The cyclocondensation reaction of equimolar amounts of SeCl2 and (Me3Si)(2)NMe in THF affords 1,3,5,7-Se-4(NMe)(4) (5b) [delta(Se-77) = 1585 ppm] in excellent yield. An X-ray structural determination showed that 5b consists of cyclic, puckered crown-shaped molecules with a mean Se-N bond length of 1.841 angstrom typical of single bonds. A minor product of this reaction was isolated as unstable orange-red crystals, which were identified by X-ray analysis as the adduct 1,5-Se-6(NMe)(2)center dot(1)/Se-2(8) (1b center dot(1)/Se-2(8)), composed of cyclic 1,5-Se-6(NMe)(2) and disordered cyclo-Se-8 molecules. A detailed reinvestigation of the cydocondensation reaction of SeCl2 and (BuNH2)-Bu-t as a function of molar ratio and time by multinuclear (H-1, C-13, and Se-77) NMR spectroscopy revealed that the final product exhibits one Se-77 resonance at 1486 ppm and equivalent (NBu)-Bu-t groups. The shielding tensors of 28 selenium-containing molecules, for which the Se-77 chemical shifts are unambiguously known, were calculated at the PBE0/def2-TZVPP level of theory to assist the spectral assignment of new cyclic selenium imides. The good agreement between the observed and calculated chemical shifts enabled the assignment of the resonance at 1486 ppm to 1,3,5,7-Se-4((NBu)-Bu-t)(4) (5a). Those at 1028 and 399 ppm (intensity ratio 2:1) could be attributed to 1,5-Se-6(NMe)(2) (1b).