Synthesis of CH₃K in the gas phase: Structural and mechanistic trends for monomeric, unsolvated CH₃M and HCCM (M = Li, Na, K)

Monomeric, solvent-free CH₃K has been made in the gas phase and its precise structure determined by millimeter/submillimeter spectroscopy, from which the following parameters were deduced: C-K and C-H bond lengths are 2.633 and 1 135 Å, respectively, and the H-C-H angle is 107.0°. The structure completes a series with those reported for CH₃Na and CH₃Li, showing the expected decrease in metal-carbon bond length (2.633 to 2.299 to 1.961 Å) ascending the periodic table, but little change in the H-C-H bond angles, which are all near 107°. The latter observation on these simple monomeric species clearly differs from the range of 104.8-108.2°reported for oligomeric CH₃M samples in the solid phase, which feature several metal- carbon contacts, showing the importance of measurements on monomeric species. Synthetic details for the methyl compounds and HCCM (M = Li, Na, K) are also discussed. Preparation of HCCLi from HCCBr and Li vapor produced LiBr but possible intermediate radical HCC· was not detected, suggesting synchronous reduction of the C-Br bond. The linear structure of HCCM is contrasted with nonlinear structures of LiCCLi (predicted theoretically), NaCN, and KCN, using Coulombic arguments to explain the different molecular geometries.