Abstract: The internal motions of β-Elemene, which is anti-cancer compound, have been investigated by using variable temperature ¹³C NMR relaxation experiments for the rirst time.The results show that the hexacyclic ring in β-Elemene is almost rigid in experimental temperature range from 298K to 318K and the self-diffusion activation energy of overall motion of the molecule is 14 kJ/mol. All the intenal rotation activation energies for side Chain molccular groups CH₂=CCH₃-and CH₂=CH-connecting with the hexacyclic ring are 19 kJ/mol.The methyl group linking directly the hcxacyclic ring has much higher internal rotation activation energy,which is 18 kJ/mol, than those in two side chain groups CH₂=CCH₃-which connect with the hexacyclic ring at different positions and moreover, the methyl groups in the two side chain groups CH₂=CCH₃-have different internal rotation activation energies which are 7,7 kJ/mol and 2.8 kJ/mol, respectively; This may result from that the three methyl groups have different space circumstances in β-Elemene.

Key words: ¹³C spin relaxation, Nuclear cross relaxation, Internal motion, Small molecule, β-elemene