| Summary: | We report here the results of a combined experimental and computational study of the negative ion photoelectron spectroscopy (NIPES) of the recently synthesized, planar, aromatic,HCPN₃⁻ ion. The adiabatic electron detachment energy of HCPN₃⁻ (electron affinity of HCPN₃•) was measured to be 3.555 ± 0.010 eV, a value that is intermediate between the electron detachment energies of the closely related (CH)₂N₃– and P₂N₃– ions. High level electronic structure calculations and Franck–Condon factor (FCF) simulations reveal that transitions from the ground state of the anion to two nearly degenerate, low-lying, electronic states, of the neutral HCPN₃• radical are responsible for the congested peaks at low binding energies in the NIPE spectrum. The best fit of the simulated NIPE spectrum to the experimental spectrum indicates that the ground state of HCPN₃• is a 5π-electron ²A″ π radical state, with a 6π-electron, ²A′, σ radical state being at most 1.0 kcal/mol higher in energy.
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