Ab Initio Quantum Chemical Study of the Coordination Preferences and Catalytic Role of Cu~+ Ions in the Dehydration Reactions of Hydroxyformaldoxime Conformers and the Oxidation of HCN to Hydroxyformaldoxime by Hydrogen Peroxide

摘要

A detailed exploration of the configurational and conformational space of hydroxyformaldoxime (hfaox) has been carried out with the aid of first principles quantum chemical techniques at the HF, MP2, B3LYP, and CCSD(T) levels of theory using the 6-31G(d), 6-311G(d,p), 6-311+G(2df,2p), and 6-311++G(2d,p) basis sets. The most stable configuration among the eight possible hfaox conformers corresponds to the (Z)-(s-cis,s-trans) configurations, while the highest energy (E)-(s-trans,s-cis) conformer was found at 15.26, 15.31, 14.34, 14.91, and 14.78 kcal/mol higher in energy at the HF, MP2, B3LYP, MP4SDQ, and CCSD(T) levels of theory, respectively, using the largest 6-311++G(2d,p) basis set. Calculated structures, relative stability, and bonding properties of the conformers are discussed with respect to computed electronic and spectroscopic properties, such as charge density distribution, harmonic vibrational frequencies, and NMR chemical shifts. From a methodological point of view, our results confirm the reliability of the integrated computational tool formed by the B3LYP density functional model. This model has subsequently been used to investigate the dehydration reactions of hfaox. Upon dehydration, hfaox could afford a number of isomeric CHNO species. All dehydration processes, except those yielding cyanic acid, are predicted to be endothermic. The most endothermic one is the dehydration reaction of the more stable (Z)-(2-cis,s-trans) conformer, 7, to formylnitrene, with the computed heat of reaction (Δ_RH) being equal to 51.34 kcal/mol. On the other hand, the most exothermic dehydration process is dehydration reaction of the (E)-(s-trans,s-cis) conformer, 14, to cyanic acid with the computed heat of reaction being equal to -26.16 kcal/mol at the B3LYP/6-311G(d,p) level of theory. The reaction pathway for the addition of water to fulminic acid is predicted to occur via an activation barrier of 68.18 kcal/mol at the B3LYP/6-311G(d,p) level. The interaction of Cu~+ ions with the CHNO isomers, as well as their precursor hfaox conformers, was also analyzed in the framework of DFT theory, illustrating that the Cu~+ ions show a higher affinity for the N donor atoms. Depending on the conformer, the computed interaction energies for the Cu-N, Cu-ON, and Cu-OC associations in the Cu(hfaox)~+ complexes were found in the range of 62.61-75.29, 44.75-57.43, and 38.84-58.46 kcal/mol, respectively. In summary, the calculations threw light on the bonding properties of the CHNO species, being the simplest model of the HN-CO peptide linkage to biologically important Cu~+ ions. Of particular interest is the prediction that Cu~+ ions catalyze the dehydration reactions of hfaox to form CHNO isomers and are involved in a novel reaction of the oxidation of organic nitriles with hydrogen peroxide to yield oximes, the latter process providing a novel synthetic route for oxime derivatives. The energetic and geometric profiles of these reactions were fully investigated in the framework of DFT theory, and their mechanisms are thoroughly discussed.