Rotational barriers in butanes

Murcko, M. A., Castejon, H., Wiberg, K. B., 1996, Carbon-Carbon Rotational Barriers in Butane, 1-Butene, and... 

Figure 5-8 Conformational energies and rotational barriers in butane, the difference in energy between the anti and gauche forms is 0.8-0.9 kcal mole-1. The energies are relative to conformation 7 as zero.

Allinger N L, R S Grev, B F Yates and H F Schaeffer III 1990. The Syn Rotational Barrier in Butane Journal of the American Chemical Society 112 114-118. 

Calculate the rotational barrier between the anti and anticlinal forms of N-butane using the AMI (or PM3 if you prefer) and HF/6-31G(d) model chemistries. Use the results for the anti form that you obtained in Exercise 6.1. Note that the anticlinal form is a transition structure you will find the Opt TS,CalcFC] keyword helpful in optimizing this structure. 

If groups A and B are methyl groups (butane), the steric hindrance between A and B leads to a rotational barrier of 25 kj ( 6 kcal) per mole. The consequence of this simple fact is that in fatty acids and related substances and in polyethylene the chains of CH2 groups tend to assume fully extended zigzag conformations. 

In practical evaluation of force-fields parameters the H... H and C... C non-bonded potentials were chosen first. Calculations of staggered and eclipsed rotamers of ethane, propane and n-butane then revealed the contribution of the non-bonded interactions to the total rotation barrier. For example, the van der Waals parameters proposed by Alhnger et al. in their first paper 40) on calculation of conformations were such that the repulsion between hydrogens on adjacent carbon atoms accounted for about 31% of the barrier in ethane. The remainder was accoimted for as a quantity which was added by considering the torsion interaction to be zero for all... 

Although the barrier to rotation in //-butane is a little higher than in ethane, it is still low enough that—at ordinary temperatures, at least—interconversion of conformers is easy and rapid. Equilibrium exists, and favors a higher population of the more stable anti conformer the populations of the two gauche conformers—... 

Some of the interesting features in silanes which are well reproduced by the calculations include the rotational barriers (T7 kcal mole-1 in methylsilane as compared with 2-9 kcal mole-1 in ethane, and correspondingly low numbers for other methyl silanes). While the torsional function about the 2,3-bond in 1-sila-butane looks almost like that in butane, the corresponding function... 

The use of molecular dynamics simulations for chemical reactions with low barriers has been so far limited to isomerization reactions. The most common system studied has been the trans-gauche isomerization of -butane. The potential energy for rotation about the central C—C bond in -butane is shown in Figure 3. The trans conformation is at a dihedral angle defined to be 0° while there are two equivalent, higher energy gauche conformations at... 

The gauche conformation of butane already shows conformational distortions in that the C—C—C—C dihedral angle is opened to about 70°, while the methyl group hydrogens are calculated to be skewed coherently by about 4.6°. The rotational barrier to anti-gauche interconversion derived spectroscopically is 3.63 kcal mol and as to the methyl eclipsing-methyl barrier, there has been considerable speculation described recently by Allinger and coworkers", who conclude it to be 4.89 kcal mol... 

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