Finally, we determined the molecular structure in detail via X-ray analysis. Recrystallization of triarylpropargyl cations 1 + TfO -,2 + TfO -afforded red single crystals suitable for X-ray diffraction measurements. In the case of 1 + TfO -, two crystallographically independent molecules 1 + (mol.1 and mol.2) exist, each of which adopts a similar structure. The X-ray structures of triarylpropargyl cations 1 + TfO -,2 + TfO -exhibit a unique isosceles triangle-like structure as shown in Fig. . The dihedral angles θ1 and θ2 for 1 + TfO -,2 + TfO -determined by X-ray analyses are small as predicted by calculations (Fig. ). Especially, 2 + TfO -adopts a completely planar geometry. While the counter anion is positioned near the cation due to electrostatic stabilization, formation of a strong ion pair with the C1 and C3 atoms was not observed. This molecular design based on the insertion of a -C≡C-bond into the triarylmethyl cation is useful for the construction of stable acceptor molecules with higher planarity.