S8 ring content in liquid sulfur determined from x-ray diffraction

The S8 ring content in liquid sulfur has been determined from the 4.47 Å peak in the x-ray pair distribution function over the temperature range -55 to 330 ºC. This peak represents the characteristic 3rd and 4th nearest atom-atom distances across an S8-ring. Three procedures were used to calculate the S8-ring fraction; scaling to crystalline β-S, a linear combination fit and a direct integration method. All methods gave similar values, indicating that approximately one third of the S8-rings break up upon melting into a distribution of short chains with an average length of 5±2 atoms. Our results contradict established models of low temperature sulfur that indicate below the λ-transition the liquid comprises almost entirely of S8-rings. In addition, we find the S8-ring fraction increases to 76±10% at -30ºC in the supercooled liquid, and decreases to 23±3% at 330ºC in the high temperature melt. First principle molecular dynamics calculations confirm the onset of dynamic ring distortion via broad bond angle distributions, associated with the interstitial region between the first and second peaks in the pair distribution function. Our results are consistent with the presence of a propensity of short chains in the low temperature liquid, that are required for the bond switching mechanism responsible for the polymerization process to occur at the λ-transition temperature, Tλ.