Electronic properties and vibrationally averaged structures of X̃ ²Σ⁺ MgOH: a computational molecular spectroscopy...

For X̃ ²Σ⁺ MgOH, we have calculated the 3D potential energy surface (PES) at the MR-SDCI+Q/[cc-pCV5Z (Mg), aug-cc-pV5Z (O), cc-pV5Z (H)] level and derived the vibrational properties from there using the discrete variable representation (DVR) method. The PES minimum is at the linear structure; hence, MgOH is a "linear molecule." The 3D PES is shallow, and MgOH tends to bend in the region immediately when either or both Mg-O and O-H bonds become longer than those of the equilibrium structure (r_e(Mg-O) = 1.7614 Å, r_e(O-H) = 0.9453 Å, and ∠_e(Mg-O-H) = 180°). The zero-point structure, determined as the expectation values over the DVR3D wavefunctions, has 〈r(Mg-O)〉₀ = 1.7837 Å and 〈r(O-H)〉₀ = 0.9948 Å, and the deviation angle from linearity 〈〉₀ = 26.4°. The harmonic frequencies ω_e for the Mg-O stretching, bending, and O-H stretching modes are 768, 142, and 4060 cm^-1, respectively, and the corresponding term values ν₁, ν₂, and ν₃ are 752, 156, and 3867 cm^-1. All the vibrational behaviors, such as quasi-linear features, unusual relationship ν₂ > ω₂, a large amplitude bending motion, etc., are elucidated in terms of the ab initio electronic wavefunctions and the DVR3D vibrational wavefunctions. We have another piece of evidence to support our postulation that a linear molecule is observed as being bent.