Ab initio study of interactions in hydrazine clusters of one to four molecules: Cooperativity in the interaction

Computations on systems consisting of up to four hydrazine molecules were p erformed by using the HF, DFT/B3LYP, and MP2 methods with the 6-31+G* basis set. The dimer was found to exhibit two minima with very similar interacti on energies (ca. 20-22 kJ/mol). The overall minimum at the MP2 level corres ponds to a structure of C-1 symmetry involving a hydrogen bond at about 2.2 Angstrom in addition to several other N ... H contacts at longer distances . The other minimum found possesses C-2 symmetry and involves two hydrogen bonds, also at ca. 2.2 Angstrom. A transition state about 4-6 kJ/mol less s table than the previous minima was also found. The trimer occurs in several structures that correspond to minima with slight energy differences among them; hydrogen bond distances are shorter than those in the dimer as a resu lt of the stronger interactions between molecules. The contribution of coop erative phenomena to the interaction energy is quite significant (it amount s to as much as 12% of the overall interaction energy). The two minima of t he tetramer behave similarly to those of the trimer; the contribution of no nadditive terms accounts for 10% of die overall interaction energy.