Hcn And Hnc To Participate In The Intermolecular Interaction Of Theoretical Research

Intermolecular interactions play a crucial role in fields of chemistry, physics, and biology including stabilizing biomolecular structures, modulating specificity of enzymatic reactions, and constructing supramolecular structures. Thus they have stirred up copious amounts of attention for the chemical and theoretical researchers. In this paper, we selected hydrogen cyanide (HCN) and hydrogen isocyanide (HNC) as models to study the hydrogen bond, halogen bond, chalcogen bond, and the cooperative and competive behaviour between different interactions. The nature of the cooperativity of the hydrogen bond was also investigated. The main results are as follows:1:An ab initio study of the complexes formed between HM (M = O and S) and HXY (XY = CN and NC) have been investigated at the QCISD/6-311++G(2df,2p) level. For comparison, the corresponding H₂M-HXY complexes were also studied. Two minima were found for each molecular pairs. The result shows the necessary of electron correlation and larger basis set in study of the open-shell hydrogen-bonded complexes. As the proton donor and acceptor, the OH radical is favorable to form a hydrogen bond with HXY than the SH radical. The MH radical is favorable to donor proton than H₂M, whereas H₂M is favorable to donor electron than MH radical. The natural bond orbital (NBO) and atoms in molecules (AIM) analyses have been performed for these systems. It is shown that the complexes are held together mainly by an electrostatic interaction.2:An ab initio computational study of the enhancing role of methyl group in the M···H (M = S and O) hydrogen bond has been carried out at the QCISD/6-311++G(2df,2p) level. The bond lengths, frequency shifts, and interaction energies were analyzed. The methyl group in the electron donor plays a positive contribution to the formation of hydrogen bond. Its enhancing role is stronger in the O···H hydrogen bond than in the S···H one. The results show that the methyl group has a prominent effect on the strength of hydrogen bond. The interaction energy is increased by 347% in Me₂O-HCN complex relative to that in O-HCN complex. The enhancing mechanism of methyl group has been analyzed by means of natural bond orbital (NBO) theory. The electrostatic interaction is of more importance in the O···H hydrogen bond, whereas the dispersion and charge transfer interactions play a more significant role in the S···H hydrogen bond.3:An ab initio computational study of the dual functions of C=S group in the M2C=S···HCN (M = H, F, Cl, Br, HO, H₃C, H₂N) complex has been performed at the MP2(Full)/aug-cc-pVTZ level. The C=S group can act as both the electron donor and acceptor, thus two minima complexes were found for each molecular pairs. The interaction energy of hydrogen bond in the F, Cl, or Br substituted complexes is less negative than that in the corresponding H₂CS one, while the interaction energy of theσ-hole interaction is more negative. The OH substitution weakens the hydrogen bond, whereas the H₃C and H₂N substitution strengthens it. Theσ-hole interaction in the HO, H₃C, and H₂N complexes is very weak. The substitution effect has been understood with electrostatic induction and conjugation effects. The energy decomposition analysis has been performed for the halogen-substituted complexes.4:A computational study of the complexes formed between HNC and HOBr has been undertaken at the MP2/aug-cc-pVTZ theoretical level. Six dyads and thirteen triads formed through hydrogen bonding and halogen bonding were found. It was found that the halogen bond is weaker than the hydrogen bond in the HNC-HOBr dyad. The C in the HNC is found to be a better electron donor than the O atom in the HOBr. The cooperative effect is present between the hydrogen bond and the halogen bond in the triad. The most stable triad corresponds to the complex with OH···C and NH···C hydrogen bonds. However, the largest cooperative effect occurs in the structure combined with NH···O and OBr···C interactions, where the cooperative energy is 23% of the total interaction energy. The cooperative effect is negative when HOBr acts as the proton and halogen donors simultaneously.5:Ab initio calculations at the MP2/aug-cc-pVTZ level have been performed to study the cooperativity of hydrogen bonds in the homoclusters (HNC-HNC-HNC and HNC-HNC-HNC-HNC) and heteroclusters (H₃N-HNC-HNC and H₃N-HNC-HNC-HNC). The cooperative energies in the HNC-HNC-HNC and H₃N-HNC-HNC trimers are -2.05 and -2.56 kcal/mol, respectively. The result shows that the cooperativity in the heterotrimer is larger than that in the homotrimer. A similar result also happens in the tetramers. The energy decomposition scheme indicates that the orbital interaction plays a main contribution to the cooperative energy of N···HN hydrogen bond, whereas the electrostatic and orbital interactions are responsible for the C···HN hydrogen bond. The effect of HNC chain length on the strength of N···HN hydrogen bond has also been considered at the MP2/aug-cc-pVDZ level. It is indicated that the interaction energy of N···HN hydrogen bond trends to be a fixed value when the HNC number tends to be infinite and the strength of N···HN hydrogen bond is regulated mainly through the electrostatic and polarization interactions although the charge transfer interaction also has an effect on it.