In the present study, the hydrogen-bonded complexes of azole with water and hydrogen peroxide are systematically investigated by second-order Møller–Plesset perturbation theory and density functional theory with dispersion function calculations. This study suggests that the ability of pyrrolic nitrogen (NH) atom to function as hydrogen-bond donor increases with the introduction of nitrogen atoms in the ring, whereas the ability of pyridinic nitrogen (N) atom to act as hydrogen-bond acceptor reduces with successive aza substitution in the ring. With introduction of nitrogen atoms in the ring, the vibrational frequency, stabilization energy, and electron density in the σ antibonding orbitals of the X–H (X = N, C of azole) bond of the complexes all increase or decrease systematically. Decomposition analysis of total stabilization energy showed that the electrostatic energy term is a dominant attractive contribution in comparison to induction and dispersion terms in all of the complexes under study.

The second-order Møller–Plesset perturbation theory (MP2) and density functional theory with dispersion function calculations have been applied to investigate the hydrogen-bonding interaction between azines and water. The study suggests that the ability of nitrogen present in azine to act as a hydrogen-bond acceptor decreases in the order of pyridine ( PY ) > diazine ( DZ ) > triazine ( TZ ) > tetrazine ( TTZ ) > pentazine ( PZ ) > hexazine ( HZ ). Natural bond orbital (NBO) analysis, atoms in molecules, symmetry-adapted perturbation theory (SAPT), and molecular electrostatic potential studies reflect the factors important for hydrogen-bond strength as well as for the structural, electronic, and vibrational changes occurring during complexation. NBO analysis reflects that upon gradual addition of nitrogen atoms, hyperconjugation leads to an increase in the population of antibonding O–H bond, thus causing elongation and weakening of O–H bond in complexes incorporating N···H–O W interaction, whereas rehybridization leads to an increase in the s character of the carbon hybrid orbital in C–H bond, thus causing contraction and shortening of C–H bond in complexes having C–H···O W interactions. From the topological analysis, an excellent linear correlation is found to exist between stabilization energy (Δ E BSSE ), electron density (ρ c ), and its Laplacian (∇ 2 ρ c ) at the bond critical points.

The hydrogen bonded complexes of serine with water and with H 2 O 2 (HP) have been completely investigated in the present study using second-order Møller-Plesset perturbation theory (MP2) and density functional theory (DFT) in order to determine their geometries, stabilization energies, vibrational frequencies and electronic characteristics. The stabilization energies (E BSSE) span a range of −2.76 to −12.46 kcal/mol for 1:1 serine-water complexes and −4.54 to −12.73 kcal/mol for 1:1 serine-HP complexes. The E BSSE values suggest that serine-HP complexes are more stable than serine-water complexes. For all the structures, complex formation results in elongation of N-H, O-H bonds and shortening of C-H bonds thereby showing the red-shift and blue-shift for the respective bonds. The structural, vibrational and electronic features are in accordance with the fact that HP is a better proton donor and water a better proton acceptor. The excellent relationship is obtained for the variation of E BSSE values with the sum of the ρ values and the sum of laplacian at the BCPs for the HBs. The E (2) values are also in concordance with the calculated E BSSE values.

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