Orbital Methods Applied to Conformational Analysis

The molecular mechanics approach to conformational analysis has the virtue of analyzing conformational factors in terms of molecular properties that are easy to describe in physical terms. Contributions from bond-length distortions, bond-angle strain, van der Waals repulsions, and dipole-dipole interactions can be calculated. The use of carefully chosen potential functions can give highly precise information as to the relative energies of various conformations. The accuracy and reliability of the calculation depend on the potential functions and parameters that describe the various interactions. 

The analysis of conformational equilibria can also be approached from the molecular orbital viewpoint. The most stable conformation can be identified by searching for the energy minimum as a function of molecular geometry. Conformational barriers can be evaluated by calculation of the energy of the presumed transition state for the conformational change.  

The interpretation of molecular orbital calculations on conformational isomers is not as straightforward as for molecular mechanics methods. Because MO calculations treat all of the bonding forces of the molecule, the difference between two conformations represents only a small part of the total energy. Furthermore, unlike the molecular mechanics model in which energies are assigned to specific interatomic interactions, the energy of a specific molecular orbital may encompass contributions from a number of intermolecular interactions. Thus, the identification of the structural features responsible for the energy difference between two conformers may be very difficult. 

The preference for the gauche arrangement is an example of the anomeric effect. An oxygen lone pair is anti to fluorine in the stable conformation but not in the unstable conformation. 

SECTION 3.6. MOLECULAR ORBITAL METHODS APPLIED TO CONFORMATIONAL ANALYSIS 

Bond lengths are given in A. The axial C-Q bond length is 1.82 A the equatorial C-Cl bond length is 1.78 A. 

The normal carbon-chlorine bond distance is 1.79 A, and, as can be seen, the equatorial C-Cl distance is observed to be almost exactly that. The axial C-Cl distance, however, is significantly longer (1.82 A), consistent with electron release into a carbon-chlorine antibonding orbital, weakening the C-Cl bond. The carbon-oxygen bond lengths, moreover, depend on the orientation of the chlorine on carbon in a way that is completely consistent with the explanation proposed. Electron delocalization is more important when the chlorine is axial the carbon-oxygen bond has more double-bond character and is shorter (1.39 versus 1.43) than when the chlorine is equatorial. 

Molecular Orbital Methods Applied to Conformational Analysis 

The importance and generality of the anomeric effect emphasize the fact that molecular conformation is not solely governed by nonbonded steric interactions. Both attractive and repulsive electronic interactions which have specific geometric (stereoelectronic) requirements are also a significant factor in determining molecular conformation. 

The molecular mechanics approach to conformational analysis has the virtue of describing molecular properties in terms that are physically easily understood. 

Several of the more common MO methods were mentioned in Chapter 1, and it was noted that most of the methods were very good at predicting minimum-energy geometries. There have been several calculations of the rotational energy barrier in ethane and related molecules that have matched the experimentally determined barriers quite well. Rather than elaborating on all the conformational equilibria that have been treated by MO methods, the emphasis in this section will be on the unique information available through MO calculations that is not directly provided by molecular mechanics calculations. 

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