Theoretical Investigation On The Excited States Of CuCH₃ And Their Positive And Negative Ions

The bonding of a carbanion to a metal affects the structure and reactivity of the carbon fragment in extremely useful ways. Organocopper compounds are textbook reagents for achieving 1, 4-addition. This remarkable difference in reactivity, controlling the regioselectivity of carbon-carbon bond formation, it could be said that the discovery of these and other useful reactions of organocopper compounds spurred the development of organometallic chemistry as a tool for organic chemists. Copper is a cheap, relatively nontoxic metal, and the continuing relevance of organocopper chemistry is highlighted by uses in catalysis of carbon-heteroatom bond formation, carbene transfer, and recent revelations about the role of various additives in conjugate addition. Equally striking is the fact that structural data on organocopper complexes in general and alkylcoppers in particular are sparse.Complete active space self-consistent field (CASSCF) multiconfiguration second-order perturbation theory (CASPT2) calculations with ANO-RCC basis set in Cs symmetry were performed for a large number of electronic states of the CuCH3 free radical and their positive and negative ions, respectively. The active space for CASSCF calculations contains 18 electrons and consists of (9a′, 4a″) 13 molecular orbitals that are all valence orbitals and 3d orbitals of copper.All calculated states are valence states and their characters are discussed in detail. The calculated Harmonic frequencies of ground and excited states of CuCH3 as calculated by MCLR,ν1=426cm-1 (represents the C-Cu stretch),ν2=1154 cm-1(v2″represents the symmetric C-H bend(out-of-plane in CH3)),ν3=2931 cm-1 (represents the symmetric C-H stretch),ν4=1490 cm-1 represents the asymmetric C-H bend,ν5=3017 cm-1 (represents the asymmetric C-H stretch),ν6=628 cm-1 (represents the Cu-CH3 tilt), which are in agreement with previous experimental data.Furthermore, the results (CASPT2 adiabatic excitation energies and CASSI oscillator strengths) suggest that the calculated transitions of CuCH3 at 303,274,611nm are attributed to the 21A′→11A′,1A″→11A′,31A′→11A′, respectively. The calculated first vertical/adiabatic ionization energies for the CH3Cu are 11.68/8.97 eV, which are in agreement with previous experimental data, which is discussed in detail.Potential energy curves (PECs) for Cu-loss dissociation from the 1A′and 1A″states were calculated at the CASSCF //CASPT2 level and the electronic states of the Cu and CH3 as the dissociation products were carefully determined by checking the relative energies and geometries of the asymptote products along the PECs. The Cu-loss PEC calculations for CuCH3 indicate that Cu-loss dissociation occurs from the1A′states [correlating with CH3 (2A′) +Cu (2S)].