Gas-phase synthesis, submillimeter spectra, and precise structure of monomeric, solvent-free CuCH₃

The most-studied organocopper, methylcopper (CH₃Cu), has been synthesized for the first time as a monomer, free of other ligands or coordinated solvent, and detected using millimeter-wave rotational spectroscopy. The molecule was created in the gas phase by the reaction of copper vapor and several different precursors, including CH₄ and tetramethylsilane. An obvious symmetric top pattern was observed, indicative of C_3v symmetry. For confirmation, rotational spectra of four isotopically substituted species, ⁶⁵Cu¹²CH₃, ⁶³Cu ¹²CD₃, ⁶³Cu¹³CH₃, and ⁶⁵Cu¹³CH₃, were recorded as well, following large-scale syntheses of specifically labeled precursors. From this data set, accurate spectroscopic constants and a structure for CH₃Cu were derived. The r_m⁽²⁾ geometry shows the shortest alkyl carbon-copper(I) bond length known (1.8809 Å) and a rather large H-C-H angle in the methyl group (109.88°). The measured rotational and distortion constants are well reproduced by QCISD calculations. This study should allow further refinement of theoretical calculations on methylcopper and other organocopper derivatives, which are valuable synthetic intermediates in organic chemistry.