High-pressure Raman study of a one-dimensional hydrogen-bonded crystal of cyanoacetylene

At room temperature, cyanoacetylene undergoes a liquid–solid transition at 70±30 MPa and further a polymerization reaction at 1.5 GPa. Raman spectra indicates that the crystal structure of the high-pressure phase is isomorphic with the monoclinic low-temperature structure (P21/m, Z=2), in which linear cyanoacetylene molecules are joined with CH NC hydrogen bonds to form infinite one-dimensional chains. The internal stretching frequency of the C–H proton donor decreases wtih increasing pressure, while that of the C 3/4 N proton acceptor increases. The librational modes associated with the rotational motions of the rigid molecules show large shifts to higher frequencies with pressure. The observed frequency shifts are attributed to the strengthening of the CH⋅⋅⋅NC hydrogen bond in the one-dimensional molecular chain.