Search results for "force"

Low-temperature methyl group dynamics of hexamethylbenzene in crystalline and glassy matrices as studied by 2H NMR

1991

Abstract 2 H NMR spectra of hexamethylbenzene (HMB) in protonated crystalline and amorphous matrices at low temperatures are presented. All spectra reveal lineshape changes which can be attributed to methyl group tunnelling. Compared to neat HMB, a drastic increase of the tunnelling frequency is found for all systems. This indicates that the hindering potential originates predominantly from intermolecular forces. We studied the temperature dependence of these spectra and the spin-lattice relaxation in order to exclude a distribution of motional correlation times describing a thermally activated process. In addition, we find a distortion of the methyl tetrahedron.

Designing Solid Solutions of Enantiomers: Lack of Enantioselectivity of Chiral Naphthalimide Derivatives in the Solid State

2017

The enantiomers of a previously reported naphthalimide derivative are shown in this study to form a solid solution; furthermore, on the basis of the knowledge of solid solution structural aspects other naphthalimide derivatives have been synthesized and shown to lack the enantioselectivity in the solid state. The structural origin of solid solution formation is the same as observed in most of the cases in the literaturequasi-centrosymmetric structures form at nonracemic compositions where the most abundant enantiomer adjusts its conformation to mimic the absent one. Such solid solutions belong to the type showing some enantioselectivity. An extended single crystal X-ray diffraction study o…

Theoretical Study of the Twisted Intramolecular Charge Transfer in 1-Phenylpyrrole

2000

Ab initio results for the electronic spectra of 1-phenylpyrrole are presented. Vertical, emission, and nonvertical excitation energies have been computed using multiconfigurational second-order per...

ChemInform Abstract: Microwave-Assisted Organocatalytic Enantioselective Intramolecular Aza-Michael Reaction with α,β-Unsaturated Ketones.

2012

The combination of an epi-hydroquinine catalyst and pentafluoropropionic acid accelerates the process.

Cyclizations via Frustrated Lewis Pairs: Lewis Acid Induced Intramolecular Additions of Amines to Olefins and Alkynes

2010

ChemInform Abstract: Intramolecular Michael Reaction of tert-Butylsulfinyl Ketimines: Asymmetric Synthesis of 3-Substituted Indanones.

2012

The reaction proceeds in the presence of the Ruppert—Prakash reagent (Tms-CF3) with either TBAT or TBAF as a base.

Do the Intramolecular π Interactions Improve the Stability of Ionic, Pyridine-Carbene-Based Iridium(III) Complexes?

2013

Throughout the last years one of the most intensive research topics in light-emitting electrochemical cells (LECs) focused on the design of blue-emitting, ionic iridium(III) complexes. To this end, the most recent strategy is the use of carbene-based ancillary ligands. Although blue LECs have been successfully fabricated, the stability has been noted as the main drawback. To overcome this problem, Zhang et al. have recently explored the use of π interactions to enhance the strength of pyridine-carbene-based complexes. The authors suggested that the use of intramolecular π–π stacking interactions by means of pendant phenyl rings to improve the stability of LECs is not as effective as in devi…

Intramolecular pi-stacking in a phenylpyrazole-based iridium complex and its use in light-emitting electrochemical cells.

2010

A novel iridium(III) complex, [Ir(dmppz)(2)pbpy][PF(6)] (Hdmppz = 3,5-dimethyl-1-phenylpyrazole and pbpy = 6-phenyl-(2,2'-bipyridine)), is reported. The complex shows an intramolecular face-to-face pi-stacking between the phenyl ring of the dmppz ligand and the pendant phenyl of the pbpy ligand. This interaction provides a supramolecular cage formation that holds also in the excited states. Light-emitting electrochemical cells (LECs) using the novel complex show extraordinary lifetimes of approximately 2000 h. The high stability is favored by the presence of pendant methyl groups on the dmppz ligands that hinder the entrance of water molecules rendering the degradation of the complex more d…

Scanning Force Microscopy of Artificial Membranes

2002

Visualization of biological membranes by scanning force microscopy (SFM) has tremendously improved the current understanding of protein ‐ lipid interactions under physiological conditions. SFM is the only tool to directly image processes on surfaces in aqueous solution at molecular resolution. Besides being a supportive means to confirm results on lipid phases and domains obtained from fluorescence spectroscopy, calorimetry, and X-ray crystallography, SFM has contributed distinct aspects on the formation of 2D crystals of various membrane-confined proteins and morphological changes of membranes due to the interaction of peptides and proteins. This review will focus on recent results in SFM …

Adhesion and adhesion hysteresis of mica surfaces covered with bola-amphipiles in dry and humid air

1998

Abstract Bola-amphiphiles with spherical hydrophilic chinuclidinium head groups form laterally ordered monolayers on mica by adsorption from solution. To measure the surface adhesion, JKR-type compression/decompression cycles have been performed with the surface forces apparatus (SFA). In an inert atmosphere, the adhesion energy on increasing the contact area is much lower than predicted by wetting experiments. Furthermore, the pronounced adhesion hysteresis varies from sample to sample in a wide range, indicating a defect-rich surface. On increase of the relative humidity (rh), the reproducibility improves and the values for the adhesion energy measured on compression and decompression con…