Photoelectron imaging of cyanovinylidene and cyanoacetylene anions

Negative ions of cyanoacetylene and cyanovinylidene are generated simultaneously via the competing 1,1H₂⁺ and 1,2-H ₂+ abstraction channels of O- reaction with acrylonitrile. The two stable isomeric forms of the anion, CCHCN- and HCCCN-, are separated by a large (∼2 eV) potential energy barrier. Their photodetachment provides access to both the reactant and the product sides of the neutral cyanovinylidene -* cyanoacetylene rearrangement reaction, predicted to involve only a very small barrier. Using photoelectron imaging spectroscopy at 532 and 355 nm, the adiabatic electron affinity of the reactive intermediate :C=CHCN (X ¹A'), is determined to be 1.84 ± 0.01 eV. The photoelectron spectrum of CCIICN-exhibits a vibrational progression attributed to the excitation of the CCII bending mode. The observed spectral features are reproduced reasonably well using a Franck-Condon simulation under the parallel-mode approximation. In contrast to unsubstituted acetylene, cyanoacetylene has a stable anionic state, which is adiabatically weakly bound, but has an experimentally determined vertical detachment energy of 1.04 ± 0.05 eV. This measurement, along with the broad, structureless photoelectron spectrum, of HCCCN-(with no identifiable origin), reflects the large geometry difference between the w-shaped structure of the anion and the linear equilibrium, geometry of HCCCN.