Temporary Anion States and Dissociative Electron Attachment to Isothiocyanates

Abstract

The temporary anion states of isothiocyanates CH3CH2=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 pi-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 pi*C=S and pi*N=C character lie at higher energy than the corresponding anion states of noncumulated pi-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 1 order of magnitude larger.

Can the Electron-Accepting Properties of Odorants Be Involved in Their Recognition by the Olfactory System?

“…Empirical rules for the linear scaling of VOEs 57 as SVOE = 0.8065 × VOE + 0.9194 58 are used to obtain SVOE values to predict experimentally measured vertical attachment energies (VAEs), i.e., the energies of incoming electrons at which formation of TNI states is observed. VAE values for II were earlier measured 59 using electron transmission spectroscopy 60 and are in good agreement with the SVOEs in Table 2, and thus, the predictions for the structurally similar compounds I and III−V are expected to be adequate. Anion currents observed for both S − and SCN − fragments in II−V at around 4 eV can be ascribed to the decay of shape resonances 41 from the π 5 * MO, with mainly totally antibonding benzene character (see Table 2).…”

The role of electronic properties to the mutagenic activity of 1,6- and 3,6-dinitrobenzo[a]pyrene isomers

“…In the present case, UB3LYP scheme was able to remove a large percentage of the spin contamination from the pseudo wavefunction (S 2 ∼ 0.75). Recently, Modelli et al have employed UB3LYP/6-31+G* level of calculation to obtain the EA values for many PAHs [32] and isothiocyanates [33], reproducing satisfactorily experimental data. Atomic charges were estimated through atomic polar tensor (APT) [34,35], Mulliken [31], Merz-Singh-Kollman electrostatic potential (ESP) [36] and natural bond order (NBO) [37] population analyses.…”

Electron capture activation of the disulfide bond. The role of the asymmetry and electronegativity

“…Later, Sobczyk and Simons, 20 by examining the diabatic potential energy surfaces for electron transfer to the lowest and excited Rydberg orbitals of the NH 3 + site and to the S-S s* orbital, showed that resonant through-bond ET can occur. Also, the temporary anion states of diphenyl disulfide were recently characterized by means of dissociative electron attachment spectroscopy 21 and the dissociation mechanism of the S-S bond in a,a 0 -dinaphthyl disulfide radical anion in solution was also investigated in transient absorption measurements. 22 However, in most of the models used, the environment of the S-S bond is strictly symmetric and, to the best of our knowledge, only Antonello et al 23 have addressed the ECD in RSSR 0 unsymmetrical disulfides, although in all cases the R and R 0 substituents are alkyl or phenyl groups.…”