Solvent Dependence of n−π* Absorption in Acetone

Abstract

A barely resolved structure can cause considerable loss of precision when locating the maxima of broad spectra. The centers of band envelopes were determined for the acetone n-pi* absorption in vapor, pure liquid, and solutions at 293 K. A complementary method of "band-halving" is proposed for accurate measurement of solvent-induced displacements. Solvent shifts span from -280 cm(-1) in CCl(4) to 465 cm(-1) in acetonitrile, and further to 1785 cm(-1) in hydrogen-bonding water, being negligible in n-alkanes. In the latter case the dispersive and induction shift components almost cancel mutually. Unbiased values of absolute frequency shifts will provide a reference to quantum chemical calculations, in particular, in nonpolar and weakly polar solvents where much controversy exists. The dipole moment in the excited state (mu(e) = 1.81 +/- 0.2 D, Deltamu = -1.81 D) and polarizability change (Deltaalpha = 0.6 +/- 0.2 A(3)) were estimated. A solvent set where the Onsager model is apparently applicable was used for the determination of mu(e). Remarkably, the bandwidth at half-maximum (fwhm) decreases in liquids, including water (6270 cm(-1)), as compared to vapor (6680 cm(-1)).