An investigation of hydrogen-bonding effects on the nitrogen and hydrogen electric field gradient and chemical shielding tensors in the 9-methyladenine real crystalline structure: A density functional theory study

Hydrogen.-bonding effects in the real crystalline structure of 9-methyladenine, 9-MA, were studied using calculated electric field gradient, EFG, and chemical shielding, CS, tensors for nitrogen and hydrogen nuclei via density functional theory. The calculations were carried out at the B3LYP and B3PW9I levels with the 6-311++13** basis set via the Gaussian 98 package. Nuclear quadrupole coupling constants, C-Q, and asymmetry parameters, eta(Q), are reported for N-14 and H-2. The chemical shielding anisotropy, Delta(sigma), and chemical shielding isotropy, sigma(iso), are also reported for N-15 and H-1. The difference between the calculated parameters of the monomer and heptameric layer-like cluster 9-MA shows how much H-bonding interactions affect the EFG and CS tensors of each nucleus. This result indicates that N(10) (imino nitrogen) has a major role in H-bonding interactions, whereas that of N(9) is negligible. There is good agreement between the present calculated parameters and reported experimental data. Although some discrepancies were observed, this could be attributed to the different conditions which were applied for calculation and the experiments.