Ab Initio and Density Functional Evaluations of the Molecular Conformations of β-Caryophyllene and 6-Hydroxycaryophyllene

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Abstract
The four conformations of β-caryophyllene (αα, αβ, βα, and ββ) were investigated ab initio at the 6-31G*/HF and MP2 levels and additionally with density functional methods (B3LYP/6-31G*), as it concerns their relative thermodynamic stabilities. The αα is predicted to be the most stable geometry, in agreement with low-temperature NMR measurements. In the case of 6-hydroxycaryophyllene, the αα is still the most stable conformation when the configuration at C-6 is S, but when the configuration is reversed to R the ββ geometry becomes the most stable one. This is again in agreement with NMR data. On the other hand, for both molecules the AM1 semiempirical model Hamiltonian fails to predict the αα as a low-energy geometry, mainly due to an incorrect description of the cyclobutane ring puckering. The interconversion paths among the different minima are also analyzed and discussed. The solvent effect (either chloroform or water) on the stability of the different conformers of β-caryophyllene and 6-hydroxycaryophyllene was studied in the polarizable continuum model framework.