Interstellar CH3CCD

We report the detection of a new interstellar molecule: CH₃CCD, a deuterated isotopomer of methyl acetylene. Analysis of lines detected at seven positions along the ridge of the dark cloud TMC-1 indicates that the CH₃CCD column density and fractionation exhibit strong spatial gradients, with D/H ratios ranging from a minimum of 0.04 at the cyanopolyyne peak (CP) position to as high as 0.18. We find that the column density in CH₂DCCH is almost constant along the ridge, whereas that of CH₃CCD increases and that of CH₃CCH decreases in moving away from the CP. Chemical mechanisms that may explain the observed fractionation trends in CH₃CCD and CH₂DCCH are briefly discussed.