Jan 21st

Major contributions which determine stability of trans conformation of amides are (1) conjugation effect from a planar geometry and (2) non-bonded interactions (carbonyl oxygen --- aromatic hydrogen)
From a small test set with six different but similar amides, it was found that even among the similar molecules, cis/trans energy differences vary.
To avoid defining additional torsion terms with very specific SMIRKS pattern, → better description of non-bonded interaction and different strength of conjugation effect → (1) vdW parameter fitting to QM torsion scan or (2) using WBO interpolated parameters for the spring constant of one-fold cosine function or (3) both (how to fit the two types of parameters should be considered. either step by step or simultaneously)

Except for one torsion scan (scan1), other torsion scans from the smaller basis set(DZVP) agree well enough with those from larger basis sets(def2-TZVP(-f), cc-pVTZ(-f))
Current choice of method is not unreliable, but some cases(such as scan1) support our concerns on the size of the current basis set to some extent.

To see if the usage of WBO interpolated parameter for the spring constant of one-fold cosine function(which is for describing the cis/trans conformation preference), linear dependences of cis/trans energy differences and WBOs from different methods(OFF AM1-Wiberg method, DFT) were plotted.

Note that there’s a disagreement of the order of AM1 WBOs with the order of WBOs obtained from DFT.

To remove the non-covalent contributions from the energies and see the linear relationship between covalent contributions to the energies and WBOs, two approaches are used.

Force Fields