9-(Arylselanyl)triptycenes (1: p-YC6H4SeTpc) should supply the planar structure (pl) around the p-YC6H4Se (ArSe) group in the ground state, irrespective of the p-Y substituents. 1 with Y = H (a), NMe2 (b), OMe (c), Cl (d), CN (e) and NO2 (f) are prepared. Structures of 1a-d and 1f are determined by X-ray analysis and dynamic 1H NMR spectroscopy is applied to 1a, 1c, 1e and 1f. For convenience of discussion, the structure of 1 is defined as follows: a conformer around the triptycyl group in 1 is called A where the Se-CAr bond is placed in the bisected area between two phenyl planes of the triptycyl group and it is B where the bond is on a phenyl plane of the triptycyl group. A conformer for the Ar group is named pl where the Se-CTpc bond is on the Ar plane, while it is pd if the bond is perpendicular to the plane. The structure of 1 is confirmed to be (A: pl) in the ground state by X-ray analysis. 1 (A: pl) changes to the equivalent one via a transition state of 1 (B: pd) (the gear process). The activation energies are determined by dynamic 1H NMR spectroscopy: the values are 36.4, 41.6, 42.3 and <34 kJ mol-1 for 1a, 1e, 1f and 1c, respectively. The MP2 level of calculations reproduced the observed values: they are evaluated to be 34, 39, 40 and 29 kJ mol-1 for 1a, 1e, 1f and 1b′ (Y = NH2), respectively, where 1b′ is employed in place of 1c. Another process (the isolated rotation process) is also operating for the interconversion of 1 (A: pl), which proceeds via1 (A: pd). The activation energies for the process are predicted to be 25, 30, 30 and 20 kJ mol-1 for 1a, 1e, 1f and 1b′, respectively, at the MP2 level. The results demonstrate that (A: pl) is the global minimum in 1: the 1 (A: pl) structure is well established for all Y examined in the ground state. © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2009.