Ab initio studies of the conformers and conformational distribution of the gaseous hydroxyamino acid threonine

Systematic and extensive conformational search has been performed to characterize the gas-phase threonine structures. A total of 1296 unique trial structures were generated by allowing for all combinations of internal single-bond rotamers. All the trial structures were optimized at the B3LYP/6-311G* level of the theory and then subjected to further optimization at the B3LYP/6-311++G** level. A total of 71 conformers were found and their rotational constants, dipole moments, zero-point vibrational energies, harmonic frequencies and vertical ionization energies of all the conformers were determined. Single-point energies were also calculated at the MP2/6-311G(2df,p) and B3LYP/6-311G(2df,p) levels. Characteristic H-bonding types were classified and listed for all the conformers. The conformational distributions of gaseous threonine at various temperatures were calculated.