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 α-pinene (42.2%), β-pinene (10.6%), germacrene D (9.7%), β-copaen-4α-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

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... ainst 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.