Shape of the proton potential in an intramolecular hydrogen-bonded system. Part II

Exploring Intra- and Intermolecular Interactions in Selected N-Oxides-The Role of Hydrogen Bonds

Intra- and intermolecular interactions have been explored in selected N-oxide derivatives: 2-(N,N-dimethylamino-N-oxymethyl)-4,6-dimethylphenyl (1) and 5,5'-dibromo-3-diethylaminomethyl-2,2'-biphenol N-oxide (2). Both compounds possess intramolecular hydrogen bonding, which is classified as moderate in 1 and strong in 2, and resonance-assisted in both cases. Density Functional Theory (DFT) in its classical formulation as well as Time-Dependent extension (TD-DFT) were employed to study proton transfer phenomena. The simulations were performed in the gas phase and with implicit and explicit solvation models. The obtained structures of the studied N-oxides were compared with experimental data available. The proton reaction path was investigated using scan with an optimization method, and water molecule reorientation in the monohydrate of 1 was found upon the proton scan progress. It was found that spontaneous proton transfer phenomenon cannot occur in the electronic ground state of the compound 1. An opposite situation was noticed for the compound 2. The changes of nucleophilicity and electrophilicity upon the bridged proton migration were analyzed on the basis of Fukui functions in the case of 1. The interaction energy decomposition of dimers and microsolvation models was investigated using Symmetry-Adapted Perturbation Theory (SAPT). The simulations were performed in both phases to introduce polar environment influence on the interaction energies. The SAPT study showed rather minor role of induction in the formation of homodimers. However, it is worth noticing that the same induction term is responsible for the preference of water molecules' interaction with N-oxide hydrogen bond acceptor atoms in the microsolvation study. The Natural Bond Orbital (NBO) analysis was performed for the complexes with water to investigate the charge flow upon the polar environment introduction. Finally, the TD-DFT was applied for isolated molecules as well as for microsolvation models showing that the presence of solvent affects excited states, especially when the N-oxide acceptor atom is microsolvated.

N-oxide Derivatives: Car-Parrinello Molecular Dynamics and Electron Localization Function Study on Intramolecular Hydrogen Bonds

First-principle molecular dynamics simulations (CPMD) and electron localization function (ELF) topological analysis were performed for two N-oxides, 2-[( N, N-dimethylamino- N-oxy)methyl]-4,6-dimethylphenol (I) and 5,5'-dibromo-3-[(diethylamino)methyl]-2,2'-biphenol N-oxide (II). Special attention was paid to hydrogen bond dynamics and spectroscopic features. Simulations were carried out in vacuo and in the crystalline phase. It was found that for compound I proton transfer phenomena do not occur spontaneously. An opposite situation was noticed for one of the two intramolecular hydrogen bonds of compound II. The influence of the neighboring molecules (including microsolvation) and the crystal field on the hydrogen bond properties cannot be neglected. Hydrogen bond spectroscopic signatures were quite well reproduced by power spectra of atomic velocity. Metric and spectroscopic theoretical findings were compared with experimental data available, especially in a view of the fact that experimental X-ray diffraction and FT-IR data of compound II indicate strong delocalization of the N-oxide bridge proton. ELF analysis showed that for compound II the N-oxide bridge belongs to the topological category of strong hydrogen bonds with the separate ELF basin for the bridged hydrogen atom.

Car-Parrinello simulation of an O-H stretching envelope and potential of mean force of an intramolecular hydrogen bonded system: application to a Mannich base in solid state and in vacuum

Car-Parrinello molecular dynamics (CPMD) study was performed for an anharmonic system-an intramolecularly hydrogen bonded Mannich-base-type compound, 4,5-dimethyl-2(N,N-dimethylaminemethyl)phenol, to investigate the vibrational spectrum associated with the O-H stretching. Calculations were carried out for the solid state and for an isolated molecule. The classical CPMD simulation was performed and then the proton potential snapshots were extracted from the trajectory. The vibrational Schrodinger equation for the snapshots was solved numerically, and the (O-H) envelope was calculated as a superposition of the 0-->1 transitions. The potential of mean force for the proton stretching mode was calculated from the proton vibrational eigenfunctions and eigenvalues incorporating statistical sampling, nuclear quantum effects, and effects of the environment. Perspectives for application of the presented methodology in the computational support of biocatalysis are given in the study.