The Microwave Spectra and Molecular Structures of Methyl Bromoacetylene and Methyl Iodoacetylene

Pure rotation spectra of H3CCCBr79, H3CCCBr81, and H3CCCI have been measured. For H3CCCBr79, B0=1561.11 Mc/sec, IB=537.379×10−40 g cm2, and DJK=+11.4 kc/sec; for H3CCCBr81, B0=1550.42 Mc/sec, IB=541.082×10−40 g cm2, and DJK=+11.1 kc/sec; for H3CCCI, B0=1259.02 Mc/sec, IB=666.316×10−40 g cm2, and DJK=+7.2 kc/sec. The nuclear quadrupole coupling factors (eqQ) for the halogens are: for Br79, +647 Mc/sec; for Br81, +539 Mc/sec; for I127, −2230 Mc/sec. On the basis of reasonable assumptions for dC≡C, dCH, and ∠HCH, it is shown that the C–C and C–Br bonds in H3CCCBr are shortened to extents comparable with those found in methyl acetylene and cyanogen bromide, respectively, while the IB of methyl iodoacetylene is consistent with a similar shortening of both C–C and C–I. This is supported by the force constants for the stretching of the C–Br and C–I bonds. Bond lengths and nuclear quadrupole couplings are discussed in relation to the structures of these molecules.