HCNH+: a new interstellar molecular ion.

A new molecular ion, HCNH+, or protonated HCN, has been detected in the interstellar medium. The J = 1-0, 2-1, and 3-2 rotational transitions of this species have been observed toward Sgr B2. Using a large velocity gradient model calculation, the column density of HCNH+ is found to be approximately 4 x 10(14) cm-2, about one order of magnitude less than that estimated for HCO+ and HCN in this source. Such a column density implies a fractional abundance for HCNH+ in Sgr B2 of approximately 3 x 10(-10), one to several orders of magnitude greater than the values predicted by ion-molecule models of interstellar chemistry. The high observed abundance of this species, relative to theoretical calculations, suggests that the destruction of HCNH+ by dissociative recombination is slower than expected, or that the formation rate of the ion has been underestimated.