Gas-Phase Electron-Transfer Equilibrium Studies on Tetraalkylhydrazines: Geometry Effects on Ionization Thermochemistry, Relaxation Energies, and Ion Solvation Energies
Equilibrium constants for electron-transfer equilibration between 35 pairs of tetraalkylhydrazines and their cation radicals were measured by high-pressure mass spectrometry and provided the data allowing calculation of aIP and ΔS0 of ionization for 31 tetraalkylhydrazines.Photoelectron spectroscopy measurements of vIP allow calculation of their enthalpies of relaxation, which range from a highest value of 38.3 kcal/mol for the trans-dipseudoequatorial conformation of 1,2-dimethylpyrazolidine (19) to a low of 18.9 kcal/mol for bridgehead anti-diazasesquinorbornane (30).The effects of changeing alkyl groups on both vIP and aIP are discussed.Comparison of the gas-phase equilibrium constants (extrapolated to 25 deg C) with cyclic voltammetric measurements of E0' in solution show that the enthalpy of solvation is strongest for the compounds with the smallest alkyl groups, tetramethylhydrazine (1) and (19), and about 7 kcal weaker for 30 but that the change in solvation energy correlates with the gas-phase charge-delocalizing ability of the alkyl groups and not with ease of approach of solvent molecules to the nitrogen atoms.
Nelsen, Stephen F.,Rumack, Daniel T.,Meot-Ner, Michael
p. 7945 - 7952
(2007/10/02)
Kinetic Effects of an Unusually Large Neutral to Radical Cation Geometry Change. Slow Electron-Transfer Reactions between Alkylhydrazines
High pressure mass spectrometry was used to measure the kinetics for electron transfer between 54 pairs of tetraalkylhydrazines containing acyclic and f-ve- to seven-membered cyclic and bicyclic rings.Rate constants for electron transfer vary between 18 and 0.03 x 10-11 cm3molecule-1s-1 at 550 K.Variable-temeprature mesuremeants were made on five pairs over a 77-120 deg range.The (Me2N)2(1+), (EtMeN2)2 pair gave Ea=2.7 kcal/mol.The association energy for (Me2N)2 was measured at ΔH0=-13.0 kcal/mol near room temperature.These data are combined to estimate an energy separation of about 15.7 kcal/mol between associated (Me2N)2(1+)/(EtMeN)2 dimer complex and the transition state for electron transfer.The observed Bronsted α value of about 0.5 suggests a large barrier to electron transfer, and the kinetics suggests that the components largely retain their original structures in the associated complex, but that significant distortion is required to reach the transition-state geometry.The effect of alkyl group changes on the electron-transfer rate and comparison of these data with solution experiments are discussed.
Nelsen, Stephen F.,Rumack, Daniel T.,Meot-Ner (Mautner), Michael
p. 1373 - 1379
(2007/10/02)
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