Simple unsaturated and cyclopropylic isocyanides are synthesized by an efficient and simple approach. These compounds with gradually increasing distance between the unsaturated moiety and the isonitrile group are studied by UV photoelectron spectroscopy and quantum chemical calculations, and also compared to the corresponding nitriles. The first photoelectron band of the unsaturated compounds is linked to removal of an electron from the HOMO, which corresponds to CC multiple-bond ionization in antibonding interaction with the π-isocyanide bond (in the same plane) for conjugated systems, or in antibonding interaction with the pseudo-π-CH2 group for isolated systems. For the 1-ethenyl derivatives, both cyano and isocyano groups act as a π-electron acceptor from the vinyl group, but the isocyano π system is much more strongly destabilized (ionization energies (IEs) shift to smaller values) by vinyl (3.12 eV) than the cyano π system is (2.70 eV). In comparison with the 1-ethynyl derivatives, a less pronounced destabilization (2.69 eV) of πNC by the ethynyl system (1.86 eV for πCN), and nearly the same order of magnitude of the energetic gap between the total antibonding (πCC−πNC) and the total bonding (πCC+πNC) IEs for ethenyl and ethynyl compounds are noted. The huge values of these last-named data for H2CCHNC (3.85 eV) and for HCCNC (4.04 eV) reflect the strong interaction between the unsaturated carbon–carbon moiety and the isocyanide group, and thus more efficient conjugation than for the corresponding nitriles.
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