13C‐NMR Spectroscopic and Conformational Studies on Rotations in Substituted Ethanes

Possibilities and results of13C‐DMR line shape analysis are explored with 1X‐2,2‐dimethylpropanes (neopentanes,1), 2X‐3,3‐dimethylbutanes (2) and 2X‐2,3,3‐trimethylbutanes (3) with X = H, Hal, OR or CH3. The observed barriers ΔG* consistently increase in this sequence, e.g. for X = Cl from 6.5 (1) to 7.7 (2) to 10.4 (3) kcal/mol. Molecular mechanics force field calculations for1, although giving barriers too low by 0.5–1.5 kcal/mol, correctly reproduce the observed trends and explain the puzzling low barriers observed for X = CH3in comparison to X = Hal, as the result of the higher ground state energy of the methyl compounds. Conformational equilibria are investigated with the non‐topomeric 2X‐2,3‐dimethylbutanes. The ΔG0values obtained from low temperature signal integration agree with force field predictions only for X = H and F, which dominate, as X = OR does, in the all‐gauche conformation; in contrast, the predominance of the trans conformer for X = Cl, Br and J is not rationalized. The calculated ground state geometries show no significant geometry distortion upon change of the substituents X, which provides a basis for the13C shielding comparison to corresponding cyclohexanes of similar substitution pattern. The observed substituent effects on the13C shifts are in agreement with other models and mechanistic proposals for the screening constant variations.