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Structures of cycloserine and 2-oxazolidinone probed by X-ray photoelectron spectroscopy

The electronic structures and properties of 2-oxazolidinone and the related compound cycloserine (CS) have been investigated using core and valence photoelectron spectroscopy and theoretical calculations. Isomerization of the central oxazolidine heterocycle and the addition of an amino group yields cycloserine. Theory correctly predicts the C, N and O 1s core spectra, and additionally we report theoretical natural bond orbital (NBO) charges. The valence ionization energies are also in agreement with theory and previous measurements. Although the lowest binding energy part of the spectra of the two compounds show superficial similarities, analysis of the charge densities of the frontier orbitals indicates substantial reorganization of the wave functions as a result of isomerization. The Highest Occupied Molecular Orbital of CS has leading carbonyl π character with contributions from other heavy atoms in the molecule, while the Highest Occupied Molecular Orbital of 2-oxazolidinone has leading nitrogen, carbon and oxygen pπ character. At laboratory temperatures, CS is predicted to exist mainly as two low energy conformers in the gas phase, separated in energy by 4.6 kJ mol-1, whereas 2-oxazolidinone is present as a single conformer in the gas phase. However the experimental spectra do not provide proof of the presence of these conformers.....