Butanes conformations

The populations of the eclipsed forms of butane, like the eclipsed forms of ethane, are small and represent energy maxima for the molecule as rotation occurs about the central C-C bond. The energy differences between the butane conformations are represented diagrammatically in Figure 5-8. The valleys correspond to staggered forms and the energy difference between the anti and gauche forms is 0.8-0.9 kcal mole-1. 

S. Flumbel, S. Sieber and K. Morokuma, The IMOMO method Integration of different levels of molecular orbital approximations for geometry optimization of large systems test for n-butane conformation and s(n)2 reaction RC1 + C1, J. Chem. Phys., 105 (1996) 1959-1967. 

Butane conformations. Rotations about the center bond in butane give different molecular shapes. Three of these conformations have specific names. 

Table 2. Grid Coordinates for Carbon Atoms in Staggered n-Butane Conformers ...

Population distributions for butane conformers in the gas phase (solid line) and in water (dashed line) at 25°. The units of the vertical axis are relative mole fraction per degree of rotation. (Adapted from reference 41.)... 

Now let us consider rotation about the C2—C3 bond of butane. The barriers to rotation about the C2—C3 bond in butane are larger than for rotation about the C—C bond in ethane, but still not large enough to prevent the rotations that lead to all possible butane conformers. 

Internal relaxations of the chain it g) are infrequent in the time scale of such simulations, but the conformational equilibrium constant appears to differ in the liquid from the gas-phase value, gauche conformations being more favoured. Rebertus et al. alleviated this problem of infrequent transitions in a model of butane in CCI4 with an exact statistical mechanical theorem, " suggesting that the assumed spherical shape of the solvent favours the compact gauche states of butane, and conversely found that the solvent structure is sensitive to butane conformation. ... 

Potential energy of butane conformers as a function of the degree of rotation about the C-2 — C-3 bond. Green letters refer to the conformers (A-F) shown on page 123. 

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