Vibrational Properties of the Disulfur Dinitride Molecule, S₂N₂: IR and Raman Spectra of the Matrix-Isolated Molecule

Abstract

The IR and Raman spectra of disulfur dinitride, S(2)N(2), are reported not only for the solid condensate but also for the molecules isolated in solid noble gas, N(2), or CH(4) matrices at low temperatures. The results imply that the isolated S(2)N(2) molecule has much the same geometry as in the crystalline solid with a virtually square-planar structure conforming to D(2h) symmetry, a conclusion confirmed by isotopic enrichment in (15)N and by the results of earlier as well as fresh quantum chemical calculations. These calculations also support the results of normal coordinate analysis of the experimental data in giving potential constants suggestive of a relatively rigid S(2)N(2) molecule consistent with its description as a 2π-electron aromatic, while appearing to maintain a formal S-N bond order close to 1.

Vibrational Spectrum and Gas‐Phase Structure of Disulfur Dinitride (S2N2)

“…The rule of mutual exclusion applies and the three b u modes are infrared active. In agreement with previous gas‐phase2 and matrix‐isolation work9 only two strong IR bands were observed at $\tilde \nu $ =789.7 (ν 6 , b 3u ) and 473.8 cm −1 (ν 4 , b 1u ). In addition we detected the very weak ν 5 (b 2u ) band at $\tilde \nu $ =655.6 cm −1 as well as four additional combination bands of ν 5 and ν 6 , respectively (Figure 1).…”

“…All these bands are unequivocally attributed to S 2 N 2 ( D 2 h ) due to their uniform photo‐destruction under UV irradiation ( λ =248 or 255 nm) 11. The position of ν 5 in solid Ar is consistent with a reported value of $\tilde \nu $ =660.5 cm −1 for polycrystalline S 2 N 2 ( D 2 h ) at 77 K,9 and with a tentatively assigned, very weak A‐type feature observed in the gas‐phase spectrum at $\tilde \nu $ =652 cm −1 2…”

“…These estimates account for anharmonic effects taken from the averages of the VPT2 and VCI results. The deviations of these estimates from previously reported Raman frequencies9 are remarkable, whereas the gas‐phase to Ar‐matrix shifts for the two measured IR gas‐phase frequencies (see below) is less than 3 cm −1 .…”

“…The ambiguity arising from a correlation of SN bond lengths to suggested bond orders has already been mentioned 26. However, one would not readily expect, that the SN bond lengths of S 2 N 2 are longer, and the corresponding stretching force constant are smaller than those of the dimer S 4 N 4 ,9 revealing polar S + N − single bonds 26. This difference is slightly pronounced when the R g bond lengths obtained from an electron diffraction study of S 4 N 4 (1.628(4) Å)27 is compared to semi‐experimental R g bond lengths in S 2 N 2 (1.6489(8) Å), where the semi‐experimental R e value have been corrected by Calcd Δ R = Calcd R g (0 K)− Calcd R e by using R g (0 K) and R e values obtained within the VPT2 approach at the CCSD(T)/pCVTZ level (Table 5).…”

“…A recent Raman and IR study of natural and 15 N‐isotopically enriched matrix‐isolated S 2 N 2 at 15–35 K confirmed the apparent D 2 h molecular symmetry by 1) the mutual exclusion of observed IR and Raman bands and 2) the presence of symmetrically equivalent N atoms proven by 14/15 N isotope splitting 9. On the other hand, a computational study highlighted unexpected low bond pseudorotation barriers of ≤2 kJ mol −1 for the planar S 2 N 2 ring structure, indicating a large deformational flexibility typical of a floppy molecule 10…”