The geometric and electronic structures, energetics and spectroscopy of different complexes formed between cytosine and a molecule of [C(n)mim]Br (n = 2,4,6,8,10) are investigated in detail using the M06-2X and omega B97XD methods with 6-311 + + G(2d, p) basis set. A great number of stable C-[C(2)mim]Br and C-[C(4)mim]Br complexes were obtained, these structures are classified as three types: approximately coplanar, parallel and perpendicular. The BSSE-corrected binding energies of the most stable structures (C(n)1) at the M06-2X method are -108.3 to -97.3 kJ.mol(-1), which higher than the results of omega B97XD. The aqueous solvation environment has an extremely weaken impact on the cytosine interaction with imidazolium bromide ionic liquid.