Fine structures of 8-G-1-(arylethynylselanyl)naphthalenes (G = H, Cl, Br): Factors to control the linear alignment of five GSe–CC–CAr atoms in crystals and the behavior in solution

8-G-1-(p-YC₆H₄CCSe)C₁₀H₆ 2 (G = Cl) and 3 (G = Br): Y = H (a), OMe (b), Me (c), F (d), Cl (e), CN (f) and NO₂ (g)] have been prepared and the NMR spectra measured, in addition to 1 (G = H). Structures have been determined by X-ray crystallographic analysis for 2b, 2e and 2g, which are all type B (B), where the Se–C_sp bond is placed in the naphthyl plane in B. The type is classified as A if the Se–C_sp bond is perpendicular to the naphthyl plane. Structures around the p-YC₆H₄ (Ar) group are pd (perpendicular) for Y = OMe (2b) and Cl (2e) and pl (planar) for Y = NO₂ (2g), where the Se–C_Nap bond is placed in the aryl plane in pl and perpendicular to the plane in pd. The 1b (A: pd) structure changes dramatically on going to 2b (B: pd) with G = Cl at the 8-position. The effect is called the G-dependence in 2. The G-dependence arises from the energy lowering effect of the n_p(Cl)σ^*(Se–C_sp) 3c–4e interaction. Structures are both (B: pd) for 1e and 2e and both (B: pl) for 1g and 2g. One may realize that the structures are unchanged by G = Cl in place of G = H for Y = Cl and NO₂ at a first glance. However, the B structures in 2e and 2g must be much more stabilized by the G-dependence of the n_p(Cl)σ^*(Se–C_sp) 3c–4e interaction or the GSe–C_sp–C_sp–C_sp2 5c–6e type interaction. The structures of 2 and 3 are examined in solution based on the NMR parameters. The results show that 2 and 3 behave very similarly to each other and the structures are predominantly B, with some equilibrium between pd and pl around the aryl groups in solution. Quantum chemical calculations support the observations.