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

被引:20
作者
GAO, YD [1 ]
LIPKOWITZ, KB [1 ]
SCHULTZ, FA [1 ]
机构
[1] INDIANA UNIV PURDUE UNIV, DEPT CHEM, INDIANAPOLIS, IN 46202 USA
关键词
D O I
10.1021/ja00153a015
中图分类号
O6 [化学];
学科分类号
0703 ;
摘要
M(tacn)(2)(3+/2+) redox couples (M = Fe, Ni, Co; tacn = 1,4,7-triazacyclononane) exhibit different extents of M-N bond lengthening upon electrochemical 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)) obtained 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 oscillator model of M-N bond elongation. In an attempt to resolve this discrepancy we have carried out molecular mechanics calculation of Delta H double dagger(is) for M(tacn)(2)(3+/2+) couples using MMX and CHARMM force fields. The procedure for doing so involves intersecting potential energy curves of oxidized and reduced reactants generated from the force field parameters required to optimize the ground state structure of each oxidation state. MMX barrier heights estimated in this way are in close correspondence with the harmonic oscillator approximation widely used in Marcus theory calculation of inner-shell reorganization energies. The vibrational entropies of the molecules are calculated, and differences in these quantities correlate with the half-reaction 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 complexes are thought to arise from changes in M-N frequencies upon reduction [Richardson, D. E.; Sharpe, P. Inorg. Chern, 1991, 30, 1412]. Poor correspondence between measured and calculated activation enthalpies remains in cases where the electrode reaction exhibits a large half-reaction entropy. The molecular mechanics force fields are used to partition the energy of the molecules into component terms, and it is found that the majority of the inner-shell barrier derives from M-N bond stretching.
引用
收藏
页码:11932 / 11938
页数:7
相关论文
共 41 条
[1]   KINETICS OF THE REDUCTION OF NITRO TO HYDROXYLAMINE GROUPS BY DITHIONITE IN A COBALT(II) CRYPTAND COMPLEX - X-RAY-ANALYSIS OF [1,8-BIS(HYDROXYAMINO)-3,6,10,13,16,19-HEXAAZABICYCLO[6.6.6]EICOSANE]COBALT(III) CHLORIDE TETRAHYDRATE [J].
BALAHURA, RJ ;
FERGUSON, G ;
RUHL, BL ;
WILKINS, RG .
INORGANIC CHEMISTRY, 1983, 22 (26) :3990-3992
[2]   MOLECULAR MECHANICS CALCULATIONS OF TRANSITION-METAL COMPLEXES [J].
BERNHARDT, PV ;
COMBA, P .
INORGANIC CHEMISTRY, 1992, 31 (12) :2638-2644
[3]   CRYSTALLOGRAPHIC STUDY OF THE LOW-SPIN IRON(II) AND IRON(III) BIS COMPLEXES OF 1,4,7-TRIAZACYCLONONANE [J].
BOEYENS, JCA ;
FORBES, AGS ;
HANCOCK, RD ;
WIEGHARDT, K .
INORGANIC CHEMISTRY, 1985, 24 (19) :2926-2931
[4]   A FORCE-FIELD FOR MOLECULAR MECHANICS MODELING OF COBALT(II) AMINE COMPLEXES AND A NEW MODEL OF ELECTRON-TRANSFER FOR COBALT(III)-COBALT(II) REDOX COUPLES [J].
BOND, AM ;
HAMBLEY, TW ;
SNOW, MR .
INORGANIC CHEMISTRY, 1985, 24 (12) :1920-1928
[5]  
BOWEN JP, 1991, REV COMPUTATIONAL CH, V2, pCH3
[6]   ELECTRON-TRANSFER REACTIONS WITH SIGNIFICANT INNER REORGANIZATION ENERGIES - REDUCTION OF SOME FULLY ALPHA-METHYLATED CYCLOALKANE-1,2-DIONES [J].
BRIELBECK, B ;
RUHL, JC ;
EVANS, DH .
JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 1993, 115 (25) :11898-11905
[7]   MOLECULAR MECHANICS CALCULATIONS IN COORDINATION CHEMISTRY [J].
BRUBAKER, GR ;
JOHNSON, DW .
COORDINATION CHEMISTRY REVIEWS, 1984, 53 (JAN) :1-36
[8]  
BURKERT U, 1982, MONOGRAPH AM CHEM SO, V177, P86
[9]   SYNTHESIS, STRUCTURE AND REDOX PROPERTIES OF NICKEL-COMPLEXES OF CAGE AMINE LIGANDS [J].
CLARK, IJ ;
CREASER, II ;
ENGELHARDT, LM ;
HARROWFIELD, JM ;
KRAUSZ, ER ;
MORAN, GM ;
SARGESON, AM ;
WHITE, AH .
AUSTRALIAN JOURNAL OF CHEMISTRY, 1993, 46 (01) :111-126
[10]  
Clark T., 1985, HDB COMPUTATIONAL CH