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 ref- erence to the Rydberg (nd sigma, nd sigma' <-(3a ''/13a)), n >= 3 members of the Rydberg (nd pi'(a')<-(3a ''/13a)) and n = 3 members of the Rydberg (np sigma' <- (10a'/12a)) transitions. The experimental absolute photoab- sorption 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)-O-center dot 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 angstrom for the RO-H and 2.2 angstrom for the RC-OH bond lengths, indicating the rather complex multidimensional character of the potential energy surfaces involved. (C) 2022 Elsevier Ltd. All rights reserved.