Rydberg transitions in the bichromophoric molecules tetramethyl-1,3-cyclobutanedithione (TMCBDT) and tetramethyl-3-thiocyclobutan-1-one (TMTCB)

The vacuum ultraviolet spectra of tetramethyl-1,3-cyclobutanedithione (TMCBDT) and tetramethyl-3-thio-cyclobutan-1-one (TMTCB) in the 140-240 nm range are reported. Each spectrum is analysed and shown to consist of various Rydberg series. While transitions from the non-bonding orbital on the C=S to ns S , np S and nd S Rydberg orbitals dominate the spectra of both compounds, a short series assigned to the non-bonding C=O to ns O orbitals is observed in TMTCB. The splitting of the n orbitals in TMCBDT is considerably smaller than that in TMTCB or TMCBD. The difference in the n orbital splitting in the two symmetrical molecules, TMCBD and TMCBDT, is attributed to the difference between the orbital energy spacings (n CO or n CS ) and the closest-lying σ relay orbital responsible for the through-bond interaction. The large splitting in TMTCB is due to the difference in ionization potential between sulphur and oxygen. For this same reason the two highest occupied orbitals in TMCBDT fit into the conventional concept of non-bonding orbitals localized on the heteroatom. In TMCBD the non-bonding orbitals are shown to be ring orbitals with some oxygen p character mixed in.