Experimental and theoretical studies on the electronic properties of acetyl pseudohalides CH3C(O)X (X=NCO, NCS and N3)

The HeI photoelectron spectra of acetyl pseudohalides: CH3C(O)NCO, CH3C(O)NCS and CH3C(O)N3 have been obtained and completely analyzed. After optimizations of the structures for stable conformers at different levels of theory, a complete theoretical study involving the calculation of the ionization energies using orbital valence Green's function (OVGF) was performed. Calculations of the radical-cationic forms were carried out in order to compare their properties with those of the neutral molecules. All calculations predict that the cis planar conformer is energetically favorable for all molecules investigated and it retains planar structure after ionization. The first vertical ionization potentials for the three molecules are 10.72 {18a′(nO, π′NCO)}−1, 9.75 {5a″(πNCS)}−1 and 10.54 eV {4a″(πN3, nO)}−1, respectively, which are in accordance with the electronegativity of the three substituents: NCO>N3>NCS. The first eight highest occupied molecular orbitals (HOMOs) with valence electrons for three molecules may be divided into two groups. One is the out-of-plane π-type MOs with a″ symmetry; and the other is the in-plane σ-type MOs with a′ symmetry.