No abstract

No abstract

Ozone adsorbed on silica gel at -78 C was passed very slowly between -5 0 -60 'C through a solution of Br2 in CFCI, with a stream of argon. until the brown color of the bromine in solution had almost disappeared. The lemon-ycllow solid was freed from solvent and ozone under vacuum and dissolved in ii little CH,CI, at -78 C. The undissolved colorless bromine oxide was removed by low-temperature centrifugation When the orange solution wiis cooled to -90 C. Br,O, crystallized in needles of the same color. Raman spectrum (solid. krypton ion laser. 647.0 nm). i. = 156. 167 m. sh. 325 OJ BrOZ) s. 387 w. 449 v(Br0Br) s. 588 v(Br0Br) s. X49 v(Br0,) s. 890 v,,.(BrOL) M cm ~ '. Crystal structure analysis: o =1186.6(2), h =762.9(1). = 869.3(2) pm. /j =106.4(1) . T = -145 C. P2,'c. Z = 8, /i = 20Xcm-'. 3029 measured reflections in the range 11 = 2 -35 . + / I , + k , + I, 2800 unique reflections. R,,, = 0.020. 2302 reflections with 12 3a(/). R = 0.046. R, = 0.039. Further details of the crystal structure investization may be obtained from the Fachinformationstentrum Karlsruhe. Gesellschaft fur wissenschaftlich-technische Information mbH. D-76344 Eg_pen~tein-Leopoldshafen (FRG), on quoting the depositor! number CSD-57429. thc names of the authors. and thc journal citation. 1634 0 VCH VerlagsgeseNschaf~ mbH, 0-69451 Wernherm, 1993 0570-0833/93/1iil-i634 $ iO.OO+.ZSjO Angen. Chem. In1 Ed. Engl. 1993, 32, N o ii

Raman microscopy and isotope labeling have been used for the first time to measure water self-diffusion in carbohydrate glasses. Together with pulsed-gradient stimulated-echo NMR, this method yielded the self-diffusion coefficients of water in amorphous maltose over 8 orders of magnitude, from the liquid to the glassy state. There are consistencies and major differences between our data and those obtained by evaporative drying. Water diffusion is remarkably fast in maltose glasses, decouples from maltose diffusion, and is not strongly affected by the glass transition.

The pterin-containing molybdenum enzymes are comprised of three general classes which perform simple oxygen-atom transfer either to or from substrate (sulfite oxidase and DMSO reductase families) and the oxidative hydroxylation of various substrates (xanthine oxidase family). 1 Following oxygen-atom transfer, the resting state of the enzyme is regenerated by two sequential one-electron transfers. Therefore, it is of interest to investigate electron transfer events in oxomolybdenum systems which mimic specific structural features of the active site. Here we describe the results of our initial investigations on photoinduced electron transfer in {5,10,15-tri-p-tolyl-20-[2,3-[((hydrotris(3,5-dimethylpyrazolyl)borato)oxomolybdenio)dioxy]phenyl]-porphyrinato}zinc(II) (ZnTTP(2,3-Mo V O(pzb)), 1) and {5,10,15tri-p-tolyl-20-[3,4-[((hydrotris(3,5-dimethylpyrazolyl)borato)oxomolybdenio)dioxy]phenyl]porphyrinato}zinc(II) (ZnTTP-(3,4-Mo V O(pzb)), 2), where a catechol moiety is used to covalently link the donor (porphyrin) and acceptor (Mo) sites via the meso position of the porphyrin. 2 The incorporation of oxo-Mo(V) centers as electron acceptors make these systems effective first generation models of electron transfer processes occurring at the active site of pterin-containing oxomolybdenum enzymes. 1 The structural rigidity of the catechol linker is particularly attractive since the through-bond and through-space distances, and the relative orientation of the donor and acceptor, can be easily modified by changing the spacer group from a 2,3-catechol to a 3,4-catechol. This provides a basis for testing various theoretical aspects of electron transfer and allows for detailed spectral probing of the electronic effects of linkage isomers on electron transfer rate constants. Furthermore, peripherally metalated porphyrin systems also provide a general means of probing real-time electron transfer dynamics at redox active transition metal sites. 3 In this communication we report direct evidence for photoinduced electron transfer in covalently linked donor-acceptor molecules 1 and 2.Since the [Mo V O(pzb)] 2+ fragment possesses a comparatively low oscillator strength ( < 10 3 M -1 cm -1 ), the ground-state electronic absorption spectra of 1 and 2 are dominated by the intense π f π* transitions of the porphyrin. The roomtemperature electronic absorption spectra of the nonperipherally metalated {5,10,15-tri-p-tolyl-20-[2,3-dimethoxyphenyl]por-phyrinato}zinc(II) (ZnTTP(2,3-(OMe) 2 ), 3) and {5,10,15-tri-ptolyl-20-[3,4-dimethoxyphenyl]porphyrinato}zinc(II) (ZnTTP-(3,4-(OMe) 2 ), 4) and the peripherally metalated derivatives 1 and 2 in DMSO are virtually identical. This indicates no significant porphyrin excited state (S 1 or S 2 ) electronic structural differences occur upon covalent attachment of the pendant [Mo V O(pzb)] 2+ subunit. However, steady-state emission studies show that covalent attachment of the [Mo V O(pzb)] 2+ pendant to the porphyrin periphery in 1 and 2 results in complete quenching of the porphyrin fluorescence as o...