Rotamers, anti/gauche

Figure 3.68 Optimized rotamers of triaminomethyl fluoride 29 (a) gauche gauche-gauche, (b) anti-gauche-gauche, (c) anti-anti-gauche, and (d) anti-anti anti. (The aaa rotamer is a transition state between equivalent aag rotamers).

Provided that the mutual size differences between H, M and L are significant, it is rather easy to predict the populations of the rotamers. Among the an/i-rotamers, anti-1, with antiperipla-nar hydrogens, is the only one devoid of the L1... L2 interaction. In the syn series, syn-2, with the hydrogens gauche, lacks the L1... L2 interaction. Therefore, it is expected that anti-1 and syn-2 dominate in their respective equilibrium, as would the respective coupling constants ... 

Ketones also favor eclipsed conformations. The preference is for the rotamer in which the alkyl group, rather than a hydrogen, is eclipsed with the carbonyl group because this conformation allows the two alkyl groups to be anti rather than gauche. Electron diffraction studies of 3-pentanone indicate the conformation shown to be the most stable rotamer, in accord with this generalization. ... 

In the reaction between anti-W2Cl2(NEt2) and alkyllithium reagents LiR (2 equiv), substitution of Cl-by-R occurs with retention of configuration (18). Kinetically anti-WgRgfNEtg) is formed which then isomerizes to a mixture of anti and gauche rotamers, with the latter being the favored rotamer. This substitution reaction can be viewed as an example of an S 2 process in... 

Many of the trisilanes exist as mixtures of rotational rotamers, as can be proved with variable temperature Raman spectroscopy. As an example Fig. 2 shows the v(SiCl) region of the Raman spectrum of ClMe2SiSiMe2SiMe3, which exists as a mixture of anti- and gauche- rotamers at room temperature. 

All of the ground-state total-energy differences, regardless of basis set or correlation treatments, give essentially the same result. For example, MBPT(2) calculations with the 6-3IG basis favor the gauche conformation over the anti conformation by just 0.04 kcal/mol. The barrier between these two conformers is 0.6 kcal/mol, but the barrier between the two gauche forms is 1.2 kcal/mol. Steric factors will dominate considerations of the relative energies of substituted rotamers. 

Further studies of the Raman spectra of MeX2SiSiX2Me and Me2XSiSiXMe2 revealed that all these disilanes exist as mixtures of anti and gauche rotamers. 

Hindered rotation has frequently been observed in halogenated alkanes. The increased barrier probably arises from a combination of steric and electrostatic interactions. 2,2,3,3-Tetrachlorobutane (5-8) exhibits a 2 1 doublet below —40°C from anti and gauche rotamers that are rotating slowly on the NMR time scale. 

The situation is more complicated for 3/hcoh involving the C-l-O-l bond, because in one of the rotamers about this bond, HO-1 is anti to 0-5, which (by Booth s effect) reduces the value of 3/hcoh in this rotamer, causing the magnitudes of the two gauche... 

Figure 17. Destabilization of anti and gauche rotamers [about C(2) conformers of 55,108, and 109.

---