MOLECULAR MECHANICS CALCULATION OF INNER-SHELL ACTIVATION BARRIERS TOHETEROGENEOUS ELECTRON-TRANSFER IN M(TACN)(2)(3+ 2+) REDOX COUPLES (M=FE, CO, NI, TACN=1,4,7-TRIAZACYCLONONANE)/

M(tacn)(2)(3+/2+) redox couples (M = Fe, Ni, Co; tacn = 1,4,7-triazacy clononane) exhibit different extents of M-N bond lengthening upon elec trochemical reduction and standard heterogeneous rate constants (k(s,h )) that decrease systematically in accord with this structural feature . Inner-shell enthalpies of activation (Delta H double dagger(is)) obt ained from temperature-dependent measurements of k(s,h) [Crawford, P. W,; Schultz, F. A. Inorg. Chem. 1991, 33, 4344] equal 1.7, 1.9, and 13 .2 kcal mol(-1) for M = Fe, Ni, and Co, respectively, in contrast with values of 0.2, 2.2, and 6.0 kcal mol(-1) calculated by the harmonic o scillator model of M-N bond elongation. In an attempt to resolve this discrepancy we have carried out molecular mechanics calculation of Del ta H double dagger(is) for M(tacn)(2)(3+/2+) couples using MMX and CHA RMM force fields. The procedure for doing so involves intersecting pot ential energy curves of oxidized and reduced reactants generated from the force field parameters required to optimize the ground state struc ture of each oxidation state. MMX barrier heights estimated in this wa y are in close correspondence with the harmonic oscillator approximati on widely used in Marcus theory calculation of inner-shell reorganizat ion energies. The vibrational entropies of the molecules are calculate d, and differences in these quantities correlate with the half-reactio n entropy (Delta S degrees(rc)) of the M(tacn)(2)(3+/2+) couples. Non- zero, metal-dependent values of Delta S degrees(rc) for these complexe s are thought to arise from changes in M-N frequencies upon reduction [Richardson, D. E.; Sharpe, P. Inorg. Chern, 1991, 30, 1412]. Poor cor respondence between measured and calculated activation enthalpies rema ins in cases where the electrode reaction exhibits a large half-reacti on entropy. The molecular mechanics force fields are used to partition the energy of the molecules into component terms, and it is found tha t the majority of the inner-shell barrier derives from M-N bond stretc hing.