Structure and spectra of tetrasulfur S4 – an ab initio MO study

The structures and relative stabilities of eight S4 isomers were investigated by G3X(MP2), CCSD(T)/cc-pVTZ and MRCI/CASSCF calculations. The cis-planar (C2v) structure is confirmed to be the global minimum of S4, while the rectangular D2h structure is calculated to be a transition state. The predicted stability order of various singlet S4 isomers is C2v>C2h>Cs>D2d>D3h. Calculated electronic absorption spectra [CIS/6-311 + G(3df)] and vibrational spectra [B3LYP/6-31G(2df)] allow a convincing assignment of the observed green absorbing species as the cis-planar (C2v) structure (global minimum) and the red absorbing species as the trans-planar (C2h) isomer, in distinct contrast to the previous assignments.