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accelerate S.H. nucleus formation and consequently F.H. conversion.3 Greenberg and Chang's chief argument for a "solution theory," the complete dissolution of C B in very dilute suspensions (0.25 g./l.), may be understood from the point of view mentioned : a t such concentrations as pertain in the water phase of very dilute C3Ssuspensions during the first minutes of the hydration, any F.H. layer retarding the dissolution of C3S is expected to be converted soon. In view of this more complicated mechanism, it is not surprising that Greenberg and Chang's data do not fit either a simple dissolution equation or an equation based on diffusion through a hydrate layer as the rate-determining step. It is of course not intended to regard these remarks as a conclusive proof for the mechanism proposed by the present authors; it will, however, be noted that it is consistent with all data available a t present including Greenberg and Chang's data. TECHNOLOGIC.AL VNIVERSITY EINDHOVEN, NETHERLANDS RECEIVEDMAY21, 1965
H. N. STEIN J. M. STEVELS
Solvent Shifts in Charge-Transfer Complex Spectra
Sir: Mulliken'sl description of donor-acceptor complexes is generally accepted. The following expressions represent the wave functions for the ground and excited states of a complex $2;
#E
= =
a#o (D, A)
+ b#i (D+-A-)
(D+--4-) - b*#o (D, A)
~ * # 1
where a2 >>; b2 and a*2 >> b*2. Since the excited state of the complex is considerably more polar than the ground state, it has been predictedJ2 despite the Franck-Condon constraints, that increasing solvent polarity should favor stabilization of the excited state, thereby reducing the energy requirements for charge- transfer electronic transitions. Scattered efforts3 have failed to confirm the validity of these predictions, using dielectric constants as a measure of solvent polarity. I t is the purpose of this paper to report the results of our studies on aromatic hydrocarbon-TCSE complexes. I n order to avoid potential difficulties in this initial study, polar donors were excluded. This was also considered desirable in view of Davis and Symons'4 recent attempt to interpret solvent shifts as a function of solvent interaction with donor and acceptor species. T h e Journal o f Phvsical Chemistrg
Table I : Absorption Maxima" of Charge-Transfer Transitions in Various Solvents nCHsOH CHBCN Hexane
Db
33.6 1.331 263 244 228 194 208 192
nDC Toluene-TCNE o-Xylene-TCXE MesityleneTCNE HhIB-TCNE Naphthalene-TCNE Fluorene-TCNE Wave number, ern.-' X Index of refraction a t 20".
375 1 9 1 344 1 375 262 231 243 236 227 220 192 190 200 187 192 181 Dielectric
CHZCls
CClr
9 1 2.2 1 424 1 463 246 244 233 231 217 216 184 185 182 182 175 177 constant a t 20".
Spectroscopic data were obtained on a Gary Model 11 spectrophotometer equipped with a four-place digital voltmeter and wave length maxima were determined to a precision of * l mp. Data for the frequency maxima of the charge-transfer bands are given in Table I, which also contains values for dielectric constants and refractive indices. Since the donors and ground state complexes are nonpolar, it may be expected that the excited state of the complexes should experience the major coulombic interactions with polar solvents. In the absence of electronic excitation, the solvent molecules are randomly oriented with respect to solute. The interval of an electronic transition to a Franck-Condon state does not allow sufficient time for effective orientation of the permanent dipoles of the qolvent. Therefore, stabilizing interactions must be attributed to the polarizability of the solvent molecules. Our data are consistent with this view. KO correlations between can solvent dielectric constants and the position of v, be discerned. As can be seen in Table I, v,,,, decreases with increasing values of refractive index (within experimental error). Since the index of refraction is related to polarizability, we suggest that this solvent property can provide a useful basis for predicting solvent spectra shifts of donor-acceptor complexes having nonpolar ground states. (1) R. S. -Mulliken, J. chim. p h y s . , 20 (1963). (2) (a) J. N. Murrell, Quart. Rev. (London), 15, 191 (1961); (b) S. F. Mason, ibid., 15, 287 (1961). (3) (a) G. Briegleb, "Electronen Donator-Acceptor Komplexe," Springer-Verlag, Berlin, 1961, p. 38; (b) R. Foster and D. L. Hammick, J. Chem. floc., 2685 (1954). (4) K . M ,C. Davis and 11. C . R. Symons, ibzd.,2079 (1965).
AIR FORCE MATERIALSLABORATORY H. 11.ROSENBERG RESEARCH & TECHNOLOGY DIVISION D. HALE WRIGHT-PATTERSON AIR FORCE BASE,OHIO RECEIVEDMAY14, 1965
