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 $\alpha $ 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 $\alpha _{1}$, $\alpha _{2}$, $\alpha _{3}$ and ($\alpha _{4}$ + $\alpha _{5}$) for acetylene are in line with the corresponding values for the deuterated derivatives, and using B$_{0}$=1$\cdot $1766 lead to B$_{3}$+1$\cdot $1817. If bond lengths r$_{e}$(CH)=1$\cdot $058$_{5}$, r$_{e}$(CC)=1$\cdot $204$_{7}$ angstrom are assumed, the results for all three isotopic molecules are mutually consistent. Values have also been obtained for the l-doubling coefficients, q$_{4}$ and q$_{5}$.