Gauche minima

As in the case of ethane, eclipsed forms of /7-butane do not exist, and correspond only to hypothetical structures in between anti and gauche minima. Unlike ethane, which is a single pure compound, any sample of /7-butane is made up of two distinct compounds, anti /7-butane and gauche /7-butane. The relative abundance of the two compounds as a function of temperature is given by the Boltzmann equation (see discussion following). 

A rather large barrier (20 kJ/mol)174 has been estimated between anti and a possible second conformer. A recent theoretical study 7S leads to an additonal,but much lower barrier of 2 kJ/mol at the syn position, thus separating the two gauche minima, estimated to be at about 40 ° from syn. This potential would lead to a high degree of flexibility in the syn region. A qualitative representation of the total potential based upon the theoretical values 75, 76, is given in Fig. 9. 

Systems with bulky substituents on sp -hybridized atoms are often found to have four minima, the anti conformation being split into two minima with torsional angles of "approximately 170°. Other systems, notably polyfluoroalkanes, also split the gauche -minima info two, often ealled gnuchp. tangle of approximately +50°) and ortho (angle... 

Halosubstituted 1,3-butadienes provide useful information on the configurational problem about the central single bond the observed stable forms of halo-1,3-butadienes are listed in Table 26. On this basis the potential function for rotation about the central bond has three minima, corresponding to the frans-planar form and to the right and left gauche nonplanar forms (a dihedral angle of about 50° from the cis) as has been predicted by the SCF calculations for 1,3-butadiene. The relative stabilities of the two forms depend sensitively on the type of substitution (Table 26). It appears that 1,1,3-substitutions make the gauche minima lower than the trans. 

Conformation energies computed for this molecule in the neighbourhood of the trans and gauche minima for bond 2 indicate that the gauche minimum occurs at 112.5° rather than the expected 120° and has a value of 2215 J moC. ... 

The tg and g t minima are equivalent to those for w-butane. The g g and g g minima (not shown) occur in the vicinity of (p2 = (p3 = ll0°. Thus the gauche minima for two adjoining bonds in gauche conformations of the same sign are mutually displaced a few degrees from the values ( 112.5°) which would be assumed by each if both of its neighbours were trans. 

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