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Ab initio IGLO studies of the conformational dependences of 13 C NMR chemical shifts and α‐through ε‐substituent effects in 1‐substituted pentanes
Author(s) -
Barfield Michael
Publication year - 1998
Publication title -
magnetic resonance in chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.483
H-Index - 72
eISSN - 1097-458X
pISSN - 0749-1581
DOI - 10.1002/(sici)1097-458x(199806)36:13<s93::aid-omr293>3.0.co;2-h
Subject(s) - dihedral angle , chemistry , substituent , chemical shift , ab initio , computational chemistry , steric effects , carbon 13 nmr , torsion (gastropod) , isotropy , crystallography , methylene , molecule , stereochemistry , organic chemistry , hydrogen bond , medicine , physics , surgery , quantum mechanics
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 13 C NMR chemical shifts to n ‐hexane and three 1‐substituted pentanes XCH 2 CH 2 CH 2 CH 2 CH 3 (X=CN, OH, F). Isotropic shifts were obtained as a function of the torsion angles φ 1 , φ 2 and φ 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 13 C chemical shifts and substituent effects, averaged over the torsional motions, compare favorably with the experimental data. The computed stereochemical dependencies of α‐ through ε‐effects are compared with experimental values in several series of bicyclic molecules which encompass a range of the three torsion angles. Emphasized here are δ‐ and ε‐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 δ‐methylene carbons which can be shielded or deshielded depending on φ 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. © 1998 John Wiley & Sons, Ltd.