Electronic structure of amino acids and ureas

The outer electronic levels of glycine, alanine, glycine ethyl ester, urea, and thiourea have been investigated by means of photoelectron spectroscopy using He I (584 Å) and He II (304 Å) radiation and CNDO/2 molecular orbital calculations. In the amino acids the molecular orbital (MO) ordering ha been found to be [σ core and carbonyl π > a″(O) a′(O) > a″(N)] with a first ionization potential of ～8.8 eV. Glycine and alanine are foun to exist as the undissociated amino carboxylic acids rather than zwitterions in the high temperature vapor. In urea the three lowest energy molecular o are near-degenerate [σ(4b₁) ～ π(1a₂) ～ π(2b₂)] while in thiourea only the two lowest energy MO's are near-degenerate [π(1a₂ σ(4b₁) ～ π(2b₂)]. The first ionization potentials of urea and thiourea are 9.7 and 7.9 eV respectively.