Vibronic Spectroscopy of a Nitrile/Isonitrile Isoelectronic Pair: <i>para</i>-Diisocyanobenzene and <i>para</i>-Isocyanobenzonitrile

The ultraviolet spectroscopy of isoelectronic pair <i>para</i>-diisocyanobenzene (<i>p</i>DIB) and <i>para</i>-isocyanobenzonitrile (<i>p</i>IBN) has been studied under gas-phase, jet-cooled conditions. These molecules complete a sequence of mono and disubstituted nitrile/isonitrile benzene derivatives, enabling a comparison of the electronic effects of such substitution. Utilizing laser-induced fluorescence (LIF) and resonant two-photon ionization (R2PI) spectroscopy, the S₀–S₁ electronic origins of <i>p</i>DIB and <i>p</i>IBN have been identified at 35 566 and 35 443 cm^–1, respectively. In <i>p</i>DIB, the S₀–S₁ origin is very weak, with b_3g fundamentals induced by vibronic coupling to the S₂ state dominating the spectrum at 501 cm^–1 (ν₁₇, isocyano bend) and 650 cm^–1 (ν₁₆, ring distortion). The spectrum extends over 5000 cm^–1, remaining sharp and relatively uncongested over much of this range. Dispersed fluorescence (DFL) spectra confirm the dominating role played by vibronic coupling and identify Franck–Condon active ring modes built off the vibronically-induced bands. In <i>p</i>DIB, the S₂ state has been tentatively observed at about 6100 cm^–1 above the S₀–S₁ origin. In <i>p</i>IBN, the S₀–S₁ origin is considerably stronger, but vibronic coupling still plays an important role, involving fundamentals of b₂ symmetry. The bending mode of the nitrile group dominates the vibronically-induced activity. Calculations carried out at the TD-DFT B3LYP/6-31+G­(d) level of theory account for the extremely weak S₀–S₁ oscillator strength of <i>p</i>DIB and the larger intensity of the S₀–S₁ origins of <i>p</i>IBN and <i>p</i>DCB (<i>para</i>-dicyanobenzene) as nitrile groups are substituted for isonitrile groups. In <i>p</i>DIB, a nearly perfect cancellation of transition dipoles occurs due to two one-electron transitions that contribute nearly equally to the S₀–S₁ transition. The spectra of both molecules show no clear evidence of charge-transfer interactions that play such an important role in some cyanobenzene derivatives