Coupled-cluster electronic spectra for the Ca+–acetylene π complex and comparisons to its alkaline earth analogs

Abstract

Equation of motion coupled-cluster (EOM-CCSD) predictions of structures and electronic excitation energies for the recently detected Ca+–acetylene π-complex confirm three experimental state assignments, but suggest reinterpretation of the signals associated with the (2) B12 and (2) B22 states that correlate to the P2←2S Ca+ atomic transition. The originally assigned 000 band for the (2) B12 state corresponds to the computed excitation energy to the (2) B22 state and simple reassignment is proposed. The true (2) B12 state was not assigned in the original spectrum. However, the computed oscillator strength is large and its optimized geometry is similar to that of the ground state. Furthermore, the experimental band tentatively attributed to the onset of the symmetric C–H stretching progression of the assigned state has a relative energy conspicuously close to the computed electronic energy for the unassigned (2) B12 state. Based on the computed energy separations of the optimized EOM-CCSD structures, reassignment of this vibronic band to the 000 line of the (2) B12 state is proposed. The newly assigned bands are also compared to the analogous transitions in the beryllium–and magnesium–acetylene π complexes.