INFRARED MATRIX-ISOLATION STUDY OF HYDROGEN-BONDS INVOLVING C-H BONDS - ALKYNES WITH BASES CONTAINING 2ND-ROW AND 3RD-ROW DONOR ATOMS

The matrix isolation technique, combined with infrared spectroscopy, has been employed to isolate and characterize hydrogen-bonded complexes between 1-alkynes and a series of second- and third-row bases. Distinct evidence for hydrogen bond formation was observed in each case, with a characteristic shift to lower energies of the alkynic C-H stretching mode. These shifts ranged from 25 to 150 cm-1, with complexes of CF3CCH consistently having the largest shifts. For a given acid, the magnitude of the shift of the C-H stretching vibration, as a function of the base, varied in the order P(CH3)3 > (CH3)2S > (CH3)2Se > PH3 > H2S > AsH3. Some indication of perturbation of the triple bond stretching mode and the CCH bending mode of the alkyne was also observed. For the very weak bases H2S and AsH3, a second complex in which the alkyne serves as a proton acceptor and H2S or AsH3 serves as the proton donor was also detected. The shifts observed in this study were significantly smaller than observed in previous studies and have been rationalized in terms of hard/soft acid/base theory. © 1990 American Chemical Society.