4267-00-9

Laser flash photolysis (308-nm) of acetophenone (AcP)-aliphatic amine mixtures in the cavities of NaY zeolite generates AcP triplets, ketyl radical and dimers of amine radical cations. In contrast, photolysis of the same AcP-amine system in acetonitrile-water allows to detect exclusively AcP triplets and AcP-·. In the zeolite cavities amine radical cation appear to interact with a second neutral amine to afford the dimer radical cation detected as a longer-lived species absorbing from 480 to 580-nm. These radical cations of amine dimers have never been observed in solution except for special cyclic diamines having two nitrogen atoms rigidly held in close proximity Product studies have revealed the formation of hydrazines in the photosensitized AcP-triethylamine irradiation adsorbed in NaY, but not in acetonitrile solution.

Second-order rate constants k(ij)(obsd) measured at 25 °C in acetonitrile by stopped-flow spectrophotometry for forty-four electron transfer (ET) reactions among fourteen 0/+1 couples [three aromatic compounds (tetrathiafulvalene, tetramethyltetraselenafulvalene, and 9,10-dimethyl- 9,10-dihydrophenazine), four 2,3-disubstituted 2,3-diazabicyclo[2.2.2]octane derivatives, six acyclic hydrazines, and the bridgehead diamine 1,5- diazabicyclo[3.3.3]undecane] and seventeen compounds and forty-seven reactions from a previous study (J. Am. Chem. Soc. 1997, 119, 5900) [three p- phenylenediamine derivatives, four ferrocene derivatives, and ten tetraalkylhydrazines] are discussed. When all 91 k(ij)(obsd) values are simultaneously fitted to Marcus's adiabatic cross rate formula k(ij)(calcd) = (k(ii)k(jj)K(ij)f(ij))(1/2), ln f(ij) = (ln K(ij))(2/4) ln((kii)k(jj)/Z2), best-fit self-exchange rate constants, k(ii)(fit), are obtained that allow remarkably accurate calculation of k(ij)(obsd); k(ij)(obsd)/k(ij)(calcd) is in the range 0.5-2.0 for all 91 reactions. The average difference without regard to sign, |ΔΔG(+)(ij)|, between observed cross reaction activation free energy and that calculated using the k(ii)(fit) values and equilibrium constants is 0.13 kcal/mol. The ΔG(+)(ii)(fit) values obtained range from 2.3 kcal/mol for tetramethyltetraselenafulvalene(0/+) to 21.8 kcal/mol for tetra-n-propylhydrazine(0/+), corresponding to a factor of 2 x 1014 in k(ii)(fit). The principal factor affecting k(ii)(fit) for our data appears to be the internal vertical reorganization energy (λ(v)), but k(ii)(fit) values also incorportate the effects of changes in the electronic matrix coupling element (V). Significantly smaller V values for ferrocenes and for hydrazines with alkyl groups larger than methyl than for aromatics and tetramethylhydrazine are implied by the observed ΔG(+)(ii)(fit) values.

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Low temperature NMR spectra allowed us to "freeze" some of the internal motions in a number of di-, tri- and tetraalkylhydrazines and to measure the corresponding free energies of activation.In particular, tetramethylhydrazine (Me2N-NMe2) was found to have, at -150 deg C, two pairs of diastereotopic methyls : this is due to the fact that both N-inversion and N,N-rotation are slow at this temperature and that a gauche conformation is adopted.The observed barrier (6.0 kcal mol-1) has been attributed to N,N-rotation, the barrier due to N-inversion being higher and not measurable via NMR in the presence of a concomitant fast rotation.In other cases, notably PriMeN-NH2, Me2N-NPri2 and Pri2N-NHMe, two different motions (inversion and rotation) were detected.In the case of Me2N-NHMe it was also possible to observe the first example of anisochronous behaviour of nitrogen-bonded methyls (Me2N) induced by an aminic nitrogen that becomes chiral at low temperature.