Quantum mechanic study of basis set effect on NMR chemical shielding and hydrogen bonding of some amino acids in gaseous phase and solvent

ABSTRACT

Amino acids can be defined separately by the unique characteristic rendered to each amino acid molecule as a result of the varying reactive abilities of their side chains. Structural analysis is performed using ab initio calculations in GAUSSIAN 98 program. The behavior of every atom as well as that of molecular fragments COOH, CH2, NH2 show that atoms in molecules [AIM] and Natural Population Analyses [NPA] charges exhibit not only a good conformational independence but also provide a meaningful picture of the electron charge distribution in this system. In this paper relative energies, dipole moments, rotational constants, vibration frequencies, charges of important atoms in structure of amino acids, NMR chemical shielding [symmetric and asymmetric delta sigma, eta and hydrogen bonded have been studied by systematic ab initio calculations in three levels of theory Hartree- Fock[HF], DFT [B3LYP] and MP2. Nine amino acids consist of alanine, asparagine, glycine, cysteine, methionine, serine, proline, threonine and valine were studied in gaseous phase and in two aprotic Solvent and different basis sets 6-31g*, 6-311+g*,2-21g* and6-311++g** were tested and compared to determine. Which basis set is the most efficient to model these particular amino acids. A series of gauge independent atomic orbiter's [GIAO] magnetic shielding calculations also have been done in solution and gaseous phase