Electronic structure and spectroscopy of ClSO+

Recent synchrotron photoionization mass spectrometry studies have identified ClSO+ as a prominent product in the VUV photochemistry of Cl2SO and revealed the vibronically structured near-threshold ionization of the ClSO radical. Motivated by these advances, we provide a focused, high-accuracy theoretical characterization of ClSO+. Coupled-cluster calculations including all-electron scalar-relativistic and explicitly correlated protocols yield equilibrium structures and benchmark adiabatic (9.47 eV) and vertical (9.67 eV) ionization energies for ClSO. The ClSO+ ground-state potential is predicted to be strongly bound, with a Cl-S bond dissociation energy of 3.21 eV at the CCSD(T)/aug-cc-pV(5+d)Z level. MRCI+Q and EOM-CCSD methods define the low-lying singlet manifold of ClSO+. We report rotational constants and IR intensities for key isotopologues, quantify geometry changes upon ionization, and predict vertical electronic excitations. This reveals a dominant allowed transition in the near-UV, alongside low-lying dark states that relax strongly along the Cl-S stretching coordinate. Our results provide a molecular framework for interpreting the threshold ionization spectra of ClSO and for assigning or anticipating ClSO+ features in VUV/UV experiments.