Hydrazine cation radicals

Figure 9.30. Electron transfer pathways in bis(hydrazine) radical cations (adapted from reference 3).

The N2H4 energy and coupling constant hypersurfaces and their application to approximate the structure of trialkyIsilyl-stibstituted hydrazine radical cations in solution. 

Example III The Hydrazine Radical Cation Total Energy and Coupling Constants Hypersurfaces and Their Application to Approximate the Structure of Organosilicon Derivatives in Solution. 

The results of the careful hypersurface calculations were surprising hydrazine with its dihedral angle ui=90° and an NN bond distance of 145 pm, on loss of one out of its 14 valence electrons, should form a completely planar (D2h) radical cation with the NN bond length shrinking by 17 pm ( ) to 128 pm ( ). Luckily enough, we dared to publish this hard-to-believe result (23), which a few months later has been completely confirmed by S.F. Nelson and collaborators (25), who succeeded in isolating crystals of the tetraalkyl hydrazine radical cation (3) and obtaining its X-ray structure, which exhibits an NN bond distance of 127 pm, i. e. close to the hypersurface prediction ... 

Figure 3. INDO open shell hypersurface calculations for the hydrazine radical cation (23,24) contracted to 3 angular coordinates of freedom (cf. text) (A) INDO total energies vs. the B/w coordinate pair, and hypersurface maps for the dependence of the ESR hyperfine coupling constants, ajj (B and C) and ajj (D and E) on the dihedral angle w and the HNH bond angle a (B and D) or the out-of-plane bending angle B (C and E).

Temperature Effects on Electron Transfer within Intervalence Bis(Hydrazine) Radical Cations. 

Phelps et al. [180] have studied the influence of seven solvents on the rate constants for three sesquibicyclic hydrazine-radical cation redox systems. The authors have found that the electrode kinetics of these systems depends on the overdamped solvent dynamics, though the activation barrier due to reactant vibrational rearrangements is substantial. 

Scheme 6 Neutral precursors of additional bis(hydrazine) radical cations prepared for optical studies.

Nelsen, S.F., Konradsson, A.E. and Teki, Y. (2006). Charge-localized naphthalene-bridged bis-hydrazine radical cations. J. Am. Chem. Soc. 128, 2902-2910... 

Suicide inactivation that might be unique to EAL would be the reaction with hydroxyethylhydrazine (HEH). The complex of EAL and coenzyme reacts rapidly with HEH to produce a complex of EAL with cob(II)alamin, 5 -deoxyadenosine, and the hydrazine radical cation shown in Figure 15. Analysis of the ERR spectra for the effects of in HEH and D2O as the solvent allowed the definitive identification of the radical cation in the inactive complex. EAL in the complex could be partially reactivated by precipitation in acidic conditions, implying that the protein portions of the enzyme might be intact, with the inactivation being due to blockage of the active site by cob(II)alamin. 

The mechanism in Figure 16 explains the available information. Initial homolytic dissociation of the Co-C5 bond in coenzyme B12 and abstraction of hydrogen from Cl of HEH by the 5 -deoxyadenosyl radical are thought to be similar to early steps in the catalytic deamination of a substrate by EAL. Elimination of the hydrazine radical cation in the last step in Figure 16 is unlike the normal deamination process in that it involves homolytic cleavage of the C-N bond in HEH. This mode of cleavage appears to occur because of the intrinsic stability of the hydrazine radical cation. 

Analogues to the five-electron bonded 2a/27t/l7t disulfide radical cations in nitrogen-based systems, namely, hydrazine radical cations could be generated by one-electron reduction of trialkyl diazenium salts (eq. 49) and subsequent equilibration of the neutral product radical with its protonated form. The pK of the R=t-butyl substituted species, incidentally, is 7.0, 2.6 units below that of the parent hydrazine compound. The results emphasize the importance of structural parameters, in particular those which control orbital orientation and overlap. >46,147 Jhe alternative possibility to generate such... 

FIGURE 10.1 Plot of log(feobs) versus log(fecaic ) for the 206 reactions studied here. The slowest reaction is for tetraisopropyl hydrazine reacting with tetracyclohexyl hydrazine radical cation, which has zero driving force and is between two of the slower couples (entries 6 and 11 in Table 10.1). 

Figure 10.5 compares color-coded calculated electron density at the van der Waals surfaces projected onto the van der Waals surfaces for four hydrazine radical cations, the Hab values obtained from our analysis of the cross-rates for these couples (from Table 10.4), and how these Hab value changes would affect the rate constants if only differences in Hah affected the rate constants (labeled Ar rei)- It is seen that although Ay changes are by far the most important factor in determining the 2 x lO " range in / n(fit) obtained for the couples studied, changes in Hab lead to clearly detectable effects on intrinsic rate constants. 

Ab Initio Calculations on the Intramolecular Electron Transfer Rates of a Bis(hydrazine) Radical Cation... 

We have put considerable effort over the past several years into study of the early intermediates expected in hydrazine (I) oxidation. As indicated in Scheme 1, electron and proton loss should alternate in the presence of a one electron oxidant, giving hydrazine radical cations (II), hydrazyl radicals (III), and trisubstituted diazenium cations (IV). Other types... 

The most studied example of II was tetraphenylhydrazine cation radical, whose purple color led Hiinig to coin the term "violenes" ( ) for the series of vinologous species his group has studied extensively (3). The solution ESR spectrum of hydrazine radical cation had been recorded in a flow system (4). Aromatic examples of III had received extensive study (5), and diphenyl-picrylhydrazyl was the first example of a neutral radical stable both to dimerization and reaction with oxygen. The chemistry of 1,1-dimethyldiazenium cation had been studied (6), and interesting electrochemical work on arylhydrazine oxidations had been carried out by Cauquis and Genies (7). 

The Hydrazyine,Hydrazine Radical Cation Redox Equilibrium... 

Our work on the first electron transfer, I 11, became possible when it was realized that tetraalkyl-hydrazine radical cations have a reasonably long lifetime in solution, even at room temperature. This discovery was made completely by accident, when tetra-methylhydrazine radical cation, 1 was observed by ESR... 

---