Torsional energy profile

Just as quantum chemical calculations are able to locate and quantify both the stable conformers and the transition states connecting stable conformers for a flexible molecule, so too are they capable of obtaining the full torsional energy profile. This may then be fitted to whatever series is appropriate. Indeed, modern programs like Spartan automate the process. The quality of fit of the actual data to the empirical form should be a good criterion for selecting the functional form. 

At the time the CLAP force field was proposed, many of the existing ionic liquid models used to borrow parameters from different, not always compatible, sources. For instance, it was common to see parameterizations of the cation and of the anion using information from different force fields [10,11,13], In the development of the CLAP force-field, in order to respect internal consistency, ab initio calculations were used extensively to provide essential data for the development of an internally consistent force field. This included molecular geometry optimization and the description of electron density using extended basis sets, leading to the evaluation of force field parameters such as torsion energy profiles and electrostatic charges on the interaction centers. 

In 2004, Canogia Lopes et al. introduced a new force field for CnCnim + cations. The model was based on the OPLS-AA/AMBER parameters. In order to obtain several terms in the force field that had not been defined in the literature, ab initio calculations were performed [60], These included torsion energy profiles and distributions of atomic charges. Validation was carried out by comparing simulated and experimental data on 14 different salts. 

Figure 12.16. Fully optimized (B3LYP/6-311G ) oligomeric fragment of the SSC PFSA pol5mer with seven -CF2- units in the backbone separating the side chains. Arrows indicate the six different bonds for which torsional energy profiles were calculated (see Figs. 17 and 18).

These non-bonded parameters were subsequently applied to evaluate the torsional energy profile of the two model compounds. Following the methodology outlined by Cornell et al., the low energy conformers of the... 

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