Theoretical Determinations of Ionization Potential and Electron Affinity of Gaseous alfa-Alanine

Adiabatic (vertical) ionization potential (IP) and valence electron affinity (EA) of gaseous α-alanine have been determined by density functional theory (B3LYP), ab initio Hartree-Fock (HF) calculations and ab inilio third order algebraic diagrammatic construction [ADC(3)] Green function method with the 6-311++G** and 6-311G** basis sets, respectively. Eighteen possible conformers of α-alanine and its charged states have been optimized, employing density functional theory (B3LYP) with 6-311++G** and 6-311G** basis sets, respectively. In the gas phase, the IPs of all α-alanine calculated have positive values and all the EAs are negative, except for VEA 2 (+0.555 eV) of 6A, which indicates that all anions except for 6A - are unstable with respect to electron autodetachment vertically and adiabatically. All kinds of chemical quantities, associated with the IP and EA, such as electronegativity, chemical potential, chemical hardness, and chemical softness, have also been determined. Finally, we also report the infrared spectrum frequencies and vibrational modes for neutral states of α-alanine 1A, 2A, 3A, 5A and their optimized cationic and anionic states in seven highest frequency regions, and analyzed the relationship between the vibrational frequencies when receiving or donating an electron.