Conformation, approximate optimization from

Approximate Conformation Optimization from Fragment Models... 

In this chapter, we first present a brief overview of the experimental techniques that we and others have used to study torsional motion in S, and D0 (Section II). These are resonant two-photon ionization (R2PI) for S,-S0 spectroscopy and pulsed-field ionization (commonly known as ZEKE-PFI) for D0-S, spectroscopy. In Section HI, we summarize what is known about sixfold methyl rotor barriers in S0, S, and D0, including a brief description of how the absolute conformational preference can be inferred from spectral intensities. Section IV describes the threefold example of o-cholorotoluene in some detail and summarizes what is known about threefold barriers more generally. The sequence of molecules o-fluorotoluene, o-chlorotoluene, and 2-fluoro-6-chlorotoluene shows the effects of ort/io-fluoro and ortho-chloro substituents on the rotor potential. These are approximately additive in S0, S, and D0. Finally, in Section V, we present our ideas about the underlying causes of these diverse barrier heights and conformational preferences, based on analysis of the optimized geometries and electronic wavefunctions from ab initio calculations. 

The moments of inertia A, B, C may be calculated from the optimized structures and the vibrational energy levels v,-, and these are available from the second-derivative matrix if a full-matrix Newton-Raphson refinement is used124,511. However, the approximations involved in the calculation of the entropies, that so far have been used for the computation of the conformational equilibria of coordination compounds, have led to considerable uncertainties163,86-881. 

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