Understanding the thermal [1s,5s] hydrogen shift isomerization of ocimene

α-Ocimene, β-ocimenes and alloocimenes are iso- meric monoterpenes occurring naturally as oils within several plants and fruits. These thermally unstable compounds are employed in the pharmaceutical and fine-chemicals industries due to their natural plant defense properties and pleasant odors. In this work, and in the context of a recent revival in attention on the subject, we provide new theoretical insights concerning the nature of the electronic reorganization driving the decomposition ofcis-β-ocimenetoalloocimene.Our find- ings support the experimental proposal of a rearrangement via a six-membered cyclic transition state in a one-step concerted and highly synchronic process.