Infrared spectral evidence of N≡C–C≡C–N≡C: Photoisomerization of N≡C–C≡C–C≡N 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 ν1 (2287.1 cm−1), ν2 (2203.6 cm−1), ν3 (2044.8 cm−1), ν4 (1202.3 cm−1), and ν5 (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 ν4 (2114.9 cm−1) and ν5 (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 1Σg+←3Σu+ 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.