Abstractl Density functional theory (DFT) was used to investigate the effects of intra-moecular interactions and implicit water molecules on the relative stability and the NMR shielding tensors of hallucinogenic harmine in the monomeric and dimeric states. Results represented that the relative stability and the NMR shielding tensors are dependent on the resonance interactions and chemical environment However, their values are obviously different in the considered states and the media. The energetic data demonstrated that dimeric and monomeric states of harmine in water medium are more stable than they are in the gaseous form. Based on NBO interpretation and NMR calculations it can also be concluded that by increasing lone pair electrons contribution of nitrogen atoms in resonance interactions and aromaticity development in the monomeric dimeric states, the values of NMR chemical shielding around them increase. Keywords: Harmine, NMR chemical shielding, NBO interpretation, monomer, dimer β-Carboline derivatives (H-pyrido [3, 4-b] indol derivatives) (BC) make up a group of drug-binding alkaloids, widely distributed in nature. Harmine [C13H12N2O] (MeO-1-1Me-9H-7pyrido [3,4-b]-indole ) is a well known member of the β-caroline alkaloid family تفاصيل المقالة