Stoichiometrical hydrogen-bonding

Tetraalkylammonium fluoride (R4N 1 F ) is well known as being highly receptive to protic compounds such as hydrogen halides and water, affording non-stoichiometric hydrogen-bonded adducts, R N F (HY) , in non-polar solvents. This property reasonably accounts for the hygroscopic nature of ammonium fluorides. However, under strictly anhydrous conditions, intramolecular interactions are predominant and result in self-destruction of the tetraalkylammonium cation via Hoffman elimination to furnish tetraalkylammonium bifluoride, trialkylamine, and olefin (Scheme 9.12) [22]. Consequently, the resulting tetraalkylammonium bifluoride,... 

FTIR measurements and ab initio calculations show that iV-methylpyrrole interacts with hexafluoroisopropanol, trifluoroethanol, 2-chloroethanol, and 1-butanol to form 1 1 stoichiometric hydrogen-bonded complexes in which the OH group acts as H-donor and the aromatic 7t-system as acceptor <2003CPH(290)69>. 

However, more recently, the application of fluorescence techniques has attracted attention. Anufrieva et al. applied the polarized luminescence method to the stoichiometric hydrogen bond... 

The differential heats of adsorption of acetone adsorbed on H-ZSM-5 (at 360 K) and silicahte (at 350 K) over a wide range of surface coverage were reported by Sepa et al. [76]. The results were compared with ab-initio calculations of the reaction of acetone with model zeolite structures to form a stoichiometric hydrogen-bonded cluster-molecule complex [76]. The differential heats of acetone adsorption on H-ZSM-5 were approximatively constant around 130 kJ mol up to a coverage of one molecule per Al, after which the heats dropped to ca. 105kjmol [71], while on silicalite the heats of adsorption were constant over the entire range examined and equal to ca. 67 kJ mor. ... 

Fig. 13. SAXS patterns as a function of temperature for stoichiometric supermolecuies consisting of poly(4-vinyl iodine) stoichiometrically hydrogen-bonded to pentadecylphenol. Adapted from J. Ruokolainen, M. Ibrlckeli, R. Serimaa, E. B. Komanschek, G. ten Brinke, and O. Ikkala, Macromolecules 30,2002 (1997).

Another difficulty is that the extent to which hydrogen bonded association and ion-pairing influence the observed kinetics has yet to be determined. However the high order of the reaction in the stoichiometric concentration of nitric acid would seem to preclude a transition state composed only of a nitronium ion and an aromatic molecule. 

Gas hydrates are non-stoichiometric crystals formed by the enclosure of molecules like methane, carbon dioxide and hydrogen sulfide inside cages formed by hydrogen-bonded water molecules. There are more than 100 compounds (guests) that can combine with water (host) and form hydrates. Formation of gas hydrates is a problem in oil and gas operations because it causes plugging of the pipelines and other facilities. On the other hand natural methane hydrate exists in vast quantities in the earth s crust and is regarded as a future energy resource. 

The aromatic spacer group of the model receptors prevent the formation of intramolecular hydrogen bonds between the opposing carboxyls yet these functions are ideally positioned for intermolecular hydrogen bonds of the sort indicated in 32. The acridine derivatives do indeed form stoichiometric complexes with oxalic, malonic (and C-substituted malonic acids) as well as maleic and phthalic acids, Fumaric, succinic or glutaric acids did not form such complexes. Though protonation appears to be a necessary element in the recognition of these diacids, the receptor has more to... 

Not surprisingly, the diacid 13 and its diamide are waterlogged with 2-4 molecules of HzO from which they are difficult to liberate. Binding experiments in CHC13, a non-competing solvent, revealed that stoichiometric complexes, e.g. 48 were formed with diketopiperazines 40) (Kh 104) and amides such as malonamide. With structures of inadequate hydrogen bonding capacity, such as sarcosine anhydride, com-plexation does not occur. 

The CO reductions generally could likely proceed through formyl intermediates, probably at a multinuclear site (420) hydride migration to a coordinated CO [e.g., as in the hypothetical scheme outlined in Eq. (72)] has not yet been observed, although metal formyl complexes have been synthesized via other methods (422-425). A ir-bonded formyl also seems plausible (426), since 7r-bonded acyl groups have been demonstrated (427). A stoichiometric hydrogen reduction of CO to methanol under mild conditions via a bis(pentamethylcyclopentadienyl)zirconium complex is considered to go through a formyl intermediate (428, 429) ... 

The formation of metal-oxygen bonds has previously been found to occur for the stoichiometric hydrogenation of CO to methanol with metal hydrides of the early transition metals (20). Moreover, in ruthenium-phosphine catalyzed hydrogenation (with H2) of aldehydes and ketones, metal-oxygen bonded catalytic intermediates have been proposed for the catalytic cycle and in one case isolated (21,22). 

Several systems have been reported involving stoichiometric hydrogenation of ketones or aldehydes by metal hydrides in the presence of acids. An ionic hydrogenation mechanism accounts for most of these hydrogenations, though in some examples alternative mechanisms involving the insertion of a ketone into a M-H bond are also plausible. 

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