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Co-crystals of 2,4,6-Trihydroxybenzoic Acid with Aromatic Diazaheterocycles – Crystallographic Studies

100%

Jankowski W. , Kadzewski A. , Gdaniec M.

2007

tom Vol. 81, nr 5-6

1095-1108

Binary co-crystals of 2,4,6-trihydroxybenzoic acid with pyrazine (1:1), quinoxaline (1:1) and phenazine (1:2) were prepared and their structure determined by X-ray crystallography. In the complex with pyrazine the carboxylic group of the acid is in the syn configuration and is connected via strong hydrogen bond to the N atom of the heterocyclic base. In the complex with phenazine the carboxylic group adopts the anti form and the acid molecule binds, via the phenolic groups, to two molecules of the base, in a manner typical for interaction of resorcinol with azaheterocycles. In the complex with quinoxaline the asymmetric part of the unit cell consists of two pairs of acid-base hydrogen- bonded units. In the first pair, 2,4,6-trihydroxybenzoic acid binds to quinoxaline via the carboxylic group, which is in the syn form. In the second pair, proton is transferred from the carboxylic group to the heterocyclic base, and the ions join via N-H+źźźO–(carboxylate) interaction. In both units acidic hydrogen atom is significantly shifted towards the center of hydrogen bond. Hydrogen bonds between the carboxyl group and the pyrazine and quinoxaline N-atoms are linear and short [OźźźN distances in the range 2.570(2)–2.630(2) capital A, ring].

Molecular recognition motifs in cytidinium and 2'-deoxycytidinium salts with composite anions

100%

Gilski M. , Jaskolski M.

nr 4

In the crystal structures of N3-protonated cytidinium and 2'-deoxycytidinium salts with composite XYn anions capable of accepting hydrogen bonds through their Y atoms, the dominating motif of cytosinium...anion interactions consists of a pair of hydrogen bonds donated from the N3+-H protonation site and from the exoamino N4-H41 group cis to N3, and accepted by two Y centers of one anion. This multi-point recognition pattern is stable and robust and thus can be classified as a supramolecular synthon. In a broader group of N3-protonated, N1-substituted cytosinium salts with composite anions it occurs with 70% frequency. The C5 side of the cytosine ring mimics the N3+-H type synthon and shows a propensity to form an analogous motif in which a C5-H5...Y hydrogen bond replaces the strong N3+-H...Y interaction. Since the C-H...Y bond is much weaker, the secondary motif shows higher deformability and is less frequent (44%).