Theoretical Study of Germabutadienic Internal Rotations and π-Conjugation

The pi-conjugational strength and the internal rotation barriers of germabutadienes are investigated at the level of B3LYP/6-311+G(d,p). The calculated potential energy surfaces (PES) for internal rotations of nine germabutadienes, which include mono-, di-, tri-, and tetragermabutadienes, are found to be similar to those of silabutadienes. For 1-germabutadiene, 2-germabutadiene, 1,3-digermabutadiene, 2,3-digermabutadiene, and 1,4-digermabutadiene, whose external double bonds belong to the Ge=C and C=C bond types, the s-trans (or twisted s-trans) conformations are the global minima for their internal rotational PESs. 1,2-Digermabutadiene, 1,2,3-trigermabutadiene, 1,2,4-trigermabutadiene, and tetragermabutadiene, which possess one and two trans-bent Ge=Ge double bonds, have two different s-gauche minima. The C-2 s-gauche conformation (D(GeGeGeGe) = 74.4 degrees) of tetragermabutadiene, which corresponds to the reported X-ray crystal structure, is found to be of comparable stability (higher by 0.63 kcal/mol) to C-i s-trans. The NBO second order perturbation analysis provides a direct insight into the g-conjugation strength for germabutadiene global mimima. The pi-conjugation strengths in mono-, di-, and trigermabutadienes groups follow the trend, 1-germabutadiene > 2-germabutadiene; 1,4-digermabutadiene > 1,3-digermabutadiene > 1,2-digermabutadiene > 2,3-digermabutadiene; and 1,2,4-trigennabutadiene > 1,2,3-trigermabutadiene. We found good linear correlation between the internal rotational barriers and the pi-conjugation strengths of the studied germabutadienes. We conclude that the pi-conjugations are the main factor dictating the internal rotational barrier, which is also supported by the calculated resonance energies using the stepwise hydrogenation approach.