D Groups The D set of point groups are classified as D nh , D nd , or D n , where n refers to the principal axis of rotation. Overall, the D groups are characterized by the presence of n C 2 axes perpendicular to the principal C n axis. Further classification of a molecule in the D groups depends on the presence of horizontal or vertical/dihedral mirror planes. Group Description Example Dnh n perpendicular C2 axes, and a horizontal mirror plane (σh) benzene, C6H6 is D6h Dnd n perpendicular C2 axes, and a vertical mirror plane (σv) propadiene, C3H4 is D2d Dn n perpendicular C2 axes, no mirror planes [Co(en)3]3+ is D3 C Groups The C set of point groups are classified as C nh , C nv , or C n , where n refers to the principal axis of rotation. The C set of groups are characterized by the absence of n C 2 axes perpendicular to the principal C n axis. Further classification of a molecule in the C groups depends on the presence of horizontal or vertical/dihedral mirror planes. Group Description Example Cnh horizontal mirror plane (σh) perpendicular to the principal Cn axis boric acid, H3BO3 is C3h Cnv vertical mirror plane (σv) containing the principal Cn axis ammonia, NH3 is C3v Cn no mirror planes P(C6H5)3 is C3 S Groups The S set of point groups are classified as S 2n , where n refers to the principal axis of rotation. The S set of groups are characterized by the absence of n C 2 axes perpendicular to the principal C n axis, as well as the absence of horizontal and vertical/dihedral mirror planes. However, there is an improper rotation (or a rotation-reflection) axis collinear with the principal C n axis. Group Description Example S2n improper rotation (or a rotation-reflection) axis collinear with the principal Cn axis 12-crown-4 is S4