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

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 C-13 NMR chemical shifts to n-hexane and three 1-substituted pentanes XCH2CH2CH2CH2CH3 (X = CN, OH, F). Isotropic shifts were obtained as a function of the torsion angles phi(1), phi(2) and phi(3) measured about the C1-C2, C2-C3 and C3-C4 bonds, respectively, with molecular structures optimized at the HF/6-31G* level. The calculated 13C chemical shifts and substituent effects, averaged over the torsional motions, compare favorably with the experimental data. The computed stereochemical dependencies of alpha- through epsilon-effects are compared with experimental values in several series of bicyclic molecules which encompass a range of the three torsion angles. Emphasized here are delta- and epsilon-effects, 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 delta-methylene carbons which can be shielded or deshielded depending on phi(3). Analysis of the IGLO local bond contributions in sterically crowded conformations suggests a general tendency in which C-H bonds pointing toward and away from the substituent lead to deshielding and shielding, respectively. (C) 1998 John Wiley & Sons, Ltd.