Pyridine-chloroform, hydrogen bonding

Shindo studied the hydrogen-bonding ability of a fairly long series of substituted pyridine 1-oxides with methanol in chloroform solution and found that the OH frequency of the hydrogen-bonded OH group in methanol is well correlated with the <7-values. For four compounds, the intensity of the same band is also well correlated. In a similar study the OH frequencies of phenol vary monotonically with the CT-values, but not in a linear fashion. 

In a similar manner, intermolecular hydrogen bonds will cause a deshielding, as is found for chloroform in pyridine (Table 3.4). If the hydrogen bonds are broken, e.g. by solvation in dilute solutions with nonpolar solvents (e.g. carbon tetrachloride or cyclohexane), upheld shifts have to be expected (Table 3.4). 

One-bond coupling constants JCH may suffer from slight solvent effects. Table 3.4 shows this behavior for chloroform, whose carbon-proton coupling increases with the polarity of the medium when measured in different solvents, being 208 Hz in cyclohexane and 215 Hz in pyridine [92]. This is attributed to association between chloroform and solvents susceptible to hydrogen bonding. 

Treatment of (3-methylglutaconic anhydride with MgCh/pyridine followed by the addition of methyl chloroformate gives methyl 6-hydroxy-4-methyl-2-oxopyran 5-carboxylate 31, stabilised in this enol form by hydrogen bonding. The crystal structure of this enol of an... 

A combination of two different supramolecular forces, namely metal-metal bonds and the ureidopyrimidones were investigated by Schubert et al. [136,137] (Fig. 35). A poly(g-caprolactame) was prepared—with a ter-pyridine moiety at one end and an ureidopyrimidone unit at the other end—via tin-octanoate catalyzed polymerization. Together with iron(ll) ions, double supramolecular polymers formed from chloroform solution. Again, solid-like behavior was observed, similar to results for the purely hydrogen-bonded polymers. 

A hydrogen-bond acidity scale has been constructed by Frange and coworkers and by Sraidi based on log K values for complexation with pyridine N-oxide in cyclohexane (equations 22 and 23). Values for phenols and, for comparison, thiols, chloroform, pyrrole and alcohols are collected in Table 5. There is a fair measure of agreement between log K and log Kf n = 16, r = 0.992) and log (n=9,r = 0.972). 

N.m.r. studies of hydrogen-bonded complex formation include work on the systems chloroform—pyridine, phenol-base (bases... 

An ion pair, associated through a hydrogen bond, is formed by the reaction of protonic acid with bases in non-dissociating solvents. Thus, in chloroform solutions, acetic acid and pyridine give... 

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