Infrared matrix isolation study of hydrogen bonds involving C-H bonds : alkynes with bases containing second- and third-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{sup {minus}1}, with complexes of CF{sub 3}CCH 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(CH{sub 3}){sub 3} > (CH{sub 3}){sub 2}S > (CH{sub 3}){sub 2}Se > PH{sub 3} > H{sub 2}S > AsH{sub 3}. For the very weak bases H{sub 2}S and AsH{sub 3}, a second complex in which the alkyne serves as a proton acceptor and H{sub 2}S or AsH{sub 3} 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.