Optical Spectra of an Isoelectronic Series; the π*←π Transition in Phenyl Isocyanide, Ethynylbenzene, and Benzonitrile

In this work we study the lowest‐lying singlet π^*←π transition in the isoelectronic series phenyl isocyanide, ethynylbenzene, and benzonitrile. The spectrum of phenyl isocyanide is analyzed for the first time and is found to be very similar to that of ethynylbenzene. The $0_0^0$ band is assigned at 2723 Å. Rotational analyses are reported on the $0_0^0$ and 2${35}_0^1$ bands in phenyl isocyanide. Changes found in rotational constants for the $0_0^0$ band are $\text{Δ A = −0.00789\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}\text{, Δ B = −0.00101\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}$ , and $\text{Δ C = −0.00101\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}$ . Origin of the electronic transition is set at 36 712.96 cm⁻¹. Dipole moment changes on excitation are determined for all three molecules by the Stark effect. For benzonitrile Δμ = 0.31 D, more than twice the change in the other two molecules. The differences in the over‐all spectrum and dipole moment change for benzonitrile compared to the other two molecules are explained in terms of the possible influence of intramolecular charge transfer.