Solvent Induced Transformations of n-π* Absorption in Formaldehyde, Acetaldehyde, and Acetone

Absorption spectra of formaldehyde (FA), acetaldehyde (AA), and acetone are compared in the vapor phase, nonpolar, and polar solutions at 295 K. The vibronic n-π* transition of carbonyl chromophore is mainly composed of the overtones of >C═O stretching vibration. A new phenomenon is observed in liquid solutions, consisting of a relative increase of Franck-Condon factors for the second and third harmonics in FA, and the second to fourth replica in AA, with respect to the gas phase. In AA and acetone with poorly resolved vibronic structure, the redistribution of intensities produces a false "solvent shift" of the band maximum between the vapor and nonpolar liquid phase by -250 ± 50 cm(-1). Modification in vibronic coupling can also explain unusual narrowing of the band contour in the solution, reported earlier for acetone (Renge , I. J. Phys. Chem. A 2009, 113, 10678). No detectable shift occurs as a function of solvent polarizability (refractive index function (n(2) - 1)/(n(2) + 2)) in n-alkanes for FA, AA, and acetone, as well as for cyclopentanone and camphor. Incidentally, the bathochromic dispersive shift is almost exactly compensated by a hypsochromic induction shift. The latter is due to the diminishing dipole moment in the excited state of the carbonyl chromophore. Differences in polarizability α and dipole moments μ were estimated for FA (Δα = 0.33 ± 0.1 Å(3)), AA (Δμ = -1.05 ± 0.2 D, Δα = 0.5 ± 0.2 Å(3)), and acetone (Δμ = -1.3 ± 0.2 D, Δα = 0.65 ± 0.2 Å(3)). The increase of α by ∼10% upon excitation is plausible for a weak n-π* transition. By contrast, near doubling of α in the upper state has been reported recently for several ketones, with Δα reaching 10 Å(3) (Catalán, J.; Catalán, J. P. Phys. Chem. Chem. Phys. 2011, 13, 4072). Empirical partitioning of solvent shifts into repulsive-dispersive, induction, dipole-dipole, and hydrogen bonding contributions was proposed to serve as a benchmark in computer chemical calculations.