INFRARED SPECTRAL EVIDENCE OF NC-CC-NC - PHOTOISOMERIZATION OF NC-CC-CN IN AN ARGON MATRIX

Ultraviolet (UV) laser photolysis of dicyanoacetylene, N=C-C=C-C=N, isolated in an argon matrix at 16 K produces the iso-nitrile, N=C-C=C-N=C, in sufficient concentration for direct Fourier-transform-infrared measurements of all five stretching vibrational fundamentals. The assignments for nu1 (2287.1 cm-1), nu2 (2203.6 cm-1), nu3 (2044.8 cm- 1), nu4 (1202.3 cm-1), and nu5 (610.1 cm-1) are supported by a normal coordinate analysis using 20 vibrational frequencies from 7 isotopomers and by ab initio results of Botschwina et al. The di-isonitrile C=N-C=C-N=C is also produced in small amounts and its infrared-active asymmetric stretches assigned as nu4 (2114.9 cm-1) and nu5 (1287.5 cm-1). As isotopic vibrational frequencies for N=C-C=C-C=N were measured for the first time, the disputed assignments for the three totally symmetric stretching vibrations were re-examined through a normal coordinate analysis. Laser-induced fluorescence measured by a Fourier-transform-spectrometer during photolysis reveals the 1SIGMA(g)+ <-- 3SIGMA(u)+ electronic transition in dicyanoacetylene at T00 = 25 607 cm-1 and four vibronic transitions involving two totally symmetric stretching states of the ground electronic state.