The essential oil from the dried leaves of Chromolaena odorata (L.) R.M. King & H. Rob. was
obtained by hydrodistillation and analyzed by gas chromatography-mass spectrometry (GC-MS). The major
components were -pinene (42.2%), -pinene (10.6%), germacrene D (9.7%), -copaen-4-ol (9.4%), (E)-
caryophyllene (5.4%), and geijerene/pregeijerene (7.5%). The oil was screened for antimicrobial activity and
showed antibacterial activity against Bacillus cereus (MIC = 39 μg/mL) and antifungal activity against
Aspergillus niger (MIC = 78 μg/mL). DFT (B3LYP/6-31G*) and post-HF (MP2/6-311+G**) indicate that
pregeijerene is less stable (0.45 and 3.99 kcal/mol, respectively) than its Cope rearrangement product geijerene.