Search results for "bond"
showing 10 items of 3527 documents
Conformational properties of methylene bridged resorcarenes
1998
Abstract Conformations and conformational interconversions of resorcarene 1c have been studied by molecular mechanics calculations. As with calix[4]arenes the general stability of the four basic conformations is cone > partial cone > 1,2-alternate > 1,3-alternate. The lowest energy is calculated for a pinched cone conformer with C2v symmetry stabilised by intramolecular hydrogen bonds of the two “parallel” resorcinol units as donors. The topomerisation of the cone conformation proceeds via the partial cone and 1,2-alternate intermediates with a calculated barrier of 9.9 kcal mol−1 which is in excellent agreement with the experimental value.
A reversible hydrogen‐bond isomerization triggered by an abrupt spin crossover near room temperature.
2020
The spin crossover salt [Fe(bpp) 2 ](isonicNO) 2 ·2.4H 2 O ( 1 ·2.4H 2 O) (bpp = 2,6‐bis(pyrazol‐3‐yl)pyridine; isonicNO = isonicotinate N‐oxide anion) exhibits a very abrupt spin crossover at T 1 /2 = 274.4 K. This triggers a supramolecular linkage (H‐bond) isomerization that responds reversibly towards light irradiation or temperature change. Isotopic effects in the thermomagnetic behaviour reveal the importance of hydrogen bonds in defining the magnetic state. Further, the title compound can be reversibly dehydrated to afford 1 , a material that also exhibits spin crossover coupled to H‐bond isomerization, leading to strong kinetic effects in the thermomagnetic properties.
Enantioselective Strecker Reaction Catalyzed by an Organocatalyst Lacking a Hydrogen-Bond-Donor Function
2007
Eigenschaften isomerer Methoxylderivate der phenolischen Mehrkernverbindungen
1971
Es wurden 21 Mehrkernverbindungen hergestellt, deren phenolische Hydroxylgruppen teilweise oder vollstandig methyliert waren. Mit steigendem Methylierungsgrad fallen die Schmelzpunkte der Ather mit einer Ausnahme regelmasig ab. Die molaren dekadischen Extinktionskoeffizienten der Maxima in den UV-Spektren der Ather werden, verglichen mit den Stammverbindungen, mit zunehmendem Methylierungsgrad kleiner. Die zunehmende Methylierung ist im Vergleich mit konstantbleibenden Banden der IR-Spektren an ab- und zunehmenden Banden zu verfolgen. Isomere (metamere) Methylather mit noch freien phenolischen Hydroxylgruppen unterscheiden sich im Schmelzpunkt, in ihren UV- und IR-Spektren. Die Unterschiede…
Structure and Stability of Racemic and Enantiopure Pimobendan Monohydrates: On the Phenomenon of Unusually High Stability
2017
Study of structures and physicochemical properties of racemic (rac-H) and enantiopure (enant-H) hydrates of the active pharmaceutical ingredient pimobendan revealed that both hydrates have highly similar crystal structures and exhibit unusually high stability. Both structures contain identical two-dimensional layers and very similar conformations. The most significant difference is the stacking of these layers. The high stability of both hydrates appeared as extremely low solubility over a wide temperature range as well as an exceptionally high dehydration temperature and melting point. Study of the dehydration process showed that both hydrates have different activation energies of dehydrat…
1-[(1-ethoxypropylidene)amino]-2-ethyl-4-(4-hydroxybenzyl)imidazol-5(4H)-one.
2012
The racemic title compound, C17H23N3O3, isolated from the reaction of L-(−)-tyrosine hydrazide with triethyl orthopropionate in the presence of a catalytic quantity ofp-toluenesulfonic acid (p-TsOH), crystallizes withZ′ = 1 in a centrosymmetric monoclinic unit cell. The molecule contains two planar fragments,viz.the benzene and imidazole rings, linked by two C—C single bonds. The dihedral angle between the two planes is 59.54 (5)° and the molecule adopts a synclinal conformation. The HOMA (harmonic oscillator model of aromaticity) index, calculated for the benzene ring, demonstrates no substantial interaction between the two π-electron delocalization regions in the molecule. In the crystal …
Tris(N,N,N′,N′‐tetramethylguanidinium) nonabromodiantimonate(III)
2006
In the title compound, [NH2C(N(CH3)2)2]3[Sb2Br9], the organic cations interact with the isolated [Sb2Br9]3− anions by way of N—H⋯Br hydrogen bonds, leading to some deformations of the inorganic unit.
syn-1,6-imino-8,13-oxido[14]annulene
1990
Abstract syn -1,6-Imino-8,13-oxido[14]annulene (1) , an aromatic annulene featuring closely spaced NH and O-bridges, has been synthesized. Contrary to expectation, NH ⋯ O hydrogen bonding in I does not profit from the proximity of the functionalities noticeably.
Hydrogen-bonded 2-(2-pyridyl)imidazole (pyim) manganese(II) complexes as building blocks for molecular assembling: Syntheses and structural character…
2008
Abstract The preparation and crystal structures of three new manganese(II) complexes with 2-(2-pyridyl)imidazole (pyim) and pseudohalides as ligands, cis-[Mn(pyim)2(N3)2] (1), cis-[Mn(pyim)2(NCO)2] (2) and cis-[Mn(pyim)2(NCS)2] (3), are reported. The structural building block in each structure is a mononuclear, neutral complex unit. The metal ion in each case adopts a distorted octahedral coordination geometry with the two pseudohalide ligands in cis positions. The main differences between the molecular structures of the three compounds are associated with the orientation of the anionic ligands. Hydrogen bonds between imidazole N–H and the metal-coordinated nitrogen atom of the pseudohalide…
Exchange interactions through hydrogen-bond bridges. Crystal structure, spectroscopic characterisation and magnetic properties of the complex [{Cu(en…
1998
Abstract The structure and spectroscopic and magnetic properties of [{Cu(en)}2(μ-egta)]·4H2O (H4egta = 3,12-bis(carboxymethyl)-6–9-dio 3,12-diazatetradecanedioic acid) are described. The compound is built of centrosymmetric neutral dimeric {[Cu(en)}2(μ-egta)] entities linked through hydrogen bonds involving water molecules. -NH2 groups and -CO2 groups, affording a three-dimensional framework. The egta anion displays an opened-up structure acting as a bridging ligand between two copper atoms. The coordination geometry about the metal atoms can be viewed as 4 + 1 + 1 (CuN3OO′O″ chromophores). The EPR spectra are indicative of an essentially dv−v ground state for the copper(II) ions. Magnetic …