Blue-Shifting Intramolecular C−H···O Interactions

A Critical Overview of Current Theoretical Methods of Estimating the Energy of Intramolecular Interactions

This article is probably the first such comprehensive review of theoretical methods for estimating the energy of intramolecular hydrogen bonds or other interactions that are frequently the subject of scientific research. Rather than on a plethora of numerical data, the main focus is on discussing the theoretical rationale of each method. Additionally, attention is paid to the fact that it is very often possible to use several variants of a particular method. Both of the methods themselves and their variants often give wide ranges of the obtained estimates. Attention is drawn to the fact that the applicability of a particular method may be significantly limited by various factors that disturb the reliability of the estimation, such as considerable structural changes or new important interactions in the reference system.

Nonbonding interaction analyses on PVDF/[BMIM][BF4] complex system in gas and solution phase

The present study provides a detailed quantum chemical description of the physicochemical interactions between poly-vinylidene fluoride (PVDF) and 1-butyl-3-methyl-imidazolium tetrafluoro borate ([BMIM][BF₄]) ionic liquid (IL). Geometry optimization and frequency calculations are carried out for four monomer units of α- and β-PVDF, [BMIM][BF₄], and PVDF/[BMIM][BF₄] using dispersion corrected density functional theory. The effects of solvation on the systems under study are demonstrated for three polar aprotic solvents, namely tetra-hydrofuran (THF), acetone, and n,n-dimethyl formamide (DMF) using the integral equation formalism polarizable continuum model (IEFPCM). Calculated negative solvation free energy values suggest solution phase stability of the systems under study. Binding and interaction energies for β-PVDF/IL are found higher in magnitude than those for α-PVDF/IL. The nonbonding interaction phenomenon of β-PVDF/[BMIM][BF₄] is elucidated on the basis of natural bond orbital (NBO), Bader's quantum theory of atoms in molecules (QTAIM), delocalization indices, Hirshfeld surface, and reduced density gradient (RDG) analyses. Both anions and cations of ionic liquids are found to show weak van der Waals interaction with PVDF molecule but the anion ([BF₄]^-)/PVDF interaction is found to be stronger than cation ([BMIM]⁺)/PVDF interaction. Inter-unit C-H⋯F type hydrogen bonds are found to show improper (causing blue shifts in vibrational frequencies) nature. Frontier molecular orbital analysis is carried out, and different chemical parameters like electronegativity, chemical potential, chemical hardness and softness, and electrophilicity index are calculated using Koopmans' theorem. Thermochemical calculations are also performed, and the variation in different standard thermodynamic parameters with temperature is formulated. Graphical abstract (a) Hirshfeld surface mapped onto electron density and (b) NCI isosurfaces showing inter-unit interactions of β-PVDF/[BMIM][BF₄].

Assessment of the Presence and Strength of H-Bonds by Means of Corrected NMR

The downfield shift of the NMR signal of the bridging proton in a H-bond (HB) is composed of two elements. The formation of the HB causes charge transfer and polarization that lead to a deshielding. A second factor is the mere presence of the proton-accepting group, whose electron density and response to an external magnetic field induce effects at the position of the bridging proton, exclusive of any H-bonding phenomenon. This second positional shielding must be subtracted from the full observed shift in order to assess the deshielding of the proton caused purely by HB formation. This concept is applied to a number of H-bonded systems, both intramolecular and intermolecular. When the positional shielding is removed, the remaining chemical shift is in much better coincidence with other measures of HB strength.

Colorimetric detection of fluoride ions by anthraimidazoledione based sensors in the presence of Cu(ii) ions

A thioimidazole bearing anthraimidazoledione detects fluoride selectively in the presence of Cu²⁺with a detection limit of 0.04 ppm. The results show that the change of thioimidazole to imidazole leads to no detection.

Different zeroes of interaction energies as the cause of opposite results on the stabilizing nature of C-H···O intramolecular interactions

The interaction energy of the C-H···O intramolecular interaction is estimated by several methods for a large group of systems possessing a quasi-cyclic six-membered ring. In the case of the geometry corrected method (GCM), the related rotamers method (RRM), and Espinosa's method (EM), the linear correlations between interaction energies and the electron density at the bond critical point have close slopes. The first and the last two methods yield almost systematically opposite results concerning the stabilizing/destabilizing character of the interaction, and their main difference is their zero of the interaction energy. An investigation on the limitations of reference energies has led to the introduction of the geometry corrected related rotamers method (GCRRM), estimating both stabilizing and destabilizing C-H···O interactions. An extension of EM is proposed.

Influence of confinement on hydrogen bond energy. The case of the FH···NCH dimer

The influence of the external pressure on the energy of the intermolecular hydrogen bond is investigated by modeling the pressure effects with helium atoms located at fixed points in space around the hydrogen bonded dimer. Several methods of estimating the energy of the H···N hydrogen bond in the He···FH···NCH···He model system are proposed. They show that the energy of the H···N hydrogen bond in this confined dimer decreases continuously with the reduction of the He···He distance, thus with the increase of the pressure effect exerted on the confined dimer.