Infrared matrix-isolation study of the molecular complexes between acetonitrile and hydrogen halides. Evidence for reactivity differences within the halide series

Abstract

I.r. spectra are reported of cocondensed mixtures of H(D)X and Y and of CH₃CN and Y (X = Cl, Br or I, Y = Ar or N₂ according to the slow-spray technique at 17 K. Identification of the complexes was mainly performed through the observation of the HX submolecule stretching mode, ν_s. For the 1 : 1 species, which is predominant at high dilution in both dopants, the ν_s frequencies are (cm^–1): X = Cl, 2659 (Ar), 2567 and 2621 (N₂); X = Br, 2393 (Ar), 2304 and 2370 (N₂); X = I, ca. 2170 (Ar) and 2142 (N₂). For X = I in solid nitrogen photodissociation of the complex is observed. This process occurs with a characteristic time of ca. 2 min at 11 K in the beam of the i.r. spectrophotometer; conversely the complex can be regenerated by warming the matrix above 15 K. These results are compared with those involving H₂O as proton donor; in spite of a rather large difference in proton affinity between these two bases, the HX perturbations are remarkably close to each other. Other aggregates have been identified for X = Cl and Br, with very different spectral and structural properties according to the matrix. In argon, the spectral properties of the 2 : 1 complexes (HX)₂CH₃CN are consistent with the sequence CH₃CN–HX–HX, while in nitrogen they are better interpreted in the case of HBr as a double attack on the CN bond, with one acid molecule along and the other perpendicular to its axis. This interpretation allows one to explain the presence of two groups of bands corresponding to ν_s for the 1 : 1 HCl–CH₃CN and HBr–CH₃CN complexes trapped in nitrogen as arising from two structures, one linear and the other T-shaped. Finally, complexes involving two acetonitrile molecules have been identified by their HX stretching frequencies, which are lower than those of the 1 : 1 complexes.