A Quantum-Chemical and Correlation Study of the Ionization of Purpurin

Anthraquinoid tautomers participate in the ionization of purpurin. The tautomerism takes place in both ground and excited states of the molecules; the excited state is more sensitive to the tautomerism. The solvation and ionization shift the tautomeric equilibria. In the experimental absorption spectra of purpurin, the major bands correspond to the 9,10-, 1,4-, and 1,10-anthraquinoid tautomers; anions with the 9,10-anthraquinoid structure are not detected. The position and intensity of the π,π* bands, and also the quantum-chemical parameters linearly correlate with the degree of ionization of purpurin.     

Structure activity relationship by NMR and by computer: a comparative study

There has recently been considerable interest in using NMR spectroscopy to identify ligand binding sites of macromolecules. In particular, a modular approach has been put forward by Fesik et al. (Shuker, S. B.; Hajduk, P. J.; Meadows, R. P.; Fesik, S. W. Science 1996, 274, 1531-1534) in which small ligands that bind to a particular target are identified in a first round of screening and subsequently linked together to form ligands of higher affinity. Similar strategies have also been proposed for in silico drug design, where the binding sites of small chemical groups are identified, and complete ligands are subsequently assembled from different groups that have favorable interactions with the macromolecular target. In this paper, we compare experimental and computational results on a selected target (FKBP12). The binding sites of three small ligands ((2S)1-acetylprolinemethylester, 1-formylpiperidine, 1-piperidinecarboxamide) in FKBP12 were identified independently by NMR and by computational methods. The subsequent comparison of the experimental and computational data showed that the computational method identified and ranked favorably ligand positions that satisfy the experimental NOE constraints.     

Kinetic models for the chemisorption of organic compounds on oxide surfaces

A kinetic equation for chemisorption on amorphous oxide surfaces is proposed, and activation energies, pre-exponential factors for reactions of various organosilicon compounds with Si, Ti and Al oxide active sites are calculated. Relationships between the kinetic parameters and reagent structure were established.     

Hydrogen‐bonded frameworks of bis(2‐carboxypyridinium) hexafluorosilicate and bis(2‐carboxyquinolinium) hexafluorosilicate dihydrate

In bis(2‐carboxypyridinium) hexafluorosilicate, 2C₆H₆NO₂⁺·SiF₆²⁻, (I), and bis(2‐carboxyquinolinium) hexafluorosilicate dihydrate, 2C₁₀H₈NO₂⁺·SiF₆²⁻·2H₂O, (II), the Si atoms of the anions reside on crystallographic centres of inversion. Primary inter‐ion interactions in (I) occur via strong N—H...F and O—H...F hydrogen bonds, generating corrugated layers incorporating [SiF₆]²⁻ anions as four‐connected net nodes and organic cations as simple links in between. In (II), a set of strong N—H...F, O—H...O and O—H...F hydrogen bonds, involving water molecules, gives a three‐dimensional heterocoordinated rutile‐like framework that integrates [SiF₆]²⁻ anions as six‐connected and water molecules as three‐connected nodes. The carboxyl groups of the cation are hydrogen bonded to the water molecule [O...O = 2.5533 (13) Å], while the N—H group supports direct bonding to the anion [N...F = 2.7061 (12) Å].     

Furan ring opening-furan ring closure: cascade rearrangement of novel 4-acetoxy-9-furylnaphtho[2,3-b]furans

The cascade rearrangement of novel 4-acetoxy-9-furylnaphtho[2,3-b]furans leading to tetracyclic naphthodifurans derivatives has been developed. The reaction proceeds via double recyclization of both furan rings of the initial molecule, one of the furan rings serving as a 1,3-dicarbonyl compound equivalent.     

A Watersoluble Sulfonatomethylated Calix[4]resorcinarene as Artificial Receptor of Metal Complexes

The binding of different sized and shaped metal complexes [Co(His)₂]ClO₄(1), [Co(en)₂C₂O₄]Cl (2) and [K18-crown-6]SCN (3)(en-ethylendiamine, His-L-histidynate-anion) with a new tetrasulfonatomethylcalix[4]resorcinarene([H₈X]Na4) was investigated in neutral and alkaline aqueous media by NMR and pH-metrictitration methods and compared with those of recently studied NMe₄Br (4). The resultsobtained indicate that the outer-sphere coordination of complexes 1–3 by[H₈X]^4- proceeds via the interaction of hydrophobic fragments of the guestswith both the negatively charged rim and the hydrophobic cavity as a -base. Thenature of binding does not change for cations 1, 2 and 4 on going from[H₈X]^4- in neutral to [H₄X]^8- in alkaline media, while the inclusionof 3 decreases on going from [H₈X]^4- to [H₄X]^8-.     

Quantum-chemical and correlation study of ionization of Alizarin

According to the results of quantum-chemical and correlation studies, ionized states of Alizarin and Alizarin Red are contributed mainly by tautomeric anthraquinoid structures. The probability of their formation increases due to solvation. The experimental electronic absorption spectra of the corresponding anions contain bands typical of the 2,9-, 1,10-, and 1,2-anthraquinoid tautomers. The 9,10-anthraquinoid structure of the anions is less probable.Translated from Zhurnal Obshchei Khimii, Vol. 74, No. 10, 2004, pp. 1681–1686.Original Russian Text Copyright © 2004 by Fain, Zaitsev, Ryabov.This revised version was published online in April 2005 with a corrected cover date.     

Effects of annulation on the mass spectral behaviour of monosubstituted bicyclo[4.3.0]-3-nonenes and bicyclo[4.3.0]-3,7-nonadienes

The mass spectra of the title compounds differ considerably depending on the annulation of the bicyclic skeleton. For a series of alkyl derivatives the cis-annulated compounds show a higher abundance of the products due to the retro Diels-Alder cleavage than the corresponding trans isomers. The cis-annulated esters and alcohols lose a molecule of ethanol and water, respectively, more easily than the corresponding trans isomers. Effects of conformation and configuration are discussed.     

Synthesis and characterization of a novel tantalum chalcogen-rich molecular cluster with square planar metal core

Black single crystals of Ta(4)Se(9)I(8) are obtained in a high yield by heating Ta, Se, and I(2) at 300 degrees C in 1:2.2:1.0 molar ratio. In the structure, the tantalum atoms form a square, with four Se(2) ligands bridging the Ta-Ta edges and one capping the square. Each Ta atom has two terminal iodine atoms. The compound is diamagnetic and has only two electrons for metal-metal bonding.     

Generation of C6H4+* by laser vaporization of magnesium with o-C6H4F2 in argon carrier gas

A route to efficient generation of C6H4+*, potentially the benzyne radical cation, is presented. Laser vaporization of Mg+* and supersonic expansion in helium doped with o-, m-, or p-C6H4F2 yields, among other ions, o-, m-, p-C6H4F2Mg+* complexes, but no C6H4+*. Collision-induced dissociation experiments show that the o-C6H4F2Mg+* complex can be converted into C6H4+* in a mildly energetic collision, with a center-of-mass energy around 1-2 eV. These conditions can also be reached in the ion source when argon is used as a carrier gas. In this way, mass spectra containing the desired m/z 76 peak, i.e. C6H4+*, are obtained.