Alkanes rotational barriers

VViberg K B and M A Murcko 1988. Rotational Barriers. 2. Energies of Alkane Rotamers. An Examination of Gauche Interactions. Journal of the American Chemical Society 110 8029-8038. 

The results of a valence bond treatment of the rotational barrier in ethane lie between the extremes of the NBO and EDA analyses and seem to reconcile this dispute by suggesting that both Pauli repulsion and hyperconjugation are important. This is probably closest to the truth (remember that Pauli repulsion dominates in the higher alkanes) but the VB approach is still imperfect and also is mostly a very powerful expert method [43]. VB methods construct the total wave function from linear combinations of covalent resonance and an array of ionic structures as the covalent structure is typically much lower in energy, the ionic contributions are included by using highly delocalised (and polarisable) so-called Coulson-Fischer orbitals. Needless to say, this is not error free and the brief description of this rather old but valuable approach indicates the expert nature of this type of analysis. 

K. B. Wiberg, M. A. Murcko. Rotational barriers. 2. Energies of alkane rotamers. An examination of gauche interactions. 

Table II extends the values given originally by Pitzer and now encompasses all the n-alkanes of interest to us. The model used can be typified by that shown in Figure 5 for one octyl side chain in poly(di-n-octyl itaconate). It has been calculated O that the rotational barriers for the (>C-CO) and (O-Cj) bonds will be too high to allow free rotation at the temperature, and so this section of the chain is considered to be immobile. Rotation of the terminal methyl unit is a low energy process and will already be occurring, so one can restrict consideration to the six remaining (C-C) bonds for poly(di-/i-octyl itaconate). All of these will contribute to the heat capacity change and as there are two side chains per monomer unit, twelve bonds must be considered in the calculation. The variation of Cy as a function of temperature is shown for the homologous series of n-alkanes in Figure 6, but only the n-octyl will be considered here.

Continuing interest in rotation barriers about single bonds in acyclic radicals, as determined by the interpretation of e.s.r. data, is illustrated in a study of R CHSR radicals and alkane-, arene-, and alkoxy-sulphonyl radicals (obtained by high-intensity u.v. irradiation of sulphinic acids in the presence of di-t-butyl peroxide at low temperatures). Further evidence for hindered rotation about the C—S bond in these species is obtained, - and an unusual order of proton hyperfine splittings a 0-H)> a (a-H) a (y-H) is reported for propanesulphonyl radicals. The potential of the e.s.r. method in conformational analysis is shown in studies of radical cations (3) from 2,5-bis(alkylthio)thiophens, where the S-cis-cis conformer (3) is identified as more stable than other rotamers this is assumed to be the case too for the neutral molecule. 

Smith G D and R L Jaffe 1996. Quantum Chemistry Study of Conformational Energies and Rotational Energy Barriers in u-Alkanes. Journal of Physical Chemistry 100 18718-18724,... 

Propane, the next higher member in the alkane series., also has a torsional barrier that results in hindered rotation around the carbon-carbon bonds. The barrier is slightly higher in propane than in ethane—a total of 14 kj/mol (3.4 kcal/mol) versus 12 kj/mol. 

The energetical description of rotations around bonds with high torsional barriers (e.g. the C=C double bond) demands the evaluation of the influence of higher cosine terms. Rotations around single bonds with sixfold symmetric torsional potentials have very low barriers (18) they occur in alkylsubstituted aromatic compounds (e.g. toluene), in nitro-alkanes and in radicals, for example. 

Heats of formation assume resonance stabilizations 10.8 kcal mole-1 in ( CHaCN) 12.6 kcal mole-1 in (CH3CHCN) and in [(CH3)2CCN]. " Na = doubly bonded nitrogen in azo compounds. h This correction assumes that the barrier to rotation in the radical R is two-thirds the barrier in the corresponding alkane RH. See O Neal and Benson for further discussion of this point. AH° and to +2 cal mole-1 °K 1 for 5°. The following example shows how the table is used to calculate thermodynamic properties for the 2-butyl radical (12).41 H3C—ch—ch2—ch3 12 ... 

As we have seen, the anomeric effect confers a double-bond character to each C—0 bond of conformer D the energy barrier for a C —0 bond rotation in acetals must therefore be higher than that observed in simple alkanes. Borgen and Dale (41) may have provided the first evidence for this point by observing that 1,3,7,9-tetraoxacyclododecane (37) has a much higher conformational barrier (11 kcal/mol) than comparable 12-membered rings such as cyclododecane (7.3 kcal/mol (42) or 1,4,7,10-tetraoxacyclododecane (5.5 and 6.8 kcal/mol (43)). It was also shown that the two 1,3-dioxa groupings in 37 exist in a conformation identical to that of dimethoxymethane, i.e. the conformation D. 

Smith, G. D. Jaffe, R. L. Quantum chemistry study of conformational energies and rotational energy barriers in n-alkanes, J. Phys. Chem. 1996,100,18718-18724. 

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