The eleven monohydrates of 1, 2, 3, 4-tetrahydroisoquinoline (THIQ) are analyzed through natural bond orbital (NBO) analysis and QTAIM methods employing M06-2X functional in DFT and MP2 methods. Here, the role of OH bonds as an acceptor and donor is critically analyzed. The role of lone pairs of O is critically monitored in two of the complexes, where N–H···O hydrogen bonds are present. The relative contributions of rehybridisation and hyperconjugation are compared in detail. Popelier criteria are satisfied in all the complexes barring a few exceptions involving weak hydrogen bonds. At the bond critical points (BCP), four monohydrates show higher values of electron density (ρ C ) and negative values of total electron energy density (H C ), while Laplacian ( ∇ 2 ρ C ) remains positive. These complexes satisfy the criteria of partial covalency. All these are O–H⋅⋅⋅N-type bonds. Remaining h-bonds are weaker in nature. These are also confirmed by the smaller values of ρ C at the respective BCP. The variation of potential energy density (V C ) among the complexes seems to be the most important factor in determining the nature of non-covalent interactions.