Theoretical analysis of the IR and visible absorption spectral change by the phase transition of the crystals of 2,3-diphenylthio- and 2,3-Di(p-chlorothiophenyl)-1,4-naphthoquinones

The isomerization mechanism of 2,3-Diphenylthio- and 2,3-Di(p-chloro thiophenyl)-l,4-Naphthoquinones was determined by using ab initia calculations and compared IR and visible absorption spectra. The optimized geometries using RHF/3-21G^* method were determined with C₂ symmetry for the violet form, and with C₃ symmetry for the red one. Visible spectra were identified by the charge transfers from thiophenyl group to naphthoquinone ring of these two isomers. The lowest allowed excitated states of these isomers were characterized by the the charge transfer (CT) from next-HOMO to LUMO in the violet form, and from HOMO to LUMO in the red form. The IR active CO stretching mode in the naphthoquinone ring appear as the symmetric type on the violet form, and anti-symmetric type on the red one.