To carry out the CBS-DFT calculations, the spin densities were constrained on the two sulfur-doped rings. Initial CBS-DFT calculations utilized a range of functionals, where the magnitude of spin density constraints was derived from BS-DFT calculations using the M06- For the M06-2X and WB97X-D3 functionals the CBS-DFT values are 121.01 cm -1 and 120.52 cm -1 respectively which are minor improvements over their corresponding BS-DFT values of 123.04 cm -1 and 128.38 cm -1 (Fig. ).