The molecules MnO, MnO2, MnO3, and MnO4 have been prepared by the vaporization and reaction of manganese atoms with O2, N2O, or O3 and isolated in various inert-gas matrices at 4 °K. ESR has been used to determine magnetic parameters which are interpreted in terms of molecular geometry and electronic structure. MnO is confirmed to have a σπ2δ2, 6Σ+ ground state with g⊥=1.990(7) (assuming g∥=ge) and a zero-field splitting in accord with the gas phase value ‖D‖=1.32 cm−1. Hyperfine splittings due to the 55Mn (I=5/2) nucleus are ‖A∥‖=176(8) and ‖A⊥‖=440(11) MHz. MnO2 is a linear 4Σ− molecule with probable configuration σδ2, ‖D‖=1.13 cm−1 (assuming g∥=g⊥=2.0023), ‖A∥‖=353(11), ‖A⊥‖=731(11) MHz. MnO3 exhibits very large hf splittings ‖A∥‖=1772(3) and ‖A⊥‖=1532(3) MHz indicative of a sdz2 hybrid 2A1 ground state of D3h symmetry. The spectrum of MnO4 is consistent with a C3v molecule distorted from a 2T1 electronic state in tetrahedral symmetry by a static Jahn–Teller effect. g and A tensors are slightly anisotropic: g∥=2.0108(8), g⊥=2.0097(8), ‖A∥‖=252(3), ‖A⊥‖=196(3) MHz. The electron hole is almost entirely in an oxygen π-bonded orbital with one oxygen atom displaced along its Mn–O bond axis.
MnH and MnH2 molecules, and their deuterated counterparts, have been trapped in argon and neon matrices at 4 °K and observed via infrared, visible, and electron-spin-resonance spectrscopy. The data for MnH support the gas-phase 7Σ ground-state assignment and yield the magnetic parameters ‖A (H) ‖?20, ‖A∥(Mn) ‖=322(6), ‖A⊥(Mn) ‖=299(2) MHz, g⊥=2.001(1) (assuming g∥ =ge), D=−0.002(1) cm−1. The derived MO description is in essential agreement with the ab initio calculations of Bagus and Schaefer. Infrared data indicate that MnH2 is bent at a bond angle of 117±30°, and stretching force constants are derived. ESR spectra variations with the matrix used and with isotopic substitution indicate motional effects in some matrices. It is concluded that the ground state is 6A1 with ‖D‖=0.20(2) cm−1 and with the probable hyperfine parameters ‖A (H) ‖=36, ‖Axy(Mn) ‖ =73 MHz, where xy is an average axis perpendicular to the H–H direction. The bent molecule is justified by Walsh-type theory applied to transition-metal dihydrides. There are indications that the MnH3 molecule may have also been observed.
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