Table 2.

Optimized hexagonsX ₆R₆ with D _6h symmetry.

X	R	Stationary pointa	Imaginary Mode of 1st TS	Structureb
Si	Na	3rd TS		B
Si	K	1st TS	Our-of-plane motion of K	B
Si	Mg	2nd TS		B
Si	Ca	6th TS		B
Si	Cu	1st TS	Out-of-plane ring deformation	A
Si	Zn	1st TS	Our-of-plane motion of Zn	B
Si	C≡N	1st TS	Out-of-plane ring deformation	A
Si	MgH	MIN		A
Si	CaH	6th TS		A
Ge	BeH	MIN		A
Ge	MgH	1st TS	Our-of-plane motion of MgH	A
Ge	CaH	17th TS		A
Si	H	1st TS	Out-of-plane ring deformation	A
Si	Li	6th TS		A

Full geometry optimizations were performed at the B3LYP/6-311++ G(3df,3pd) level for K, Ca, Cu, Zn and CaH substitution, and at the B3LYP/cc-pVTZ level for H, Li, Na, Mg, C≡N, BeH and MgH substitution. ^aMIN: minimum, TS: transition state. ^bTwo D _6h structures A and B are shown in Fig. 6. More stable structure between the two D _6h structures is listed for the terminal substituent of metals (R = Na, K, Mg, Ca, Cu, Zn). For comparison, the results of Si₆H₆ and Si₆Li₆ of structure A are listed.