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

Abstract

Abstract Possibilities and results of 13 C‐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 CH 3 . The observed barriers Δ G * 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 for 1 , 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 = CH 3 in 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 Δ G 0 values 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 the 13 C shielding comparison to corresponding cyclohexanes of similar substitution pattern. The observed substituent effects on the 13 C shifts are in agreement with other models and mechanistic proposals for the screening constant variations.