14N and15N NMR spectroscopic characterization and analysis of cyclic sulfur imides

Abstract

The 14N and 15N NMR spectra of natural abuadance and 15N-enriched samples of S7NH and S4(NH)4 were obtained by direct detection and by a variety of polarization transfer techniques. Sensitivity enhancements of ca 10 (compared with direct detection) for the INEPT pulse sequence and ca 100 for the HMQC (heteronuclear multiple quantum coherence) + BIRD (bilinear rotation decoupling) sequence were observed for the imides. 15N spin–lattice relaxation constants (T1) of 10.2 ± 0.1 and 12.6 ± 0.1 s were determined for S7NH and S4(NH)4, respectively. The HMQC procedure was used to obtain the 15N and 1H NMR chemical shifts and 11J(15N, 1H) values for 1,3-,1,4- and 1,5-S6(NH)2, 1,3,5- and 1,3,6-S5(NH)3. The trends in NMR chemical shifts and coupling constants are discussed in the context of the known molecular structurts of cyclic sulfur imides. The use of NMR techniques for the analysis of mixtures of cyclic sulfur imides is compared with the well established methods of infrared spectroscopy and thin-layer chromatography.