wR--Bn H H 键角接近120° H O H R-C－C－C-R' α H H α' . . C O .. δ+ δ- H O H (H)R-C－C-H α C O 键角接近 120° π σ σ σ . . . . (H)R' C O .. . . R δ+ δ- . . C O .. δ+ δ-