Specific Features of Intramolecular Proton Transfer Reaction in Schiff Bases

The differences between the intramolecular proton transfer in Mannich and Schiff bases are discussed. The tautomeric forms being in equilibrium in both types of molecules are seriously different. In Mannich bases there are in equilibrium the forms of phenols and phenolates. In Schiff bases each of tautomers is strongly influenced by resonance between zwitterionic and keto structures. Despite the common opinion that the proton transfer forms in compounds with internal π-electronic coupling are mainly keto forms it is shown in this work, that in Schiff bases the content of keto structure is slightly less than zwitterionic one. Almost equal participation of both forms leads to effective resonance between them and stabilization of intramolecular hydrogen bond in this way

Direct observation of the substitution effects on the hydrogen bridge dynamics in selected Schiff bases-A comparative molecular dynamics study

We have studied substituent effects on the properties of the intramolecular hydrogen bond of some ortho-hydroxy Schiff bases using density functional theory (DFT) based first-principle molecular dynamics (FPMD) and path integral molecular dynamics. The studied compounds possess a strong intramolecular hydrogen bond (r((O ... N)) <= 2.6 angstrom), which can be tuned by substitution to either (i) enhance the basicity of the acceptor moiety by induction effects or (ii) decrease the hydrogen bond length through steric repulsion. DFT calculations and FPMD were employed to investigate structural and dynamical properties of the selected molecules, while quantum effects on the structural properties were assessed using path integral FPMD. The simulations were performed in vacuo and in the solid state to study the influence of the environment on the hydrogen bond and spectroscopic properties. We give computational support to the suggestion that induction effects are less effective to tune the intramolecular hydrogen bond properties of the discussed ortho-hydroxy Schiff bases than the steric or the environmental effects. (C) 2011 American Institute of Physics. [doi:10.1063/1.3528721

Interaction of Piperidin Derivative of Mannich Base with DPPC Liposomes

The long chain Mannich bases, especially with the piperidine and morpholine groups, display very promising antimicrobial activity. In order to extend our knowledge on their impact on biological systems, we examined the interactions of the 5-pentadecyl-2-((piperidin-1-yl)­methyl)­phenol (PPDP) with model lipid membrane by means of differential scanning calorimetry (DSC) and fluorescence measurements. The small unilamellar vesicles of dipalmitoylophosphatidylcholine (DPPC) with different piperidine Mannich base concentration were investigated as a function of the increase of temperature. The phase separation accompanied by the rise of the transition enthalpy of both subcomponents, the increase of the function of the GP values of Laurdan versus the wavelength of excitation in the gel phase of PPDP/DPPC systems, and no remarkable differences in the fluorescence anisotropy of PPDP molecules in lipid environment for different mixtures of PPDP/DPPC was observed. Additionally, it was shown that PPDP itself interdigitated in solid state

Intermolecular Interactions in the Solid State of Ionic Secondary Mannich Bases

Two new secondary Mannich bases, 4-bromo-2-[(aminopropyl)­methyl]-phenol (<b>1</b>) and 4-nitro-2-[(aminopropyl)­methyl]-phenol (<b>2</b>), were synthesized. Crystal structures were determined at liquid nitrogen temperature. It was found that in both compounds the proton transfer forms exist in the solid state. In the case of <b>1</b>, this was unexpected, because of the weak acidity of <i>p</i>-bromophenol being the parent component of this Mannich base. The reason for that was found to be the formation of the O^–···H–N⁺ hydrogen bonded tetramer in the solid state. Two cyclic aggregates R₄²(8) and R₂²(12) describe the pattern of hydrogen bonded interactions in the crystals of both compounds. Additionally, C–H···π interactions stabilize the crystal structures. The hydrogen bonds in <b>1</b> are slightly stronger (N···O distances 2.708 and 2.733 Å) than in <b>2</b> (2.721 and 2.765 Å, respectively) despite the fact that <i>p</i>-nitrophenol participating in <b>2</b> is a stronger acid. The influence of permittivity of surroundings and the hydrogen bonding pattern on the properties of intermolecular hydrogen bonds are discussed on the basis of B3LYP and MP2 calculations with basis sets 6-31+G­(d,p) and 6-31++G­(2d,2p). The coupling between hydrogen bonds in crystals was studied with the application of the IR spectra of isotopically diluted species. It was found that such a coupling is stronger for <b>2</b>, forming weaker hydrogen bonds. Both the theory of IR spectra and quantum chemical calculations demonstrate that the source of the observed behavior is electronic participation in vibronic absorption