Electronic structure and spectroscopy of the ground and excited states of the HMgO and HMgS radicals

Ab initio RCCSD(T) and MRCI calculations have been carried out for the stable isomer forms of magnesium monohydroxide, HMgO and monohydrosulfide, HMgS. For both molecular systems, it is found that the electronic ground state is a 2Πi state with linear/linear Renner-Teller components. At the equilibrium geometry of the ground states, the spin-orbit constants ASO have been calculated -123 cm-1 for HMgO and -312 cm-1 for HMgS. Along the isomerisation path towards the more stable structures MgOH and MgSH, the 2A' component of the Renner-Teller pair correlates with the ground state and the 2A'' component with an excited state. Using RCCSD(T) potential energy functions and a variational Renner-Teller approach, rovibronic levels of the X2Π state of both systems have been evaluated for J=1/2 and 3/2. Strong Fermi resonances are found in HMgO, but not in HMgS. The first electronically excited 2Σ+ states are calculated (Te value at RCCSD(T) level) to lie 8041 cm-1 above the ground state for HMgS and 4935 cm-1 for HMgO