Electronic structures and rovibronically averaged geometries of the X 6Ai' and A 6Ai" states of FeOH.

We have recently reported a theoretical prediction of the rovibronic spectra of the FeOH molecule. These spectra have not been observed experimentally. In the present work, we complement the previously published information by reporting the details of the electronic structure of FeOH together with rovibrationally averaged structural parameters. The electronic ground state is X (6)A(i)', which is Renner-degenerate with the A (6)A(i)" state; the two states correlate with a (6)Delta state at linearity. We have calculated the three-dimensional potential energy surfaces (PESs) of the X and A states, which are close in energy over the range of geometries studied, at the MR-SDCI+Q+E(rel)/[Roos ANO (Fe), aug-cc-pVQZ (O, H)] level of theory. The equilibrium structure of the X state is bent with r(e)(Fe-O)=1.806 A, r(e)(O-H)=0.952 A, and angle(e)(Fe-O-H)=134.2 degrees. The barrier to linearity is 273 (266) cm(-1) in the X (A) state so that FeOH is quasilinear in the X and A states. The Fe-O bonds in both states are ionic and the bending potentials are shallow, resulting in large amplitude bending motion. The rovibrationally averaged structures of the X (6)A' and A (6)A" electronic states have been calculated for the average of the X and A PESs by the variational MORBID method as expectation values in terms of rotation-vibration wave functions. FeOH is said to be quasilinear, but the rovibrationally averaged structure is bent with (0)=1.805 A, (0)=0.967 A, and (0)=141(14) degrees (where the quantity in parentheses is the quantum mechanical uncertainty), which is close to the equilibrium structure. We demonstrate that by means of the Yamada-Winnewisser quasilinearity parameter we can distinguish linear and quasilinear molecules.