ARTICLE pubs.acs.org/JPCA
Electron Spin Multiplicities of Transition-Metal Aromatic Radicals and Ions: M[C6(CH3)6] and Mþ[(C6(CH3)6] (M = Ti, V, and Co) Jung Sup Lee, Yuxiu Lei, and Dong-Sheng Yang* Department of Chemistry, University of Kentucky, Lexington, Kentucky 40506-0055, United States
bS Supporting Information ABSTRACT: Determination of electron spin multiplicities of transition-metal radicals and ions challenges both experimentalists and theoreticians. In this work, we report preferred electron spin states of M[C6(CH3)6] and Mþ(C6(CH3)6], where M = Ti, V, and Co. The neutral radicals were formed in a supersonic metal cluster beam source, and their masses were measured with time-of-flight mass spectrometry. Precise ionization energies of the radicals and metalligand stretching frequencies of the ions were measured by pulsed field ionization zero electron kinetic energy spectroscopy. CH stretching frequencies of the methyl group in the radicals were obtained by infraredultraviolet two-photon ionization. Electron spin multiplicities of the radicals and ions were investigated by combining the spectroscopic measurements, density functional theory, and FranckCondon factor calculations. The preferred spin states are quintet, sextet, and quartet for the neutral Ti, V, and Co radicals, respectively; for the corresponding singly charged cations, they are quartet, quintet, and triplet. In these highspin states, the aromatic ring remains nearly planar. This finding contrasts to the previous study of Sc(hmbz), for which low-spin states are favored, and the aromatic ring is severely bent.
1. INTRODUCTION Interactions between transition-metal atoms and aromatic molecules play fundamental roles in homogeneous catalysis involving CC and CH bond activation13 and hydrogenation.46 Although there have been numerous experimental and computational studies of metalbenzene complexes,7 electron spin multiplicities of the monobenzene species are still under debate. For Ti(benzene), both triplet8,9 and quintet1016 states have been predicted as the ground electronic state, and the electron spin preferences appeared to depend on computational methods. For instance, B3LYP calculations predicted the quintet state to be the ground state with the triplet state lying
