Vibration-rotation bands of acetylene and the molecular dimensions

In view of recent inconsistencies in the relation between the bond lengths and rotational constants in the isotopic acetylenes, some vibration-rotation bands of acetylene have been re-measured, and the results have been analyzed together with earlier work on this molecule. The α values for each of the molecular vibrations have been re-determined, taking into account possible vibrational perturbations and also anomalous changes in the centrifugal stretching constant D of the different vibrational levels. The new values of α, 1, α2, α3 and (α4 + α5) for acetylene are in line with the corresponding values for the deuterated derivatives, and using Bo = 1·1766 lead to Be + 1·1817. If bond lengths re(CH) = 1·0585, re(CC) = l·2047 Å are assumed, the results for all three isotopic molecules are mutually consistent. Values have also been obtained for the l-doubling coefficients, q4 and q5.