The molecular structures, relative stabilities, dipole moments and rotational constants of HC_3N, HC_5N and their isomers are calculated by B3LYP/6-311++G~(**). In HC_(3-x)NC_x (x = 0～3), HC_3N is found to be the most stable structure. The rotational constants B_0 values of HC_3N and HC_2NC which are obtained from B3LYP/6-311++G~(**) are almost exactly in accordance with the experimental values. In HC_(5-x) NC_x(x = 0～5), HC_5N is found to be more stable than other isomers, but its dipole moment is relatively small. For the rotational constants B_0 of HC_5N, the relative standard deviation between the calculated value and the experimental value is only 0.6%.

Key words: PROPYNE P, NITRILE, density functional theory, dipole moment