No abstract

No abstract

Self-assembly of molecules through weak forces such as hydrogen bonds and hydrophobic and electrostatic interactions to form supramolecular aggregates has become a major research area in recent years."] While conventional liquid crystals (LC) are one aspect of this subject,[21 metal-containing liquid crystals (metallomesogens, MLCs) provide new possibilities for varying the structure and properties such as color and polari~ability. [~] Designing stable MLCs remains a challenge. Many MLCs decompose at the clearing point; their applications are therefore limited. We are interested in the chemistry of gold(1). Gold(1) compounds are used in medicine, they exhibit remarkable photophysical properties, and aurophilic interactions are important in the construction of supramolecules. The above reasons and the inherent thermal stability of metal complexes of 1,3-disubstituted imidazol-2-ylidene (imy) led us to investigate the synthesis of liquid-crystalline bis(l,3-dialkylbenzimidazol-2-ylidene)gold(1) bromide [{(C,H,,+,),-bimy},Au]Br (n = 12, 14, 16).

Close interactions of the C(alpha)[bond]H- - -O type have been analyzed via X-ray crystallography and high-pressure infrared spectroscopy. The results demonstrate that the C(alpha)[bond]H- - -O interactions can offer an additional stability to the beta-sheet formation. X-ray structural data suggest that while 1-acetamido-3-(2-pyrimidinyl)-imidazolium bromide exhibits a bilayer stacking, the PF(6)(-) salt reveals a beta-sheetlike pattern. The appearance of the free-NH infrared absorption indicates that the conventional N[bond]H- - -O or N[bond]H- - -N hydrogen bonds do not fully dominate the packing for the PF(6)(-) salt. The high-pressure infrared study suggests that the C(alpha)[bond]H- - -O hydrogen bonds are the important determinants for the stability of the PF(6)(-) salt. This study also verifies that the imidazolium C[bond]H stretching frequency shifts to a longer wavelength upon the formation of the C[bond]H- - -O hydrogen bonds.

We have probed under high pressure the C-H hydrogen bonds formed by N,N(')-disubstituted imidazolium ions having PF(6) (-) and Br(-) counterions. High-pressure infrared spectral profiles, x-ray crystallographic analysis, and ab initio calculations allow us to make a vibrational assignment of these compounds. The appearance of a signal for the free-NH unit (or weakly bonded N-H...F unit) in the infrared spectrum of the PF(6) (-) salt indicates that conventional N-H...O and N-H...N hydrogen bonds do not fully dominate the packing. It is likely that the charge-enhanced C(2)-H...F interactions, combined with other weak hydrogen bonds, disturb the formation of N-H hydrogen bonds in the PF(6) (-) salt. This finding is consistent with the pressure-dependent results, which reveal that the C(2)-H...F interaction is enhanced upon increasing the pressure. In contrast to the PF(6) (-) salt, the imidazolium C-H bonds of the Br(-) salt have low sensitivity to high pressure. This finding suggests that the hydrogen bonding patterns are determined by the relative hydrogen bond acceptor strengths of the Br(-) and PF(6) (-) ions.

C–H–O interactions of a self-assembled triple helix based on the 1-acetamido-3(2-pyrazinyl)-imidazolium cation has been probed by high pressure. The infrared spectroscopic profiles and ab initio calculations allow us to make a vibrational assignment of this compound. The C–H bonds forming C–H–O interactions shorten as the pressure was elevated, while free C–H vibration modes show low sensitivity to high pressure. The pressure-dependent results can be attributed to the strengthening of C–H–O electrostatic–dispersion interactions upon increasing pressure. The appearance of the free-NH infrared absorption indicates that the conventional N–H–O hydrogen bond does not dominate the inter-strand packing in the compound. It is proposed that the charge-enhanced C–H–O interactions, forming a helical hydrogen-bonding network, disturb the formation of inter-strand N–H–O hydrogen-bonding in order to form a maximum number of hydrogen bonds. Applying high-pressure seems not to change the C=O bond length in contrast to the trend of blue-shift in frequency of C–H vibrations. London dispersion energy is suggested to be required for understanding the pressure-dependent results, although more additional terms, such as the effect in the presence of charge, are needed for the correct explanation. This work demonstrates that high-pressure studies may have the potential to provide insight into the C–H–O structural properties of biological related systems.

Interactions in Cliemi.stf:J, Wiley, New York, 1985. [16] Single-point-Rechnnngen an [Ni5(p5-S) (p2-SH)J mit der experimentell bestimmten Struktur wurden auf dem RHF-Niveau durchgefuhrt. Fur Schwefel und Wasserstoff wurde der 6-31G*-Basissatz und fur Nickel ein effektives Rumpfpotential (effective core potential, ECP) mit dem LanL2DZ-Basissatz verwendet. Bei den Rechnungen an [Ni20(ps-Se),(pc,-Se)~o(p2-SeH),,I ~ wurde sowohl fur Nickel als auch fur Selen ein ECP rnit dem LanL2MB-Basissatz verwendet.[17] G.