Microwave Spectrum and Molecular Structure of Methylchloroacetylene

The J = 6←5 rotational transitions have been observed for 10 isotopic species of CH3CCCl, including four of the fully deuterated molecule. The values for DJ were calculated from the force constants of the molecule, and are tabulated with the resulting values for B0. The structural parameters giving the best fit to the 10 moments of inertia IB are: r0(C–Cl) = 1.6371 A, r0(C≡C) = 1.2069 A, r0(C–C) = 1.4584 A, r0(C–H) = 1.117 A, ≰HCC = 110° 46′, r0(C–D) = 1.114 A, and ≰DCC = 110° 30′. The quadrupole coupling constant eqQ was determined as —79.6 Mc/sec for Cl35 and —62.6 Mc/sec for Cl37. For CH3CCCl35, CH3CCCl37, CD3CCCl35, and CD3CCCl37 experimental values of DJK and HJKK were determined from the K structure. To fit the observed fine structure it was necessary to include the third-order term HJKK in the expression for the rotational energy.