Selenium-77 NMR spectroscopic study of fluxional selenium sulfide 1,2,3,4,5-Se5S2

Indirect Nuclear 77Se?77Se Spin?Spin Coupling Constants. Application of Density Functional Theory (DFT) Calculations

“…In contrast, the corresponding dihedral angle in the diboryl substituted diselane 2a is calculated to be much larger (156.0 • ), indicating potential B-Se(pp) π interactions. The energy difference between the structure shown for the seven-member ring 3e and conformational isomers with the analogous chair conformation is calculated to be extremely small (<0.1 kcal/mol), in agreement with the fluxional behaviour of this molecule [16,22,23]. For the structure of Se 5 , see Scheme 2.…”

“…Several of the molecules shown in Scheme 1 can be considered as examples for 77 Se-77 Se spin-spin coupling across two or three bonds [ 2,3 J( 77 Se, 77 Se)]. In the cases of mixed chalcogens, some of these data have been determined experimentally using 77 Se enriched materials [16,17], and calculated values are in good agreement (included in Figure 1). It should be noted here, that the magnitude of 2,3 J( 77 Se, 77 Se) can easily exceed the magnitude for the one-bond coupling constants.…”

“…The rather small values 1 J( 77 Se, 77 Se) measured for compounds with Se-Se bonds [14][15][16][17] have left much room for speculations on the nature of the Se-Se bond including the influence of the lone pairs of electrons on the Fermi contact term (FC) [26] which was considered to be the dominant coupling mechanism [14,15]. There is no obvious relationship between calculated Se-Se bond lengths and the magnitude of 1 J( 77 Se, 77 Se).…”

“…The present calculations support this assumption of a negative sign of 1 J( 77 Se, 77 Se) in 1e. When some of the mixed chalcogens 3 had been prepared with 77 Se enrichment, the resulting complex spin systems have been analysed successfully with respect to J( 77 Se, 77 Se) [16,17]. A negative sign of 1 J( 77 Se, 77 Se) has been assumed throughout, apparently on the basis of relativistic extended Hückel MO calculations (REX) [16].…”

Selenium sulfur dihalides, ChnX2 (n = 1, 2, 3; Ch = Se, S; X = Br, Cl). Raman and 77Se NMR spectroscopic characterization

“…Recently SezClz and Se,Br, were shown to contain equilibrium amounts of Sex,, Se,X,, and Se,X, (2). The mixed halogen compounds, Se,,BrCl (n = 1,2) have also been reported (2) but mixed chalcogen dihalides have received little attention although sulfur selenium chains in general are currently of great interest (3). Early work, using chemical methods, suggested that SeSCl, has a trigonal pyramidal structure (4).…”