Millimeter-wave rotational spectroscopy of FeCN (X⁴Δ _i) and FeNC (X⁶Δ_i): Determining the lowest energy isomer

The pure rotational spectrum of FeCN has been recorded in the frequency range 140-500 GHz using millimetersub-millimeter direct absorption techniques. The species was created in an ac discharge of Fe(CO)₅ and cyanogen. Spectra of the ¹³C, ⁵⁴Fe, and ⁵⁷Fe isotopologues were also measured, confirming the linear cyanide structure of this free radical. Lines originating from several Renner-Teller components in the v₂ bending mode were also observed. Based on the observed spin-orbit pattern, the ground state of FeCN is ⁴Δ_i, with small lambda-doubling splittings apparent in the Ω = 5/2, 3/2, and 1/2 components. In addition, a much weaker spectrum of the lowest spin-orbit component of FeNC, Ω = 9/2, was recorded; these data are consistent with the rotational parameters of previous optical studies. The data for FeCN were fit with a Hunds case (a) Hamiltonian and rotational, spin-orbit, spin-spin, and lambda-doubling parameters were determined. Rotational constants were also established from a case (c) analysis for the other isotopologues, excited vibronic states, and for FeNC. The r₀ bond lengths of FeCN were determined to be r_Fe-C = 1.924 Å and r_C-N = 1.157 Å, in agreement with theoretical predictions for the ⁴Δ _i state. These measurements indicate that FeCN is the lower energy isomer and is more stable than FeNC by ∼1.9 kcalmol.