| Variable temperature proton NMR parameters (chemical shifts and coupling constants) have been obtained for cis- and trans-2,2,5-trimethyl-3-hexene and for cis-2,5-dimethyl-3-hexene using high precision experimental techniques, coupled with a powerful spectral analysis technique (NUMARIT).;Detailed structural and energetic information for the above-mentioned molecules was obtained employing a Force Field procedure.;The general approach for the interpretation of the experimental results involved the combined use of the ((DELTA)J/(DELTA)(theta)) data calculated by Rummens and Kaslander, the structural geometry calculated using the Force Field method and the theorem of Boltzmann population statistics. In many cases the technique is proven to be reliable.;No such generally useful structure-parameter stratagem is found for the interpretation of chemical shifts. Differential solvent effects are suspected to be the cause of this failure. For the same reasons, the carbon-13 spectra, obtained for cis- and trans-2,2,5-trimethyl-3-hexene were not useful for the interpretation of the rotameric changes.;The temperature dependence of coupling constants for trans-2,2,5-trimethyl-3-hexene is explained in terms of an anti-gauche equilibrium. The difference in Gibbs free energy between the anti and gauche forms could be determined as (DELTA)G('0) = 531 (+OR-) 59J.mol('-1) (127 (+OR-) 14cal.mol('-1)); the Force Field method overestimates this difference by about 4KJ.mol('-1) (lKcal.mol('-1)). No evidence of interaction between the isopropyl and tert-butyl rotors could be found.;In cis-2,2,5-trimethyl-3-hexene, it is concluded that the two substituents (isopropyl and tert-butyl groups) are in anti position in the ground state. It seems clear that there exists a second low-lying minimum ((DELTA)G('0) (TURNEQ) 2.1KJ.mol('-1) or 0.5Kcal.mol('-1)) not in concordance with the Force Field calculation.;The results on cis-2,5-dimethl-3-hexene give definitely a ground state with an anti-anti conformation. There exists a low energy second conformer ((DELTA)G('0) (TURNEQ) 1.13KJ.mol('-1) or 0.27Kcal.mol('-1)), but its structure could not be determined. The anti-syn structure obtained from the Force Field as second conformation is disproved by the NMR results. The latter point to a skew-skew structure as a likely form. |
