Selenium-77 NMR chemical shifts of five-membered selenium and sulphur heterocycles with CSe,CS,and CO groups

This paper reports the ⁷⁷Se NMR chemical shifts of 1,3-dithiole-, 1,3-thiaselenole- and 1,3-diselenole-2-ones, -thiones and -selones, of the corresponding saturated compounds 1,3-diselenolane-2-one, -thione and -selone, and the 1,3-thiaselenolium tetrafluoroborates, either unsubstituted or substituted by morpholino, ethylthio or ethylseleno groups in the 2-position. The ⁷⁷Se chemical shift values of the ring selenium and the C?Se groups are compared with the ¹³C chemical shift values of neighbouring carbon atoms. The relationships between the ⁷⁷Se chemical shifts of the C?Se groups and the wavelengths of their n→* absorption in the UV-visible spectrum are discussed with respect to the significance of the δE term in the contribution of the paramagnetic screening and the electron density distribution.