Characterization of cooperative effects in linear α-glycylglycine clusters

The aspects of N-H⋯OCNH, N-H⋯OCO and C-H⋯OCNH interactions are analyzed by applying ab initio and DFT methods as well as Bader theory. We investigated geometry, binding energies, 17O, 15N chemical shift tensors, and Atoms in Molecules (AIM) properties of α-glycylglycine (α-glygly) clusters, via MP2, B3LYP and PW91XC methods. Dimer stabilization energies and equilibrium geometries are studied in various levels of theory. MP2 and DFT calculations reveal that for α-glygly clusters, stability of N-H⋯O and C-H⋯O hydrogen bonds are enhanced significantly as a result of cooperativity effects. Furthermore, a covalent nature is also detected for some hydrogen bondings. The n-dependent trend of 17O and 15N chemical shift tensors was reasonably correlated with cooperative effects in hydrogen-bond interactions. Regarding the various N-H⋯OCNH, N-H⋯OCO and C-H⋯OCNH hydrogen bondings, capability of the α-glygly clusters for electron localization at the N-H⋯O and C-H⋯O bond critical points, depends on the cluster size. This leads to cooperative changes in the hydrogen-bond length and strength as well as 17O and 15N chemical shift tensors.