Intermolecular interactions and nature of cooperative effects in linear cis, cis-cyclotriazane clusters ( n=2–8)

Abstract

Intermolecular interactions in the linear cis, cis( c, c)-cyclotriazane clusters ( n=2–8) have been investigated via several theoretical methods including the SAPT (DFT) method is a combination of symmetry-adapted perturbation theory (SAPT) with the asymptotically corrected DFT. The SAPT(DFT) calculation indicates that the hydrogen-bonded c, c-cyclotriazane clusters, which are bound mainly by electrostatic interactions exhibit remarkable dispersion interactions. However, the dispersion interactions are only partially reproduced by DFT/B3LYP. Accuracy of the B3LYP interaction energies is markedly inferior to that of the MP2 interaction energies, which is very different from the H-bonded systems described well by B3LYP. Thereby, the B3LYP interaction energies are corrected to arrive at the same accuracy as the MP2 energies. The computation based upon the three-body SAPT shows the inductive effect contributes most importantly to the nonadditive three-body interaction. The induction nonadditivity plays a key role in stabilizing the clusters in the process of contraction in their N⋯H hydrogen bonds. Because of the weak nonadditive exchange–dispersion effect, B3LYP is able to represent well the nonadditive interaction energy even in higher cluster. The quantum cluster equilibrium (QCE) calculations illustrate that the nonadditive energies and the dispersion energies neglected by the DFT in the clusters can seriously affect properties of the c, c-cyclotriazane liquid.