The structure and vibrational frequencies of the HCNN radical and the HCNN+ cation have been calculated using ab initio methods. Both the lowest triplet and lowest singlet states of the cation have been determined. In the (X) over tilde(3)Sigma(-) state the ion is linear, whereas it is bent in the (a) over tilde(1)A' state. Using a complete basis set method, the ionization energy for the process yielding the (X) over tilde(3)Sigma(-) State of the cation is calculated as 9.63 +/-0.04 eV. That for the process yielding the (a) over tilde(1)A' state of the cation is 10.33 +/- 0.04 eV.