NMR J(¹³C,¹³C) spin–spin coupling constants in pyridine-carboxaldehydes. Experimental and DFT-B3LYP studies

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• 作者：Oscar E. Taurian ; Dora G. De Kowalewski ; Jorge E. Pé ; rez and Rubé ; n H. Contreras
• 关键词：Scalar couplings ; Stereospecificity ; Negative hyperconjugative interactions ; Geminal couplings ; Conjugative interactions
• 刊名：Journal of Molecular Structure
• 出版年：2005
• 期刊代码：62_00222860
• 类别：ch
• 出版时间：2005
• 卷：754
• 期：1-3
• 页码：1-9
• 文件大小：160 K

摘要

J(¹³C,¹³C) spin–spin couplings were measured at natural abundance in five pyridine aldehyde derivatives. Such couplings were also calculated at the DFT-B3LYP-6-311G**/EPR-III level of theory where special attention was paid to their possible stereospecific behaviors. Dielectric solvent effects were calculated on all four isotropic contributions to J(¹³C,¹³C) couplings using the polarization continuum model, PCM. It is observed that the inclusion of dielectric solvent effects in general leads to a better agreement between calculated and experimental couplings. ²J(¹³C_C,¹³C_A) couplings were observed to be sensitive to the aldehyde side-chain conformation (coupling pathway: C_C–C_i–C_A, where C_C stands for the carbonyl-, C_i is the aromatic ipso- and C_A is an adjacent aromatic-C atom). This is a positive coupling and becomes unusually large for a trans configuration between the carbonyl CO and the C_C–C_i bonds. In this conformation, there is an enhancement of conjugative interactions involving the π(CO) and π(C_CC_i) bonding and antibonding orbitals.