Ethane rotational barriers

The ethane rotational barrier and wave function analysis... 

Several explanations have been given for the physical origin of the ethane rotational barrier. One analysis attributes the barrier to the Pauli-exclusion-prindple repulsion (Section 10.3) between the eclipsing localized C—H bonding electron pairs in eclipsed ethane [R. M. Pitzer, Acc. Chem. Res., 16,207 (1983)], but several workers have rejected this explanation and offered other explanations [R. F. W. Bader et al.,... 

Recall (Section 3 1) that the rotational barrier in ethane IS only 12 kJ/mol (3 kcal/mol)... 

The rotational barrier in methylsilane (Table 3.4, entry 5) is significantly smaller than that in ethane (1.7 versus 2.88 kcal/mol). This reflects the decreased electron-electron rqjulsions in the eclipsed conformation resulting from the longer carbon-silicon bond length (1.87 A) compared to the carbon-carbon bond length (1.54 A) in ethane. 

The haloethanes all have similar rotational barriers of 3.2-3.7 kcal/mol. The increase in the barrier height relative to ethane is probably due to a van der Waals rqjulsive efiect. The heavier halogens have larger van der Waals radii, but this is ofiset by the longer bond lengths, so that the net efiect is a relatively constant rotational barrier for each of the ethyl halides. 

Changing the atom bound to a methyl group from carbon to nitrogen to oxygen, as in going from ethane to methylamine to methanol, produces a decrease in the rotational barrier from 2.88 to 1.98 to 1.07kcal/mol. This closely approximates the 3 2 1 ratio of the number of H—H eclipsing interactions in these three molecules. 

The K constant is usually taken as 1.75 this value reproduces the rotational barrier in ethane. 

The four-electron destabilization rationale The rotation barrier of ethane is sometimes explained in terms of the mnemonic energy-level-splitting diagram shown in Fig. 3.58. The figure purports to depict how two filled MOs of ethane ( and 4>+) evolve perturbatively from two... 

We conclude that the four-electron stabilization rationalization lacks both physical and numerical relevance to barrier problems and should not be taken as evidence in support of a picture of the rotation barrier of ethane based on steric repulsions. 

Table 3.23. Rotation barriers (Ecc — Estg) and leading a-cr donor acceptor stabilizations (—A Eaa ) in anti and syn orientations for methyl rotors (CH3—X, X = CH3, NH2, OH) and higher ethane-like congeners...

In comparison with previous plots of this section, the no-crco anomeric interaction of Fig. 3.65 can be seen to be a rather typical example of hyperconjugative donor-acceptor interactions. Consequently, there seems to be no valid reason to invoke a special effect for the conformational preferences of sugars, obscuring their essential conformity with a unified donor-acceptor picture of ethane-like rotation barriers. 

The strikingly different characteristics of transition-metal hyperconjugative interactions are particularly apparent in their influence on internal rotation barriers. To illustrate, let us first consider ethane-like Os2H6, whose optimized staggered and eclipsed conformations (displaying conspicuous deviations from those of ethane) are shown in Fig. 4.81. 

The nature of the rotational barrier in ethane is not easily explained. It is too high to be due to van der Waal s forces. It is considered to arise by interactions among the electron clouds of C—H bonds and quantum mechanical calculations show that the barrier should exist. 

Semi-empirical models are markedly inferior to all other models dealt with (except the SYBYL molecular mechanics model) for barrier calculations. Major trends in rotation barriers are often not reproduced, for example, the nearly uniform decrement in rotation barrier from ethane to methylamine to methanol. None of the semi-empirical models is better than the others in this regard. One the other hand, AMI is clearly superior to MNDO and PM3 in accounting for nitrogen inversion barriers. All in all, semi-empirical models are not recommended for barrier calculations. 

Delocalization of the odd electron into extended n systems results in considerable radical stabilization. The C—H BDE at C3 of propene is reduced by 13 kcal/mol relative to that of ethane. That the stabilization effect in the allyl radical is due primarily to delocalization in the n system is shown by the fact that the rotational barrier for allyl is 9 kcal/mol greater than that for ethyl. Extending the conjugated system has a nearly additive effect, and the C—H BDE at C3 of 1,4-pentadiene is 10 kcal/mol smaller than that of propene. Delocalization of the odd electron in the benzyl radical results in about one-half of the electron density residing at the benzylic carbon, and the C—H BDE of the methyl group in toluene is the same as that in propene. 

Determination of the Rotational Barrier in Ethane by Vibrational Spectroscopy and Statistical Thermodynamics 166... 

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