Structures and EPR spectra of binary sulfur–nitrogen radicals from DFT calculations

The scattered electron paramagnetic resonance (EPR) spectroscopic data for binary sulfur–nitrogen (S,N) radicals have been compiled and critically assessed.Many of these are inorganic rings or cages.For each species, possible equilibrium structures in the gas phase and the EPR hyperfine coupling (hfc) constants have been calculated with DFT using the B3LYP functional and basis sets of triple-ζ (or better) quality.Good agreement is obtained between calculated and measured values for the well characterized [S3N2]+, a planar π-radical for which the s-component of the orbitals is likely to be reasonably independent of minor geometrical changes between gas-phase and condensed-phase states.The cage compounds [S4N4]− and [S4N5]−2, for which reliable experimental EPR spectra have been reported, show larger variation between calculated and measured hfc, as a consequence of the dependence of the s orbital content of the molecular orbitals on small structural changes.The very large disagreements between the DFT calculated and experimentally claimed hfc constants for [NS], [SNS] and [S4N4]−3 in condensed phases lead us to question their assignment.Among binary S,N radicals, 33S hfc data has only been reported for [S3N2]+ (through isotopic enrichment).These values were essential for the correct identification of the EPR spectra of this important radical, which previously was misassigned to other species.Our results suggest that 33S data will be equally important for the correct identification of the EPR spectra of other binary S,N species, many of which are cyclic systems, e.g.[S3N3], [S4N3] and[S4N5].

DFT calculations are proving to be of immense value in the rapid calculation of hyperfine coupling (hfc) constants of main group element free radicals.Predictions can be made not only for the commonly observed isotopes, but also for those that are experimentally hard to observe for reasons of low abundance or spins >1, such as 33S. Using calculated hfc constants, we predict that the never-observed isotropic EPR spectrum of [S3N3] would be more diagnostic if obtained on a sample with modest (30%) enrichment in 33S content than if obtained at natural abundance, as shown in a simulation employing realistic linewidths.Figure optionsDownload as PowerPoint slide