Low-lying electronic states of ethanol investigated by theoretical and synchrotron radiation methods

We investigate the ethanol absorption spectrum in the range 5.64–10.78 eV (220–115 nm), by combining ab initio theoretical and experimental methods in order to provide the most accurate and up-to-date information about the electronic state spectroscopy of ethanol. In particular, absolute cross-section values are reported from high-resolution vacuum ultraviolet (VUV) photoabsorption measurements. The present VUV spectrum reveals several new features not previously reported in the literature, with particular reference to the Rydberg (ndσ, ndσ′ ← (3a′′/13a)), n ≥ 3 members of the Rydberg (ndπ′(a′) ← (3a′′/13a)) and n = 3 members of the Rydberg (npσ′ ← (10a′/12a)) transitions. The experimental absolute photoabsorption cross sections have subsequently been used to calculate the photolysis lifetime of ethanol in the Earth's atmosphere (0–50 km), showing that solar photolysis is expected to be a weak sink at altitudes lower than 40 km to ^●OH radical reactions. Potential energy curves for the lowest-lying excited electronic states, as a function of the O–H and the C–OH coordinates, are also obtained employing the equation of motion coupled cluster single and doubles (EOM-CCSD) and time dependant density functional theory (TD-DFT) methods. These show clear dissociation character at inter-nuclear distances greater than 2.0 Å for the R_O–H and 2.2 Å for the R_C–OH bond lengths, indicating the rather complex multidimensional character of the potential energy surfaces involved.