We compare the mesogenic properties of two-ring compounds, substituted at one end with n-alkyl groups and at the other end with alkyl, alkyloxy, omega-fluorinated alkyloxy, methylenoxy-alkyl and acyloxy groups. The different groups have different effect on the mesogeneity, and their effect is different when they are attached at benzene resp. cyclohexane rings. In order to find out rational reasons for this behaviour we performed quantum chemical calculations on the density functional theory and ab initio level. Contrary to existing opinions, in cyclohexane derivatives the calculated axial conformers were proven to have so low stability that in the thermal equilibrium they do not play a role. In alkyloxy compounds gauche conformers were found to be more stable than the most elongated all-trans conformers.