J. Phys. Chem. 1996, 100, 5603
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COMMENTS Comment on “Solvent Effects on the Electronic Spectrum of C60” Paul Suppan Institute of Physical Chemistry, UniVersity of Fribourg, CH-1700 Fribourg, Switzerland ReceiVed: August 2, 1995; In Final Form: October 2, 1995 In a recent communication1 which describes and studies the solvatochromic shifts of the absorption spectrum of C60, it has been suggested that these can be explained by the change in quadrupole moment of this highly symmetrical molecule; although the ground state is assumed to be “isotropic” (which presumably means of spherical symmetry), the excited state would be distorted so as to have a permanent quadrupole moment in some undefined direction, which would interact with the solvent to yield a spectral shift related to the solvent’s polarity function f(D) ) 2(D - 1)/(2D + 1) (D being the static dielectric constant of the solvent and f(D) being generally known as the “Onsager” polarity function). The purpose of this comment is to point out that the change in quadrupole moment from zero (ground state) to any finite nonzero excited value cannot produce a solvatochromic shift,
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simply because the solvent around a molecule of spherical symmetry is unpolarized. Random fluctuations of the solvent will of course induce fluctuating dipole moments, quadrupole moments, and higher order multipole moments, but these depend only on the polarizability of the solute molecule and are not related to any change in a permanent multipole moment. (This is known as the “solvent Stark effect”, but there is no evidence that it may result in observable solvatochromic effects.2) The crucial equations (18) and (19) of ref 1 are of little use because the function f(∆σ), stated to be “a function of change in quadrupole moment ...”, is not defined. The solvatochromic shift equations for dipole moment changes and for quadrupole moment changes always multiply the multipole of the initial state (e.g., µg or Qg) by the change (∆µ or ∆Q), and it is clear that polarization of the solvent in the initial state is a prerequisite for any solvatochromic effect related to permanent multipole moment changes. References and Notes (1) Gallagher, S. H.; Armstrong, R. S.; Lay, P. A.; Reed, C. A. J. Phys. Chem. 1995, 99, 5817. (2) Ghoneim, N.; Suppan, P. Spectrochim. Acta 1995, 51A, 1043.
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© 1996 American Chemical Society
