Ab initio IGLO studies of the conformational dependences of 13C NMR chemical shifts and α-through ε-substituent effects in 1-substituted pentanes

✍ Scribed by Michael Barfield

• Publisher: John Wiley and Sons
• Year: 1998
• Tongue: English
• Weight: 411 KB
• Volume: 36
• Category: Article
• ISSN: 0749-1581
• DOI: 10.1002/(sici)1097-458x(199806)36:13<s93::aid-omr293>3.0.co;2-h

✦ Synopsis

Ab initio IGLO (individual gauge for localized molecular orbital) methods of SCF-MO theory were used to extend studies of the conformational dependences of isotropic 13C NMR chemical shifts to n-hexane and three 1-substituted pentanes (X \ CN, OH, F). Isotropic shifts were obtained as a func-XCH 2 CH 2 CH 2 CH 2 CH 3 tion of the torsion angles and measured about the C1ÈC2, C2ÈC3 and C3ÈC4 bonds, respectively, r 1

, r 2 r 3 with molecular structures optimized at the HF/6È31G* level. The calculated 13C chemical shifts and substituent e †ects, averaged over the torsional motions, compare favorably with the experimental data. The computed stereochemical dependencies of athrough e-e †ects are compared with experimental values in several series of bicyclic molecules which encompass a range of the three torsion angles. Emphasized here are dand e-e †ects, which are sensitive to all three dihedral angles. Inclusion of the third dihedral angle improves the results over those based on the substituted n-butanes, especially for d-methylene carbons which can be shielded or deshielded depending on r 3 . Analysis of the IGLO local bond contributions in sterically crowded conformations suggests a general tendency in which CÈH bonds pointing toward and away from the substituent lead to deshielding and shielding, respectively.