Conformational Analysis and Electronic Interactions Laboratory, Institute of Chemistry, University of São Paulo, USP, CP 26077, 05513-970 São Paulo, SP, Brazil.
Molecules. 2013 Jun 27;18(7):7492-509. doi: 10.3390/molecules18077492.
The analysis of the IR carbonyl bands of some 3-(4'-substituted phenylsulfanyl)-1-methyl-2-piperidones 1-6 bearing substituents: NO₂ (compound 1), Br (compound 2), Cl (compound 3), H (compound 4) Me (compound 5) and OMe (compound 6) supported by B3LYP/6-31+G(d,p) and PCM calculations along with NBO analysis (for compound 4) and X-ray diffraction (for 2) indicated the existence of two stable conformations, i.e., axial (ax) and equatorial (eq), the former corresponding to the most stable and the least polar one in the gas phase calculations. The sum of the energy contributions of the orbital interactions (NBO analysis) and the electrostatic interactions correlate well with the populations and the νCO frequencies of the ax and eq conformers found in the gas phase. Unusually, in solution of the non-polar solvents n-C₆H₁₄ and CCl₄, the more intense higher IR carbonyl frequency can be ascribed to the ax conformer, while the less intense lower IR doublet component to the eq one. The same νCO frequency trend also holds in polar solvents, that is ν(CO)(eq)< ν(CO)(ax). However, a reversal of the ax/eq intensity ratio occurs going from non-polar to polar solvents, with the ax conformer component that progressively decreases with respect to the eq one in CHCl₃ and CH₂Cl₂, and is no longer detectable in the most polar solvent CH₃CN. The PCM method applied to compound 4 supports these findings. In fact, it predicts the progressive increase of the eq/ax population ratio as the relative permittivity of the solvent increases. Moreover, it indicates that the computed ν(CO) frequencies of the ax and eq conformers do not change in the non-polar solvents n-C₆H₁₄ and CCl₄, while the ν(CO) frequencies of the eq conformer become progressively lower than that of the ax one going from CHCl₃ to CH₂Cl₂ and to CH₃CN, in agreement with the experimental IR values. The analysis of the geometries of the ax and eq conformers shows that the carbonyl oxygen atom of the eq conformer is free for solvation, while the O[CO]…H[o-Ph] hydrogen bond that takes place in the ax conformer partially hinders the approach of the solvent molecules to the carbonyl oxygen atom. Therefore, the larger solvation that occurs in the carbonyl oxygen atom of the eq conformer is responsible for the observed and calculated decrease of the corresponding frequency. The X-ray single crystal analysis of 2 indicates that this compound adopts the most polar eq geometry in the solid. In fact, in order to obtain the largest energy gain, the molecules are arranged in the crystal in a helical fashion due to dipole moment coupling along with C-H…O and C-H…π(Ph) hydrogen bonds.
用 B3LYP/6-31+G(d,p) 和 PCM 计算以及 NBO 分析（对于化合物 4）和 X 射线衍射（对于 2）分析了一些 3-（4'-取代苯基硫基）-1-甲基-2-哌啶酮 1-6 的 IR 羰基带的分析，这些化合物带有取代基：NO₂（化合物 1）、Br（化合物 2）、Cl（化合物 3）、H（化合物 4）、Me（化合物 5）和 OMe（化合物 6）。结果表明，存在两种稳定的构象，即轴向（ax）和赤道（eq），前者对应于气相计算中最稳定和极性最小的构象。轨道相互作用（NBO 分析）和静电相互作用的能量贡献之和与气相中 ax 和 eq 构象的特征频率很好地相关。不同寻常的是，在非极性溶剂正己烷和 CCl₄的溶液中，较高的 IR 羰基频率可以归因于 ax 构象，而较低的 IR 双组分则归因于 eq 构象。在极性溶剂中也存在相同的 νCO 频率趋势，即 ν(CO)(eq)< ν(CO)(ax)。然而，从非极性溶剂到极性溶剂，ax/eq 强度比发生了反转，ax 构象的比例相对于 eq 构象逐渐减小，在极性最强的溶剂 CH₃CN 中不再可检测到。应用于化合物 4 的 PCM 方法支持这些发现。事实上，它预测随着溶剂介电常数的增加，eq/ax 种群比例会逐渐增加。此外，它表明，在非极性溶剂正己烷和 CCl₄中，ax 和 eq 构象的计算 ν(CO)频率不会改变，而 eq 构象的 ν(CO)频率从 CHCl₃到 CH₂Cl₂再到 CH₃CN 逐渐降低，与实验 IR 值一致。对 ax 和 eq 构象的几何形状的分析表明，eq 构象的羰基氧原子可以自由溶剂化，而在 ax 构象中发生的 O[CO]…H[o-Ph]氢键部分阻碍了溶剂分子接近羰基氧原子。因此，在 eq 构象的羰基氧原子中发生的较大溶剂化是导致观察到和计算出的相应频率降低的原因。2 的 X 射线单晶分析表明，该化合物在固态中采用最极性的 eq 几何形状。事实上，为了获得最大的能量增益，分子在晶体中排列成螺旋形，这是由于偶极矩耦合以及 C-H…O 和 C-H…π(Ph)氢键的作用。
