Relative energies of conformers

The HF method overestimates the barrier for linearity by 0.73 kcal/mol, while MP2 underestimates it by 0.76 kcal/mol. Furthermore, the HF curve increase slightly too steeply for small bond angles. The MP4 result, however, is within a few tenths of a kcal/ mol of the exact result over the whole curve. Compared to the bond dissociation discussed above, it is clear that relative energies of conformations which have similar bonding are fairly easy to calculate. While the HF and MP4 total energies with the aug-cc-pVTZ basis are 260 kcal/mol and 85 kcal/mol higher than the exact values at the equilibrium geometry (Table 11.8), these errors are essentially constant over the whole surface. 

Table 44. Calculated relative energies of conformers of HO2F and F2O2...

Although many satisfactory VCD studies based on the gas phase simulations have been reported, it may be necessary to account for solvent effects in order to achieve conclusive AC assignments. Currently, there are two approaches to take solvent effects into account. One of them is the implicit solvent model, which treats a solvent as a continuum dielectric environment and does not consider the explicit intermolecular interactions between chiral solute and solvent molecules. The two most used computational methods for the implicit solvent model are the polarizable continuum model (PCM) [93-95] and the conductor-like screening model (COSMO) [96, 97]. In this treatment, geometry optimizations and harmonic frequency calculations are repeated with the inclusion of PCM or COSMO for all the conformers found. Changes in the conformational structures, the relative energies of conformers, and the harmonic frequencies, as well as in the VA and VCD intensities have been reported with the inclusion of the implicit solvent model. The second approach is called the explicit solvent model, which takes the explicit intermolecular interactions into account. The applications of these two approaches, in particular the latter one will be further discussed in Sect. 4.2. 

The difference between threo and erythro penultimate effects rests with the relative energies of conformations 8A and 8D and, more importantly, the transition states leading from these conformations. Transition states deriving from 8A are favored... 

Figure 6.26 Geometries and relative energies of conformers of threo-and erythro-2,3-difluorobutane in vacuo and in water. The lower energy conformer(s) for each of the diastereomers is shown in blue.

Relative energies of conformations of GABA Comparison of PCILO vs ab initio results... 

TABLE 4.6 ENERGY COSTS FOR COMPARING THE RELATIVE ENERGY OF CONFORMATIONS... 

Fig. 2.31) provides a nice example of conformational analysis, the study of the relative energies of conformational isomers. Let s begin this analysis by constructing a Newman projection by looking down the C(2)—C(3) bond of butane... 

Table 5.4 The relative energy of conformers of the 2 -deoxyribonucleotides anions, kcal/mol...

Additionally, for flexible molecules, the presence of multiple conformations may require the consideration of solvent effects, mainly if experimental data in polar solvents are to be reproduced. The relative energies of conformers and their chiroptical properties can be largely affected by solvent effects, and thus, in some cases, the inclusion of either the polarizable continuum model (PCM) or the conductor-like screening model (COSMO) since geometry optimization steps may be beneficial." ... 

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