Improper rotation axis

{
  "id": "MLC-T-CHM-005737",
  "term": "Improper rotation axis",
  "field": "Chemistry",
  "definition": "A symmetry element made of a rotation by 360°/n about an axis followed by reflection in the plane perpendicular to that axis, denoted Sn. S1 is equivalent to a mirror plane and S2 to an inversion centre. A molecule possessing any improper axis is achiral, so chirality requires the absence of Sn for every n; methane, with three S4 axes, is achiral for this reason.",
  "subject": "Analytical Chemistry",
  "references": [
    "PAC, 2021, 93, 647. 'Glossary of methods and terms used in analytical spectroscopy (IUPAC Recommendations 2019)' on page 750 (https://doi.org/10.1515/pac-2019-0203)"
  ],
  "url": "https://mlchart.com/terminology/chemistry/improper-rotation-axis/"
}