Ionization Potential Of Small Biomolecules And Electronic Affinities Theoretical Research,

Both ionization potential (IP) and electron affinity (EA) are very important properties for atoms and molecules, as they are fundamental in assessing the electron donating and accepting ability. Many theoretical and experimental studies have been carried out to study the IP and EA of atoms or molecules. Recent developments of experimental techniques for studying ion-molecule reactions and structure-reactivity relationships made it possible to determine accurate IPs and EAs for a wide variety of organic molecules, α -Alanine is one of the about 20 amino acids which form proteins and it is also the smallest chiral amino acid in nature. The amino group is of great importance because it is the fundamental group combing peptides and proteins. Both terminal amino acids and amino amides have a free NH2 group. So, it is very important to study the structures and properties of small amino acids and amide molecules.Quantum chemistry is becoming increasingly important within the field of chemistry. Its steady growth, application and influence are evidenced by modern chemical attitudes and approaches as well as the continuous increase in the sophistication of computational techniques. In this paper, we calculate the ionization potentials and electron affinities of α -alanine, serinamide and cysteinamide employing quantum chemistry calculation methods ab initio and density functional theory (DFT). Some beneficial results have been obtained.The whole paper consists of five chapters. The first chapter is the preface and can be divided into four parts. The first part briefly introduces the history of the modern quantum chemistry theory. The second part narrates the history background and practical meaning of calculating ionization potential and electron affinity. The third part introduces the definitions of ionization potential and electron affinity and their calculation formulas. The fourth part introduces the main work and practical meanings of this paper. The second chapter introduces the related calculation methods which are used in this paper. In brief, the former two chapters are the basis and background of our studies, which offer us withuseful and reliable quantum methods.On the basis of these theories, in Chapter three, eighteen possible conformers of a -alanine and its charged states have been optimized employing density functional theory (B3LYP) with 6-311-HG** and 6-311G** basis sets, respectively. Adiabatic (vertical) ionization potential (IP) and valence electron affinity (EA) of a -alanine in the gas phase have been determined using density functional theory (B3LYP), ab initio Hartree-Fock (HF) calculations and ab initio third order algebraic diagrammatic construction [ADC(3)] Green function method with the 6-311++G** and 6-311G** basis sets, respectively. Finally, we also report the infrared spectrum frequencies and vibrational modes for neutral states of a -alanine 1A, 2A, 3 A, 5 A and their optimized cationic and anionic states in seven highest frequency regions, and analyzed the relationship between the vibrational frequencies and receiving or donating an electron.In Chapter four, eight possible conformers of serinamide and its charged states in the gas phase have been optimized employing the DFT B3LYP method with 6-311++G** and 6-311G** basis sets, respectively. Adiabatic and vertical ionization potentials (IPs) and valence electron affinities (EAs) of serinamide in the gas phase have been determined using density functional theory (DFT) B3LYP, B3P86 and B3PW91 methods with the 6-311++G** and 6-311G** basis sets, respectively. IPs and EAs of serinamide in solution have been calculated with the B3LYP method using the 6-311++G** and 6-311G** basis sets.In Chapter five, six possible conformers of cysteinamide and its charged states have been optimized employing B3LYP/6-31+G* level. Adiabatic and vertical ionization potentials (IPs) of cysteinamide in the gas phase and in solutions have been calculated with the density functional theory (B3LYP, B3P86) and ab initio HF methods using the 6-31+G* basis set, respectively. Dipole-bound electron affinities (EAs) of cysteinamide in the gas phase have also been calculated at the same level of theory.Through carrying on literature investigation of our selected researchsystems, and summarizing what we have done, we have found the innovation spot of the present paper. They are listed as follows:(1) The research systems are quite novel. Serinamide and cysteinamide are extremely important organic molecules, which have been studied by very few people.(2) In theory, we have proved the fact that six atoms of peptide bond C-N are in a plane.(3) Through comparing ionization potentials of serinamide and cysteinamide in the gas phase to the results in solutions, we have found that ionization potentials in solutions are generally smaller than the results in the gas phase and decrease with increased dielectric constants in solutions. This finding indicates that serinamide and cysteinamide are easier to oxidize in solutions than in the gas phase.(4) We have found that the conformer changes of serinamide and cysteinamide have accompanied by the ionization process. These conformer changes have been analyzed according to the changes of the lengths of the bonds, the changes of dihedral angles and the changes of the intramolecular hydrogen bonds.