Ro-vibrational properties of FeCO in the (X)over-tilde3Σ- and (a)over-tilde5Σ- electronic states: A computational molecular spectroscopy study

The present work complements our previous study of the geometry and electronic structure in the ground and low-lying electronic states of FeCO. Here, we report three-dimensional potential energy surfaces (PESs) for the (3)Sigma(-) electronic ground state and its high-spin counterpart, the excited state a(5)Sigma(-), calculated ab initio at the MR-SDCI+Q_DK3/[5ZP ANO-RCC (Fe, C, O)] level of theory. These PESs are employed in 2nd-order-perturbation-theory and DVR3D calculations of the rotation-vibration energies and ro-vibrationally averaged structures. The equilibrium structures determined from the 3D PESs have r(e)(Fe-C) = 1.7247 angstrom, r(e)(C-O) = 1.1587 angstrom, and angle(e)(Fe-C-O) = 180 degrees for the (X) over tilde (3)Sigma(-) state, and r(e)(Fe-C) = 1.8429 angstrom, r(e)(C-O) = 1.1522 angstrom, and angle(e)(Fe-C-O) = 180 degrees for the a(5)Sigma(-) state. The ro-vibrationally averaged structures, determined as expectation values over DVR3D wavefunctions, have < r(Fe-C)>(0) = 1.7303 angstrom, (r(C-O))0 = 1.1631 angstrom, and (angle(Fe-C-O))(0) = 172.6 degrees for the (X) over tilde (3)Sigma(-) state, and < r(Fe-C)>(0) = 1.8471 angstrom, (r(C-O))(0) = 1.1568 angstrom, and (angle(Fe-C-O))(0) = 171.4 degrees for the (a) over tilde (5)Sigma(-) state. The coordinate-covalent Fe-C bond in the (X) over tilde (3)Sigma(-) state, which elongates significantly as the molecule bends, is shown to exhibit normal large amplitude bending motion with strong coupling (manifested by the large value of the relevant third order force constant) between bending and Fe-C stretching modes. The ionic Fe-C bond in the (a) over tilde (5)Sigma(-) state shows anormal bending behavior due to a severe Fermi resonance which also gives rise to a large coupling between the bending and the Fe-C stretching motions, even though the corresponding third order force constant is small. The Yamada-Winnewisser quasi-linearity parameter gamma(0) is calculated to be -1.00 and -0.90, values characteristic for a linear molecule, for the (X) over tilde (3)Sigma(-) and (a) over tilde (5)Sigma(-) states, respectively. The ro-vibrationally averaged structures of the (X) over tilde (3)Sigma(-) state are discussed in detail and it is concluded that any triatomic, linear molecule will be observed as being bent on to-vibrational average. (C) 2015 Elsevier Inc. All rights reserved.