Knowledge Bank

The matrix isolation technique combined with infrared spectroscopy has been employed to isolate and characterize hydrogen bonded complexes between alkynes and a number of bases containing N, P, O and S donors. Distinct evidence for hydrogen bond formation was observed for each case, with a characteristic red shift of C-H stretching motion vs. These shifts, in the range of $30-300cm-1$, were distinct and variable with the different alkynes and bases employed. The perturbed $C\equiv C$ stretching vibration was also observed for most complexes, as was the alkynic hydrogen bending mode. In general, for any base, linear correlations were noted between the magnitude of shift of $\nu 5$ (hence the strength of interaction), the gas phase acidity of the alkynes and the Hammett substituent constants of the alkynes. For a given acid, the $\Delta \nu 2$ varied in the order $Me3N>Me3P>Me2O>Me2S;NH3>PH3>H2S$. The degree of hydrogen bonding shown by a C-H bond was linearly dependent on the reaction partner (or bases) as well as the substituent groups on the alkyne. Lack of correlation of $\Delta vs$, with either $\sigma 1$ or $\sigma R$ alone suggests that both inductive and resonance contributions to the strength of the hydrogen bonding interaction are important.