The Microwave Spectrum and Structure of Methyl Acetylene

Abstract

From measurements of the J=1→2 rotational transitions of methyl acetylene and some of its isotopic modifications, the moments of inertia, I_B (in the units 10⁻⁴⁰ gm cm²), were found for the ground vibrational state: for CH₃CCH, 98.17₂₃; for CH₃C¹³CH, 98.21₃₂; for C¹³H₃CCH, 100.91₉₃; for CH₃CC¹³H, 101.19₉₂; for CH₃CCD, 107.72₃₄; and for CD₃CCD, 124.58₀₆. The centrifugal distortion constants, D_JK, in mc/sec. are 0.16₁, 0.1₆, 0.1₆, 0.1₃, 0.1₄, and 0.0₉, respectively. The J=2→3 rotational transition of CH₃CCH was also measured, and the distortion constant D_J was found to be 0.00₅ mc/sec. Four lines of CH₃CCH corresponding to the J=1→2 transition and six lines for the J=2→3 transition in the excited vibrational state v₁₀=1 and six lines for the J=2→3 transition in the excited state v₉=1 were measured and interpreted according to Nielsen's theory of l‐type doubling in symmetric tops. The calculated structure for the ground vibrational state is: C–H (acetylenic), 1.056A; C≡C, 1.207A; C–C, 1.460A; C–H (methyl), 1.097A; ∠HCH, 108°14′.