Ultrafast charge transfer studied by femtosecond IR-spectroscopy and ab initio calculations

Femtosecond vibrational spectroscopy is a versatile method to monitor excit ed state dynamics evolving in photochemical reactions. Applying high-level calculations to analyze infrared absorption spectra allows to elucidate mol ecular structures of the transient compounds and to assign normal modes to the absorption lines. Striking differences in the experimental vibrational pattern of the locally excited states of 4-(dimethyl-amino)benzonitrile (DM ABN) and 4-aminobenzonitrile (ABN) (dissolved in acetonitrile) are explaine d on the basis of planar and pyramidal structures obtained from ab initio c omplete-active-space self-consistent-field (CAS SCF) calculations, giving e vidence for a strong sensitivity of the molecular structure on modest chang es in the substituents. Different models for the charge transfer state of D MABN are evaluated.