigorous method is to use apparent molal compressibilities3 +K which are derived from the difference between the isothermal compressibilities of the solution and that of the solvent, p - PO. These +K can also be obtained from adiabatic compressibilities using known expansitivities and heat capacities.3 94 The advantage of this quantity is that is can be written as a function of concentration, i.e.
+
$K $2 A,e"' -I- B,e (1) where 4 ~ is0 the standard apparent molal compressibility, equal to the standard partial molal cornpressi bility KO; A, the Debye-Huckel limiting slope; and BK an adjustable parameter related to all ion-ion interactions not accounted for by the limiting Debye-Huekel law. The parameter 4 ~ is0 additive and independent of ion-ion interactions while BK is nonadditive. The accuracy of experi-
(1) J. O'M. Bockris and P. P. S. Salu@ J. Phys. Chem.. 76, 2140 (1 972). (2) A. Passynski, Acta Physicochim., 8,385 (1938). (3) H. S. Harned and €3. B. Owen, "The Physical Chemistry of ElectroIyteSolutions," Reinhold, New York, N, Y . , 1958, Chapter 8 . (4) J. E. Desnayersand P. R. Philip, Can. J. Chem., 50, 1094 (1972). The Journal of Physicai Chemistry, Voi. 77, No. 4 , 1973
