Energy rotation barrier, components

Ab initio calculations on these conformers indicate that the anti geometry is 6.04 kcal/mol more stable than the syn291 Some of this energy difference is undoubtedly due to the eclipsing interaction of the hydrogens but this can not account for the total rotational barrier. Thus, the a—p+ interaction effect must be responsible for some major component of the rotational barrier. 

Benzene and other aromatics alike are stable molecules, while cyclobutadiene and other antiaromatic molecules are unstable molecules.27-76 Similarly, allylic species are stable intermediates and possess significant rotational barriers. It may appear as a contradiction that, for example, the tr-component of benzene can be distortive but it still endows the molecule with special stability or that the distortive jr-component of allyl radical can lead to a rotational barrier. We would like to show in this section that these stability patterns derive from the vertical resonance energy which is expressed as a special stability because for most experimental probes (in eluding substitution reactions) the o-frame restricts the molecule to small distortion167 in which the vertical resonance energy is still appreciable, as shown schematically in Figure 5. 

Shaik and Bar102 demonstrated that allyl anion has a distortive jr-component but at the same time exhibits a rotational barrier. This analysis was reaffirmed later for allyl radical.5 Subsequently, Gobbi and Frenking93 pointed out that the total distortion energy of allylic species is very small because it reflects the balance of jr-distortivity opposed by the a-symmetrizing propensity. They further argued that along with this jr-distortivity, the allylic species enjoys resonance stabilization which is the source of the rotational barrier. A detailed VB analysis by Mo et al.149 established the same tendency. 

Hydroxyborane, H2BOH, has been studied171 with a (9,5,1)->[4,2,1] basis set. The OH bond length and the BOH angle were optimized. Of particular interest in this study was the rotational barrier about the BO bond. The planar form of the molecule is the most stable, and the computed barrier was 68.6 kJ mol-1. A partial 7r-bond is superimposed on the c-bond. An analysis of the energy and population components was carried out. 

It is also desirable to separate the resonance component of the total stabilization energy. Wiberg addressed the issue by comparing total stabilization with the rotational barrier, which should be a measure of the resonance contribution. The resonance component for F was assumed to be zero. This analysis provides the order NHj > OH for the TT stabilization but OH > NH2 for the a component, as shown in Table 3.22. 

A case in point concerns the barriers in ethane and in ethyl fluoride. The calculated internal rotation barriers are almost identical (2.58 kcal/mol for ethane and 2.59 kcal/mol for ethyl fluoride by ab initio SCF methods), and the experimentally determined barriers are similar (2.87 0.12 kcal/mol for ethane and 3.33 0.05 kcal/mol for ethyl fluoride). Table 3.8 lists the differences in energy components for the staggered and eclipsed forms of ethane and ethyl fluoride obtained by analysis of the results of the ab initio calculations. 

Isomeric alkenes may be either constitutional isomers or stereoisomers There is a sizable barrier to rotation about a carbon-carbon double bond which corresponds to the energy required to break the rr component of the double bond Stereoisomeric alkenes are configurationally stable under normal conditions The configurations of stereoisomeric alkenes are described according to two notational systems One system adds the prefix CIS to the name of the alkene when similar substituents are on the same side of the double bond and the prefix trans when they are on opposite sides The other ranks substituents according to a system of rules based on atomic number The prefix Z is used for alkenes that have higher ranked substituents on the same side of the double bond the prefix E is used when higher ranked substituents are on opposite sides... 

Torsional barriers are referred to as n-fold barriers, where the torsional potential function repeats every 2n/n radians. As in the case of inversion vibrations (Section 6.2.5.4a) quantum mechanical tunnelling through an n-fold torsional barrier may occur, splitting a vibrational level into n components. The splitting into two components near the top of a twofold barrier is shown in Figure 6.45. When the barrier is surmounted free internal rotation takes place, the energy levels then resembling those for rotation rather than vibration. 

Figure 3.51 The barrier to internal rotation about the C—C single bond of CH2FCH=CH2, showing the total energy (solid line), localized Lewis component ,(L) (dashed line), and delocalized non-Lewis component /f(NL (dotted line) as functions of the dihedral angle c/>cccf-...

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