Rotational constants for HCN and DCN

Microwave measurements of rotational transitions within vibrationally excited states of several isotopic species of HCN have given improved values for the pertinent B_v constants. These new data have been combined with infrared measurements given in the literature (ncluding D_v terms) to arrive at a set of rovibrational constants (α and γ constants) which is consistent with all available data. Bond distances resulting from several different B_e approximations are intercompared to assess the variability of the r_e values and the importance of the γ terms. The latest r₀ and r_s bond distances are also given.