The pure rotational spectrum of CrCN (X ⁶Σ⁺): an unexpected geometry and unusual spin interactions

Abstract

The pure rotational spectrum of CrCN (X ⁶Σ⁺) has been recorded in the frequency range 250–520 GHz using direct-absorption techniques. This is the first spectroscopic investigation of the CrCN radical. This species was synthesized by reacting Cr vapour with (CN)₂. Spectra were obtained for the main isotopic species, the ⁵³Cr and ¹³C isotopologues, and the heavy atom stretch and several quanta of the bending vibration. The molecule was found to have a linear cyanide geometry and a ⁶Σ⁺ ground state. Rotational, fine structure, and l-type doubling constants have been determined. The spin–spin parametre λ was found to be small–a likely result of competing second-order spin–orbit contributions from excited ⁴Σ⁻, ⁴Π, and ⁶Π states. However, λ increased significantly in the bending mode, which may be caused by a reduction of the ⁶Σ⁺−⁴Π interaction strength due to spin–orbit vibronic coupling. In contrast, the value of γ, arising from interactions with a nearby ⁶Πstate, was independent of v ₂. The CrCN bond lengths were determined to be r _Cr–C = 2.019 Å and r _C–N = 1.148 Å. Delocalization of the 3dπ electrons into the CN 2π* orbital, which is polarized towards the carbon atom, may account for the CrCN structure.