Neon matrix isolation electron spin resonance and theoretical studies of the various isotopomers of the CH3Mg radical

The (CH3Mg)-C-12, (CH3Mg)-C-13-Mg-25, (CH3Mg)-C-12-Mg-25, (CD3Mg)-C-12-Mg-2 5, (CH3Mg)-C-13-Mg-25 and (CD3Mg)-C-13-Mg-25 radicals have been isolated in an inert neon matrix at 4.3 K and their electronic structure probed, for t he first time, using matrix isolation electron spin resonance (MI-ESR) spec troscopy. These radicals were formed from the reaction of laser-ablated mag nesium metal and an appropriately labeled derivative of acetone or methyl i odide. The spin Hamiltonian parameters, g(perpendicular to) = 1.9999(4), A( perpendicular to)(Mg-25) = -184(1) MHz, A(perpendicular to)(C-13) = 128(2) MHz and A(perpendicular to)(H) = 7(1) MHz were determined from an exact dia gonalization analysis of the experimental spectra and estimates were derive d for A(parallel to)(Mg-25) = -197(10) MHz and A(parallel to)(C-13) = 180(2 0) MHz assuming g(parallel to) = 2.0023. A model for the bonding in the CH3 Mg radical is derived using this hyperfine data. Comparisons are made betwe en the CH3Mg radical and other related magnesium and monomethylmetal radica ls, MgH, MgOH, CH3Cd, CH3Zn. and CH3Ba. Theoretical nuclear hyperfine coupl ing constants for the CH3Mg radical were evaluated using Hartree-Fock singl e and double excitation configuration interaction (HFSDCI), multireference single and double excitation configuration interaction (MRSDCI) and density functional theory (DFT) ab initio calculations. While these theoretical me thods yielded values for A(dip)(Mg-25) and A(dip)(C-13) in agreement with t he experimental values, the calculated A(iso)(Mg-25) value was low by 4%(HF SDCI) and 15% (MRSDCI). Whereas the calculated A(iso)(C-13) values were low by 50% (HFSDCI) and 32% (MRSDCI). Unrestricted DFT calculations using the B3PW91 and B3LYP functionals yielded values of A(iso)(Mg-25) low by approxi mately 15% for both functionals and values of A(iso)(C-13) in agreement wit h experiment for UB3LYP and low by 10% for UB3PW91. The discrepancy between the calculated and experimental values of A(iso)(C-13) for the CI results is attributed to the limited reference space resulting in an overestimation of the ionic character in the bonding of the CH3Mg radical.