Computational study of interactions and nuclear magnetic shielding constants in linear chains of formamide clusters.

We investigated the energetic, structural, dielectric, and nuclear magnetic shielding properties of linear n-formamide clusters, with n up to 6, to quantitatively characterize cooperative effects in model biological systems. The geometries of the complexes were optimized at the MP2 and DFT/B3LYP levels by using the pc-2 and pc-3 basis sets, while the nuclear magnetic shielding constants were calculated by employing pcS-n type basis sets, which have been optimized specifically for density functional calculations of these properties. The interaction energies show the cooperative effect, which favors the successive addition of monomers. In addition, by analyzing structural changes in the intermolecular C=O, C-N and hydrogen O⋯H bonds, as well as in the average dipole moments as cluster size increases, we found that the cooperative interaction far exceeds that expected for electrostatic interactions. Such non-pairwise-additive effects are also reflected in the changes of the nuclear magnetic shielding constants. In particular, the negativity of O shielding decreases around 23% from the monomer to the 6-formamide chain. It is possible to note the decrease in the shielding of H and in the deshielding of O as a result of their hydrogen bonding. However, the results obtained show that these variations in the extremes of formamide chains tend to zero, and the respective shielding values tend to stabilize as the number of monomers increases in the chain. Also, the cooperative effect increases in the middle of the chains, by decreasing the shielding for all atoms except that of O, which decreases its deshielding. These results could serve to guide improvements in current conventional models for simulating hydrogen bonded systems.