| Abstract
| - We investigate aspects of N−H···N hydrogen bonding in the linear trans-diazene clusters (n = 2−10) suchas the N···H and N−H lengths, n(N) → σ*(N−H) interactions, N···H strengths, and frequencies of the N−Hstretching vibrations utilizing the DFT/B3LYP theory, the natural bond orbital (NBO) method, and the theoryof atoms in molecules (AIM). Our calculations indicate that the structure and energetics are qualitativelydifferent from the conventional H-bonded systems, which usually exhibit distinct cooperative effects, as clustersize increases. First, a shortening rather than lengthening of the N−H bond is found and thus a blue ratherthan red shift is predicted. Second, for the title clusters, any sizable cooperative changes in the N−H andN···H lengths, n(N) → σ*(N−H) charge transfers, N···H strengths, and frequencies of the N−H stretchingvibrations for the linear H-bonded trans-diazene clusters do not exist. Because the n(N) → σ*(N−H) interactionhardly exhibits cooperative effects, the capability of the linear trans-diazene cluster to localize electrons atthe N···H bond critical point is almost independent of cluster size and thereby leads to the noncooperativechanges in the N···H lengths and strengths and the N−H stretching frequencies. Third, the dispersion energyis sizable and important; more than 30% of short-range dispersion energy not being reproduced by the DFTleads to the underestimation of the interaction energies by DFT/B3LYP. The calculated nonadditive interactionenergies show that, unlike the conventional H-boned systems, the trans-diazene clusters indeed exhibit veryweak nonadditive interactions.
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