ZEKE Spectroscopy of the Organometallic Radicals MgCH₃ and ZnCH₃:  Construction of a High-Resolution “Experimental” Molecular Orbital Diagram

Abstract

The resonance-enhanced multiphoton ionization (REMPI) and zero kinetic energy (ZEKE) photoelectron spectra of the organometallic radicals MgCH3 and ZnCH3 are reported. The radicals were generated in a pulsed supersonic expansion and therefore had vibrational and rotational temperatures of ca. 10 K. The REMPI experiments have resolved the spin−orbit and vibrational structure of the excited state, which is used as an intermediate state in the ZEKE experiment. Because of the intrinsically high resolution of the ZEKE technique, the photoelectron spectra of these radicals were vibrationally resolved. We also report density functional calculations of the geometries and vibrational frequencies of these radicals, and those of the closely related radicals CaCH3 and CdCH3. We have shown how the photoelectron spectra of these radicals can be used to generate high-resolution “experimental” MO diagrams of molecules that contain these fragments. We illustrate this approach with the MO diagram of Zn(CH3)2.

Consecutive fragmentations of the cubane-like zinc cluster [CH3Zn(O-i-C3H7)]4upon electron ionization

“…48 An additional factor disfavoring the formation of Zn-O bonds is associated with the positive charge of the cluster ions. The ionization energy of the CH 3 Zn radical (IE ¼ 7.273 eV), 45 for example, is notably lower than IE(ZnOH) ¼ 8.4 eV predicted by theoretical studies. 49 In the competition between C-O and Zn-O bond cleavages, the latter is therefore expected to be preferred for cationic fragments.…”

The zero kinetic energy photoelectron spectrum of the propargyl radical, C3H3

“…Thus resonantly enhanced multiphoton ionization ͑REMPI͒, the most common technique used to access the ionization threshold, is not always applicable, although ZEKE spectra via REMPI excitation were reported for a number of radicals, like benzyl with its fluorescing S 1 state, or some organometallic species of the MCH 3 (MϭCd, Zn, Mg) type. 23,24 Alternatively, one-photon excitation using tunable vacuum ultraviolet ͑VUV͒ light is possible, 21,25 but it remains difficult to tune a VUV laser source over several thousand wave numbers, in order to obtain vibrational spectra of the cation.…”

Photoionization Spectroscopy of the Zinc Monoethyl Radical and Its Cation

“…It is useful to follow the "experimental molecular orbital diagram" approach outlined by Barckholtz et al 10 Figure 1 represents the molecular orbital diagrams for the three molecular fragments: Zn atom, methyl radical, and ethyl radical as well as for the two organometallic radicals of interest: ZnC 2 H 3 and ZnC 2 H 5 . Wherever possible, the energies of molecular orbitals were taken from experimental data: i.e., the experimentally determined ionization thresholds and origins of electronic transitions.…”

“…In a recent publication, a wealth of spectroscopic data for all four species has been summarized. 10 The authors also discussed how the experimentally measured values of electronic excitation and ionization energies can be used to qualitatively estimate the strength of metal-carbon bonds and to predict that the substitution of methyl by a larger alkyl group would weaken the Zn-C bond.…”