Cyano substituent effects on enol and enethiol acidity and basicity: The protonation and deprotonation of 3-hydroxy-2-propenenitrile and its thio analogue

The gas-phase basicity and acidity of 3-hydroxy-2-propenenitrile (3-hydroxyacrylonitrile) and its sulfur-containing analogue, 3-mercapto-2-propenenitrile, have been determined by means of high-level G3B3 ab initio calculations and, in the case of the latter compound, compared with the experimental values obtained by means of FT-ICR mass spectrometry techniques, and with previous reported values for the NC–CHCH–X (X = CH3, NH2, SiH3, PH2) analogues. For both compounds the Z-isomer is the dominant species in the gas-phase. Protonation takes place in both cases at the cyano group. The loss of the proton from the substituent, was found to be systematically much more favorable than the deprotonation at the HCCH group. 3-Hydroxy-2-propenenitrile is predicted to be a stronger base by ca. 5 kJ mol−1 than its thio analogue, but a weaker acid by 26 kJ mol−1. Both compounds are stronger acids than the corresponding unsubstituted vinyl compounds, because cyano substitution stabilizes much more the deprotonated species than the corresponding neutral compound. There is a clear disagreement between our theoretical estimates for both the gas-phase basicity and the gas-phase acidity of 3-mercapto-2-propenenitrile and the corresponding experimental values, which is consistent with its isomerization to yield isothiazole.