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, IB (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, DJK, 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 DJ 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′.