Matrix-isolated FeO,NiO, and CoO: Ground-state vibrational frequencies

The infrared vibrational spectra of the FeO, NiO, and CoO molecules have been observed in Ar matrices at 14 K. Frequencies for each of the isotopomers ⁵⁴Fe¹⁶O, ⁵⁴Fe¹⁸O, ⁵⁶Fe¹⁶O, ⁵⁶Fe¹⁸O, ⁵⁸Ni¹⁶O, ⁵⁸Ni¹⁸O, ⁶⁰Ni¹⁶O, ⁶⁰Ni¹⁸O, ⁵⁹Co¹⁶O, and ⁵⁹Co¹⁸O were measured. The derived vibrational constants are:

$\text{w}{}_{\mathrm{e}}\text{= 880.02 ± 1.2 and w}{}_{\mathrm{e}}\text{x}{}_{\mathrm{e}}\text{= 3.47 ± 0.6 cm}{}^{\mathrm{?1}}\text{for}{}^{56}\text{Fe}{}^{16}\text{O; w}{}_{\mathrm{e}}\text{= 837.61 ± 1.1 and w}{}_{\mathrm{e}}\text{x}{}_{\mathrm{e}}\text{= 5.92 ± 0.6 cm}{}^{\mathrm{?1}}\text{for}{}^{58}\text{Ni}{}^{16}\text{O; and w}{}_{\mathrm{e}}\text{= 853.75 ± 1.2 and w}{}_{\mathrm{e}}\text{x}{}_{\mathrm{e}}\text{= 3.65 ± 0.6 cm}{}^{\mathrm{?1}}\text{for}{}^{59}\text{Co}{}^{16}\text{O.}$

These results support the assignment of ⁵Σ⁺ as the ground state of FeO, ³Σ^? as the ground state of NiO, and ⁴Σ^? as the ground state of CoO.