The aromatic oligoamide (arylamide) foldamer class, characterized by the repetitive aromatic-amide pattern, is one of the most intensively studied foldamer families. In this article, the potential energy profiles with regard to torsional motions around the two types of aromatic-amide bonds (CaCp and CaN) are obtained at the B3LYP/6-311G(d,p) level of theory. The effect of ortho substituents with different hydrogen bonding abilities (OCH3 vs. SCH3) on the torsional potential profiles is analyzed in detail. There are several findings that have implications in foldamer design. The ortho-SCH3 substituent on the benzene ring produces a much more flexible arylamide backbone with respect to the OCH3 substituent, as it restricts the CaCp torsion to a lesser extent. Interestingly, the rigidifying effect of the ortho-SCH3 substituent on the CaN torsion is very similar to that of the OCH3 substituent on the same linkage type. In addition, the SCH3 substituent prefers a perpendicular orientation with respect to the benzene ring to the in-plane one. It is also found that reparameterization of the corresponding torsional parameters, sometimes specific to the ortho substituent type, in the general amber force field is necessary for an accurate description of the backbone torsions in arylamides. Six sets of partial charge/torsional parameters for each linkage (CaCp or CaN)/substituent (OCH3 or SCH3) combination are obtained based on the ab initio torsional profiles. Initial assessments of these parameters show good agreement with the ab initio results. © 2011 Wiley Periodicals, Inc. J Comput Chem, 2011
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