The hydrogen-bonded dimers of formic acid derivatives XCOOH (X = H, F, Cl, and CH3) have been investigated using density functional theory (B3LYP) and second-order Møller-Plesset perturbation (MP2) methods, with the geometry optimization carried out using 6-311++G(2d,2p) basis set. The dimerization energies calculated using aug-cc-pVXZ (with X = D and T) basis have been extrapolated to infinite basis set limit using the standard methodology. The results indicate that the fluorine-substituted formic acid dimer is the most stable one in comparison to the others. Topological analysis carried out using Bader's atoms in molecules (AIM) theory shows good correlation of the values of electron density and its Laplacian at the bond critical points (BCP) with the hydrogen bond length in the dimers. Natural bond orbital (NBO) analysis carried out to study the charge transfer from the proton acceptor to the antibonding orbital of the X-H bond in the complexes reveals that most of the dimers are associated with conventional H-bonding except a few, where improper blue-shifting hydrogen bonds are found to be present.
Ab initio and density functional theoretical studies on hydrogen-bonded complexes of azabenzenes with water, acetamide, and thioacetamide have been carried out to explore the controversy involved in the relative order of their stability in a systematic way. The interaction energies of these complexes have been analyzed using the Morokuma energy decomposition method, and the nature of the various hydrogen bonds formed has been investigated through topological aspects using Bader's atom in a molecule (AIM) theory. Morokuma energy decomposition analysis reveals that the major contributions to the energetics are from the polarization (PL) and charge transfer (CT) energies. From the calculated topological results, excellent linear correlation is shown to exist between the hydrogen-bond length, electron density [rho(r)], and its Laplacian [nabla(2)rho(r)] at the bond critical points for all the complexes considered.
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