Chirality of the Conformation Attacks the Planarity of the sp2 Carbon Atom in a Covalent Bond

The
correlation of the rotation about the bond C′–Cα and the pyramidalization of the sp2 hybridized
carbon atom C′ and its three bonding partners to a flattened
tetrahedron was investigated for the substructure Cβ–CαH–C′(O)–OMe.
A search in the Cambridge Structural Database (CSD) gave 15,295 structures
with a substituted methyl acetate group at the end of the molecules.
The scatter plot of the rotation angle ψ = OC′–Cα–Cβ versus the pyramidalization
angle θ = O­(MeO)­C′Cα and the ψ/θav curve show an unusual undulating pattern with three maxima
and minima for a 360° rotation about the bond C′–Cα. There is no net chiral induction from the (Mψ)/(Pψ) conformations of the bond C′–Cα to
the (Rθ)/(Sθ) configurations of the flattened tetrahedron because
the contributions of the three maxima and minima cancel each other.
The wavelike ψ/θav curve demonstrates that
the rotation about the bond C′–Cα and
the pyramidalization of the group C′(O)­(OMe)­Cα are not independent processes. They are coupled with each other
in one common molecular motion. The ψ/θav curve
appears as the third harmonic of a sinusoidal fundamental frequency.
DFT calculations of the propanoate anion, methyl propanoate, and methyl
2-methylpropanoate confirm the results of the CSD search.

对亚结构 Cβ–CαH–C′(O)–OMe 中，围绕键 C′–Cα 的旋转与 sp2 杂化碳原子 C′及其三个成键伙伴向平面四面体的金字塔化之间的相关性进行了研究。在剑桥结构数据库（CSD）中进行搜索，得到含有分子末端取代甲基乙酸酯基团的15,295个结构。旋转角 ψ = OC′–Cα–Cβ 与金字塔化角 θ = O­(MeO)­C′Cα 的散点图以及 ψ/θav 曲线显示出一种不寻常的波动模式，该模式在围绕键 C′–Cα 的360°旋转中呈现出三个极大值和极小值。由于三个极大值和极小值的贡献相互抵消，从键 C′–Cα 的 (Mψ)/(Pψ) 构型到扁平四面体的 (Rθ)/(Sθ) 配置之间没有净手征诱导。波形的 ψ/θav 曲线表明，围绕键 C′–Cα 的旋转与 C′(O)­(OMe)­Cα 组合金字塔化并非独立的过程，它们在一种共同的分子运动中相互耦合。ψ/θav 曲线呈现出正弦基频的三次谐波。丙酸根阴离子、甲基丙酸和甲基2-甲基丙酸的大分子场计算结果证实了CSD搜索的结果。