Hydrogen bonding adducts

Although Lewis and Bronsted bases comprise the same species, the same is not true of their acids. Lewis acids include bare metal cations, while Bronsted-Lowry acids do not. Also, Bell (1973) and Day Selbin (1969) have pointed out that Bronsted or protonic acids fit awkwardly into the Lewis definition. Protonic acids cannot accept an electron pair as is required in the Lewis definition, and a typical Lewis protonic add appears to be an adduct between a base and the add (Luder, 1940 Kolthoff, 1944). Thus, a protonic acid can only be regarded as a Lewis add in the sense that its reaction with a base involves the transient formation of an unstable hydrogen bond adduct. For this reason, advocates of the Lewis theory have sometimes termed protonic adds secondary acids (Bell, 1973). This is an unfortunate term for the traditional adds. 

Triphenylsilanol also forms simple hydrogen-bonded adducts with ethers. For example, the crown ether 12-crown-4 forms a 2 1 adduct... 

Water adsorption on high silica zeolites. Formation of hydroxonium ions and hydrogen-bonded adducts... 

It was assumed that the hydrogen bond forms between the superoxo moiety and one of the OH-groups of the substrate. The rate-determining step is the redox decomposition of the hydrogen bonded adduct into hydroperoxocobaloxime and semiquinone. The latter participates in an equilibrium reaction with Co(II) ... 

The solid-state structure of ort o-carborane was determined for the first time in 1996 featuring HMPA as a weak hydrogen bond adduct in the solid state.569 The geometry of ort o-carborane, I T BjoH, is a distorted icosahedron with a bond of 1.62 A between the cage carbons (C1-C2). This bond is remarkably elastic being elongated up to 1.85 A... 

Detailed microwave spectroscopic analysis of gas-phase hydrogen-bonded adducts can yield the hydrogen-bond energy with precision, e.g. for HCN HF, (NHF) = 26.1 1.6kJmol-i (Legon et al 1980). Sadly, few systems are susceptible to this approach. 

A crystal structure of an unstable JV,JV -[bis-(d-tosylbenzyl)urea-acetone hydrogen-bonded adduct had been reported earlier Tel, R M. and Engberts, J.B.F.N. (1976)./. Chem. Soc. Perkin Trans. 2, 483 88. 

FIGURE 4. Visualization of (a) the hydrogen-bonded H2O2 dimer (IR, MP2) and (b) the predicted hydrogen-bonded adduct between the HO and H2O2 (UBSLYP) ... 

Where the lanthanide ionic radius and the macrocyclic cavity are incompatible, though in hydrous conditions the crown ether is likely to be displaced by water ligands, the crown may still be present in the structure of the crystal as a hydrogen-bonded adduct. This behaviour is seen in [Gd(N03)3(H20)3]-(18-crown-6).445 This type of compound is quite well known in the case of s block metals also, e.g. [Mg(H20)6]Cl2 (12-crown-4)454 and, a more subtle case, [Ca(nitrobenzoate)2(benzo-15-crown-5)]-3H20(benzo-15-crown-5)455 in which an apparent 2 1 complex has only half its crown ligand coordinated to Ca2+. 

Unlike the reaction of 1-ethoxysilatrane with CF3COOH, which results in the hydrogen-bonded adduct, 1-ethoxytribenzosilatrane is converted to disiloxane 168 under the same conditions (equation 173)27. 

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,... 

Figure 4. View of hydrogen-bonding adduct of /ran,v-[(NH )2Pt(9-EtA-/V7X9-McGH-iV7)l2 1 with 1-MeC. A water molecule [0(2w)] is also involved in hydrogen bonding. [Reprinted with permission...

The effect of protic additives was rationalized by the formation of hydrogen-bonded adducts, such as that between benzoic acid and the carbonyl group of the coordinated benzoate, as observed by 1H NMR. Such an adduct would be expected to facilitate the formation of a polar transition state. The reaction rate also increases upon modification of the ligand with anionic groups and in polar solvents. Radical initiators have no effect. 

In a hydrogen-bonded adduct of aqua-dichlorodiphenyltin(IV) (entry 3), the Ph2Sn(H20) CI2 unit exhibits a distorted TBP coordination geometry around tin425. The distortion is illustrated by the deviation of the axial O Sn—Cl bond angle (172.3°) from linearity. 

Hydrothermal Synthesis of Organic Channel Structures 1 1 Hydrogen-Bonded Adducts of Melamine with Cyanuric and Trithiocyanuric Acids... 

Ranganathan, A., Pedireddi, V. R. and Rao, C. N. R. (1999). Hydrothermal synthesis of organic channel structures 1 1 hydrogen-bonded adducts... 

The connection between energies and geometry must be empirical or indirect through theoretical hypotheses and calculations. This connection is further complicated by the other intermolecular interactions in crystals. The hydrogen-bond lengths observed in crystals, which provide the vast majority of the data available, are modulated by the crystal field effects and any one observation is therefore not necessarily representative of the potential energy minima of the isolated hydrogen-bonded adducts. 

Initially, a loosely bound hydrogen bonded adduct 10 is formed, from the carboxyhc acid 8 and the carbonyl compound 6. The next step is the formation of the a-adduct 11 by reaction of the isocyanide with the nucleophilic carboxylate and the electrophilic protonated carbonyl compound. a-Adduct 11 then rearranges to give oc-acyloxy amide 9. 

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