| Abstract
| - The hydrogen bonding interactions of the HNO dimer have been investigated using ab initio molecular orbitaland density functional theory (DFT) with the 6-311++G(2d,2p) basis set. The natural bond orbital (NBO)analysis and atom in molecules (AIM) theory were applied to understand the nature of the interactions. Theinterrelationship between one N−H···O hydrogen bond and the other N−H···O hydrogen bond has beenestablished by performing partial optimizations. The dimer is stabilized by the N−H···O hydrogen bondinginteractions,which lead to the contractions of N−H bonds as well as the characteristic blue-shifts of thestretching vibrational frequencies ν(N−H). The NBO analysis shows that both rehybridization and electrondensity redistribution contribute to the large blue-shifts of the N−H stretching frequencies. A quantitativecorrelations of the intermolecular distance H···O (r(H···O)) with the parameters: ρ at bond critical points(BCPs), s-characters of N atoms in N−H bonds, electron densities in the σ*(N−H), the blue-shift degrees ofν(N−H) are presented. The relationship between the difference of ρ (|Δρ|) for the one hydrogen bond comparedwith the other one and the difference of interaction energy (ΔE) are also illustrated. It indicates that forr(H···O) ranging from 2.05 to 2.3528 Å, with increasing r(H···O), there is the descending tendency for oneρ(H···O) and the ascending tendency for the other ρ(H···O). r(H···O) ranging from 2.3528 to 2.85 Å, thereare descending tendencies for the two ρ(H···O) with increasing r(H···O). On the potential energy surface ofthe dimer, the smaller the difference between one ρ(H···O) and the other ρ(H···O) is, the more stable thestructure is. As r(H···O) increases, the blue-shift degrees of ν(N−H) decrease. The cooperative descendingtendencies in s-characters of two N atoms with increasing r(H···O) contribute to the decreases in blue-shiftdegrees of ν(N−H). Ranging from 2.05 to 2.55 Å, the increase of the electron density in one σ*(N−H) withelongating r(H···O) weakens the blue-shift degrees of ν(N−H), simultaneously, the decrease of the electrondensity in the other σ*(N−H) with elongating r(H···O) strengthens the blue-shift degrees of ν(N−H). Rangingfrom 2.55 to 2.85 Å, the cooperative ascending tendencies of the electron densities in two σ*(N−H) withincreasing r(H···O) contribute to the decreases in blue-shift degrees of ν(N−H).
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