Temporary Anion States and Dissociative Electron Attachment to Isothiocyanates

The temporary anion states of isothiocyanates CH3CH2N=C=S (and CH3CH2N=C=O for comparison), C6H5CH2N=C=S and C6H5N=C=S are characterized experimentally in the gas phase for the first time by means of electron transmission spectroscopy (ETS). The measured vertical electron attachment energies (VAEs) are compared with the virtual orbital energies of the neutral state molecules supplied by MP2 and B3LYP calculations with the 6-31G* basis set. The calculated energies, scaled with empirical equations, reproduce satisfactorily the experimental VAEs. The first VAE is also closely reproduced as the total energy difference between the anion and neutral states calculated at the B3LYP/6-31+G* level. Due to mixing between the ring and N=C=S p-systems, C6H5N=C=S possesses the best electron-acceptor properties and its lowest-lying anion state is largely localized at the benzene ring. The anion states with mainly p*C=S and p*N=C character lie at higher energy than the corresponding anion states of non-cumulated p-systems. However, the electron-acceptor properties of isothiocyanates are found to be notably larger than those of the corresponding oxygen analogues (isocyanates). The dissociative electron attachment (DEA) spectra show peaks close to zero energy and at 0.6 eV, essentially due to NCS - negative fragments. In spite of the energy proximity of the first anion state in phenyl isothiocyanate to the DEA peak, the zero energy anion current in the benzyl derivative is about one order of magnitude larger. A possible cause can be traced back to the different thermodynamic requirements of the dissociative process, as indicated by the calculations.