an ab initio and chemical shielding tensors calculations for nucleotide 5’-monophosphates in the gas phase

structural and magnetic properties of purine and pyrimidine nucleotides (cmp, ump, dtmp, amp, gmp, imp) were studied at different levels of ab initio molecular orbital theory. these calculations were performed at the hartree-fock level and density functional b3lyp methods. geometries were fully optimized by following cs symmetry restrictions. the standard 6-31g** basis set which includes polarization and diffuse functions, was used for all the calculations.  the gauge-invariant atomic atomic orbital (giao) method and the continuous-set-of-gauge-transformation (csgt) procedure was employed to calculate atomic shielding tensors of the nucleotides using density theory at the b3lyp/6-31g** and hf/6-31g** level. the calculated chemical shifts were used to aid in assigning in the nmr spectra.