NMR spectra of 1,3-butadienes: VII—the conformation of 2,4-dimethyl-2,4-pentadiene

Proton magnetic resonance spectra of 2,4-dimethyl-2,4-pentadiene have been examined for a number of solution conditions and (for a CF₂Cl₂ solution) over a range of temperatures. Coupling constants for the ethylenic protons were obtained from double resonance (methyl decoupled) spectra. Further double resonance experiments established the sign of ⁴J_c. It is found that ⁴J_c = ?1·20 Hz and ⁴J_t] = 0·18 Hz. It is concluded that the compound exists in a single nonplanar conformation with an average dihedral angle (from the s-cis form) ? = 50° ± 15°. Some long range coupling constants involving the methyl group were found to be appreciable in spite of the lack of planarity of the diene chain.