de Melo Ulisses Zonta, Silva Raí G M, Yamazaki Diego A S, Pontes Rodrigo M, Gauze Gisele F, Rosa Fernanda A, Rittner Roberto, Basso Ernani A
Chemistry Department, State University of Maringa , Avenida Colombo, 5790, 87020-900, Maringa, Paraná, Brazil.
J Phys Chem A. 2015 Mar 12;119(10):2111-21. doi: 10.1021/jp512378g. Epub 2015 Feb 25.
This study reports the results of ab initio and density functional theory (DFT) electronic structure calculations as well as (3)J(HH) experimental and calculated coupling constant data obtained in the investigation of the conformational equilibrium of 3-halo-derivatives of 1-methylpyrrolidin-2-one. The five-membered ring assumes an envelope conformation owing to the plane of formation of the O═C-N-R bond, with C4 forming the "envelope lid". When the conformation changes, the "lid" alternates between positions above and below the amide plane. The α-carbonyl halogen assumes two positions: a pseudo-axial and a pseudo-equatorial. In the gaseous phase, the calculations indicate that the pseudo-axial conformer is more stable and preferable going down the halogen family. Natural bond orbital analysis showed that electronic delocalization is significant only for the iodo derivative. In the other derivatives, the electrostatic repulsion between oxygen and the halogen determines the conformational equilibrium. When the solvated molecule was taken into account, the pseudo-equatorial conformer population increased with the relative permittivity of the solvent. This variation was strong in the fluoro derivative, and the preference was inverted. In the chlorine derivative, the two populations became closer in methanol and acetonitrile. In the bromine and iodine derivatives, the percentage of pseudo-equatorial conformer increased only slightly owing to the dipole moment of the conformation: the pseudo-equatorial conformation has a greater dipole moment and thus is stable in media with high relative permittivity.
本研究报告了从头算和密度泛函理论（DFT）电子结构计算的结果，以及在对1-甲基吡咯烷-2-酮的3-卤代衍生物的构象平衡研究中获得的（3）J（HH）实验和计算耦合常数数据。由于O═C-N-R键的形成平面，五元环呈现信封式构象，其中C4形成“信封盖”。当构象改变时，“盖子”在酰胺平面上方和下方的位置之间交替。α-羰基卤素占据两个位置：一个假轴向和一个假赤道向。在气相中，计算表明假轴向构象异构体更稳定，并且在卤素族中越往下越占优势。自然键轨道分析表明，电子离域仅对碘代衍生物有显著影响。在其他衍生物中，氧和卤素之间的静电排斥决定了构象平衡。当考虑溶剂化分子时，假赤道向构象异构体的比例随溶剂的相对介电常数增加。这种变化在氟代衍生物中很强，并且优势发生了反转。在氯代衍生物中，在甲醇和乙腈中两种构象异构体的比例变得更接近。在溴代和碘代衍生物中，假赤道向构象异构体的百分比仅由于构象的偶极矩而略有增加：假赤道向构象具有更大的偶极矩，因此在相对介电常数高的介质中是稳定的。
