UVPES and ab initio molecular orbital studies on the electron donor-acceptor complexes of bromine with methylamines

Abstract

The He I photoelectron spectra of bromine, methylamine, and their complex have been obtained, and the spectra show that lone-pair orbital energy of nitrogen in methylamine is stabilized by 1.8 eV and the bromine orbital energies are destabilized by about 0.5 eV due to complexation. Ab initio calculations have been performed on the charge-transfer complexes of Br₂ with ammonia and methyl-, dimethyl-, and trimethylamines at the 3-21G^∗, 6-311G, and 6-311G^∗ levels and also with effective core potentials. Calculations predict donor and acceptor orbital energy shifts upon complexation, and there is a reasonable agreement between the calculated and experimental results. Complexation energies have been corrected for BSSE. Frequency analysis has confirmed that ammonia and trimethylamine form complexes with C_3v symmetry and methylamine and dimethylamine with C_s symmetry. Calculations reveal that the lone-pair orbital of nitrogen in amine and the σ^∗ orbital of Br₂ are involved in the charge-transfer interaction. LANL1DZ basis seems to be consistent and give a reliable estimate of the complexation energy. The computed complexation energies, orbital energy shifts, and natural bond orbital analysis show that the strength of the complex gradually increases from ammonia to trimethylamine.