Conformational diversity of the THF molecule in N2 matrix by means of FTIR matrix isolation experiment and Car-Parrinello molecular dynamics simulations

Tetrahydrofuran (THF) is a widely used chemical compound, in particular as a solvent in organic and inorganic synthesis. The THF molecule has also an interesting property, namely, undergoes pseudorotation, similar to the case of the cyclopentane. Low energy difference between the envelope (C_s symmetry) and twisted (C₂ symmetry) conformations of the THF molecule leads to the interconversion between the two conformers. We study the influence of the molecular environment (N₂) on the C_s-C₂ equilibrium of tetrahydrofuran in the THF@N₂ system utilizing nitrogen matrix isolation infrared spectroscopy. We observe a different ratio between envelope (C_s) and twisted (C₂) conformations with respect to a change of the temperature. FTIR experimental studies are supported by the results of the static density functional theory calculations and Car-Parrinello molecular dynamics simulations. We focus on the dynamics of the pseudorotation process, in particular, the lifetime of the THF conformations and their mutual rearrangements. On the basis of the THF@N₂ matrix model, with explicit nitrogen molecules, the anharmonic infrared spectra are generated from the Fourier transformation of the dipole moment autocorrelation function.

Single-walled carbon nanotubes in tetrahydrofuran solution: microsolvation from first-principles calculations

The molecular interactions between the commonly used solvent tetrahydrofuran (THF) and single-walled carbon nanotubes (SWCNT) are studied using density functional theory calculations and Car-Parrinello molecular dynamics simulations. The competitive interplay between THF-THF and THF-SWCNT interactions via C-H⋯O and C-H⋯π hydrogen bonds is analyzed in detail. The binding energies for different global and local energy minima configurations of THF monomers, dimers, trimers, and tetramers on SWCNT(10,0) were determined. The adsorbed species are analyzed in terms of their coordination to the surface via weak hydrogen bonds of the C-H⋯π type and in terms of their ability to form intermolecular C-H⋯O hydrogen bonds, which are responsible for the self-aggregation of THF molecules and a possible dimerization or tetramerization process. A special focus is put on the pseudorotation of the THF molecules at finite temperatures and on the formation of blue-shifting hydrogen bonds.

Structural flexibility of the sulfur mustard molecule at finite temperature from Car-Parrinello molecular dynamics simulations

Sulfur mustard (SM) is one of the most dangerous chemical compounds used against humans, mostly at war conditions but also in terrorist attacks. Even though the sulfur mustard has been synthesized over a hundred years ago, some of its molecular properties are not yet resolved. We investigate the structural flexibility of the SM molecule in the gas phase by Car-Parrinello molecular dynamics simulations. Thorough conformation analysis of 81 different SM configurations using density functional theory is performed to analyze the behavior of the system at finite temperature. The conformational diversity is analyzed with respect to the formation of intramolecular blue-shifting CH⋯S and CH⋯Cl hydrogen bonds. Molecular dynamics simulations indicate that all structural rearrangements between SM local minima are realized either in direct or non-direct way, including the intermediate structure in the last case. We study the lifetime of the SM conformers and perform the population analysis. Additionally, we provide the anharmonic dynamical finite temperature IR spectrum from the Fourier Transform of the dipole moment autocorrelation function to mimic the missing experimental IR spectrum.

A priori calculations of the free energy of formation from solution of polymorphic self-assembled monolayers

Modern quantum chemical electronic structure methods typically applied to localized chemical bonding are developed to predict atomic structures and free energies for meso-tetraalkylporphyrin self-assembled monolayer (SAM) polymorph formation from organic solution on highly ordered pyrolytic graphite surfaces. Large polymorph-dependent dispersion-induced substrate-molecule interactions (e.g., -100 kcal mol(-1) to -150 kcal mol(-1) for tetratrisdecylporphyrin) are found to drive SAM formation, opposed nearly completely by large polymorph-dependent dispersion-induced solvent interactions (70-110 kcal mol(-1)) and entropy effects (25-40 kcal mol(-1) at 298 K) favoring dissolution. Dielectric continuum models of the solvent are used, facilitating consideration of many possible SAM polymorphs, along with quantum mechanical/molecular mechanical and dispersion-corrected density functional theory calculations. These predict and interpret newly measured and existing high-resolution scanning tunnelling microscopy images of SAM structure, rationalizing polymorph formation conditions. A wide range of molecular condensed matter properties at room temperature now appear suitable for prediction and analysis using electronic structure calculations.

A computational study of intramolecular hydrogen bonds breaking/formation: impact on the structural flexibility of the ranitidine molecule

Ranitidine is a histamine H2-receptor antagonist that reduces gastric acid secretion. We studied the flexibility of the ranitidine molecule with the special focus on the network of diverse intramolecular hydrogen bonds: N-H⋯O, N-H⋯N, C-H⋯O, C-H⋯N and N-H⋯S. We performed static density functional theory calculations of global and local minima and analyzed their stability at finite temperature in the Car-Parrinello molecular dynamics simulations. We observed intramolecular H-bonds breaking/formation crucial for the structural rearrangements leading to the folding process. The lifetimes of the closed structures of ranitidine were also estimated. The existence of hydrogen bonds and their strength were confirmed on the basis of topological parameters in the bond critical points utilizing Quantum Theory of Atoms in Molecules.