Microwave to Terahertz-Wave Spectroscopy of Transient Metal-Containing Molecules: Hydrides, Hydrosulfides, and Methyl Halide Insertion Products

Abstract

Metal-ligand interactions play an essential role in various areas of chemistry, including catalysis, biochemistry, coordination chemistry and materials science. However, little is known about their fundamental properties. In order to elucidate the nature of the metal to ligand chemical bond, high-resolution laboratory spectroscopy measurements of small alkali, alkaline earth and transition metal-containing molecules were carried out using pulsed Fourier transform microwave (FTMW) techniques combined with millimeter/Terahertz-wave direct absorption methods. Novel gas-phase synthetic techniques such as laser ablation and DC/AC glow discharges were employed to synthesize these reactive species. Rotational spectra of LiCCH, NaCCH, KCCH, ScN, YN, BaNH, CaH, MgH, ZnH, FeH, LiSH, NaSH, KSH, ZnSH, ClZnCH₃ and IZnCH₃ were recorded in the 4–60 GHz (FTMW) and 200–850 GHz (direct absorption) frequency range. Measurements of the weaker isotopologues, including LiCCD, NaCCD, KCCD, Sc¹⁵N, Y¹⁵N, CaD, ²⁴MgH, ²⁵MgH, ²⁶MgH, ⁶⁶ZnH, ⁶⁷ZnH, ⁶⁸ZnH, ⁷⁰ZnH, FeD, KSD, ⁶⁶ZnSH, ⁶⁸ZnSH, ⁶⁴ZnSD, I⁶⁶ZnCH₃, IZn¹³CH₃ and IZnCD₃ were also carried out. Due to short molecular lifetimes as well as the presence of fine/hyperfine structure, these data were particularly challenging to analyze as often weak signals with complex rotational patterns had to be identified amongst hundreds of contaminant molecular lines. The spectra were fit using an effective Hamiltonian consisting of rotational, electron spin-rotation, electron spin-orbit, electron spin-spin, magnetic hyperfine and electric quadrupole terms to derive spectroscopic constants. Based on such results, molecular geometries were determined as well as electronic structure information and the degree of covalent/ionic bonding character in the metal–ligand bond.