Silabutadienes. Internal rotations and pi-conjugation. A density functional theory study

The potential energy surfaces (PESs) for internal rotation around the central single bond of nine silabutadienes, which include all possible mono-, di-, tri-, and tetrasilabutadienes, are investigated computationally by using DFT with the B3LYP functional and the 6-311+G(d,p) basis set. For 1-silabutadiene (3), 2-silabutadiene (4), 1,4-disilabutadiene (5), 2,3-disilabutadiene (6), and 1,3-disilabutadiene (7), the s-trans rotamer is the most stable. For 1,2-disilabutadiene (8), 1,2,3-trisilabutadiene (9), and 1,2,4-trisilabutadiene (10), all having a trans-bent SiSi double bond, the most stable conformers are those having an antiperiplanar (ap) structure. For tetrasilabutadiene (11), the global minimum is the gauche rotamer. The internal rotation barriers (RB) (relative to the global minimum) follow the order (kcal/mol) 5 (10.0) > 3 (7.4) > 1,3-butadiene (12, (6.6)) > 10 (4.9) > or = 7 (4.4) > or = 4 (4.0) approximately = 8 (3.9) > 9 (2.7) approximately = 6 (2.6) > 11 (2.4). The barriers are slightly smaller at CCSD(T)/cc-PVTZ, but the trend remains the same. The size of the rotation barrier is mainly dictated by the length of the central single bond; that is, it is the largest for dienes with the shorter C-C central bond (5, 3, and 12), and it is smaller for dienes with the longer Si-C and Si-Si central bonds. The strength of pi-conjugation in the s-trans conformers of silabutadienes was estimated by resonance stabilization energies (RE) calculated by using the Natural Bond Orbital (NBO) and Block Localized Wave function (BLW) methods and bond separation equations. A linear correlation is found between the barrier heights for internal rotation and pi-conjugation energies. The calculated RBs are significantly smaller than the corresponding REs, indicating that pi-resonance energies are not the only factor that dictate the RB, and therefore, RBs, although suitable for estimating trends in pi-conjugation in a series of compounds, cannot be used for estimating absolute resonance energies.