Hydrogen bonding interactions in PN···HX complexes: DFT and ab initio studies of structure, properties and topology |
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Authors: | Pradeep Risikrishna Varadwaj |
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Institution: | (1) Department of Chemistry, Faculty of Sciences, Okayama University, Tsushima-naka 3-1-1, Okayama-Shi, Okayama-ken 700-8530, Japan |
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Abstract: | Spin-restricted DFT (X3LYP and B3LYP) and ab initio (MP2(fc) and CCSD(fc)) calculations in conjunction with the Aug-CC-pVDZ
and Aug-CC-pVTZ basis sets were performed on a series of hydrogen bonded complexes PN···HX (X = F, Cl, Br) to examine the
variations of their equilibrium gas phase structures, energetic stabilities, electronic properties, and vibrational characteristics
in their electronic ground states. In all cases the complexes were predicted to be stable with respect to the constituent
monomers. The interaction energy (ΔE) calculated using a super-molecular model is found to be in this order: PN···HF > PN···HCl
> PN···HBr in the series examined. Analysis of various physically meaningful contributions arising from the Kitaura-Morokuma
(KM) and reduced variational space self-consistent-field (RVS-SCF) energy decomposition procedures shows that the electrostatic
energy has significant contribution to the over-all interaction energy. Dipole moment enhancement (Δμ) was observed in these
complexes expected of predominant dipole-dipole electrostatic interaction and was found to follow the trend PN···HF > PN···HCl
> PN···HBr at the CCSD level. However, the DFT (X3LYP and B3LYP) and MP2 levels less accurately determined these values (in
this order HF < HCl < HBr). Examination of the harmonic vibrational modes reveals that the PN and HX bands exhibit characteristic
blue- and red shifts with concomitant bond contraction and elongation, respectively, on hydrogen bond formation. The topological
or critical point (CP) analysis using the static quantum theory of atoms in molecules (QTAIM) of Bader was considered to classify
and to gain further insight into the nature of interaction existing in the monomers PN and HX, and between them on H-bond
formation. It is found from the analysis of the electron density ρ
c
, the Laplacian of electron charge density ∇2ρc, and the total energy density (H
c
) at the critical points between the interatomic regions that the interaction N···H is indeed electrostatic in origin (ρc > 0, ∇2ρc > 0 and Hc > 0 at the BCP) whilst the bonds in PN (ρc > 0, ∇2ρc > 0 and Hc < 0) and HX ((ρc > 0, ∇2ρc < 0 and Hc < 0)) are predominantly covalent. A natural bond orbital (NBO) analysis of the second order perturbation energy lowering,
E(2), caused by charge transfer mechanism shows that the interaction N···H is n(N) → BD*(HX) delocalization. |
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Keywords: | |
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