Robert J. Donohoe

Hole delocalization in oxidized cerium(IV) porphyrin sandwich complexes

Optical absorption, electron paramagnetic resonance, and resonance Raman spectra are reported for the one-electron oxidation products of the Ce(IV) sandwich porphyrins, Ce(OEP)/sub 2/ and Ce(TPP)/sub 2/ (OEP = octaethylporphyrin; TPP = tetraphenylporphyrin). These data, in conjunction with electrochemical measurements, indicate that the holes of both oxidized species are delocalized on the vibrational and probably the electronic time scales. In Ce(OEP)/sub 2//sup +/, the hole is delocalized through purely porphyrin-porphyrin interactions. In Ce(TPP)/sub 2//sup +/, delocalization is further facilitated by participation of the f orbitals of the Ce(IV) ion. The different characteristics of the redox orbitals of Ce(OEP)/sub 2//sup +/ and Ce(TPP)/sub 2//sup +/ are accounted for by the fact that oxidation occurs from an a/sub lu/-like orbital in the former complex and from an a/sub 2u/-like orbital in the latter.

The mid-infrared signature of photo-induced defects in the quasi-one-dimensional mixed-valence solid [PtII(en)2][PtIV(en)2Cl2][ClO4]4

Abstract The observation of photo-induced absorption features immediately to the red of the band edge in the halogen-bridged mixed-valence linear chain {[Pt II (en) 2 ][Pt(en) 2 Cl 2 ][ClO 4 ]} x (S. Kurita, M. Haruki & K. Miyagawa, J. Phys. Soc. Japan , submitted for publication) has led to the prediction of an associated absorption in the mid-infrared. We have detected this band via difference IR spectroscopic measurements and verified that it tracks the red region absorptions in rate of growth upon photolysis, saturation and in recycling upon warming the sample to room temperature. Based on theoretical predictions, we tentatively assign the mid-IR feature to absorption due to a photo-induced electron polaronic defect.

The red and near-infrared resonance Raman spectroscopy of photo-induced defects in the mixed-valence linear chain complex [PtII(en)2][PtIV(en)2Cl2][ClO4]4

Abstract Resonance Raman (RR) spectra of the quasi-one-dimensional solid [PtII(en)2][PtIV(en)2Cl2][ClO4]4 were obtained wth red excitations (1.65 and 1.41 eV) as a function of photolysis of the solid with energy within the intervalence charge transfer band (2.41 eV). The 1.65 eV excitation corresponds to a polaronic local gap absorption which is known to increase in intensity upon photolysis [S. Kurita & M. Haruki, Synthetic Metals 29, F129 (1989)]. However, the RR spectrum which best reveals the photo-induced defects is obtained with 1.41 eV excitation and indicates the presence of both hole and electron polaron defects.

Phenomenological description of the longitudinal vibrations of the quasi-one-dimensional solid PtCl: calculation of the valence defect frequencies

The longitudinal vibrations of the PtCl linear chain and those of various valence defects are described by a one-dimensional model in a first-nearest-neighbour interaction approximation. The analysis is performed as a function of the K2/K1 ratio (K1 and K2 representing the PtClIV-Cl and PtII-Cl force constants, respectively), which is shown to play an important role in the vibrational characteristics of the defects while the infrared frequencies are not strongly dependent on a particular ratio. A coherent description of the frequencies of the modes of the perfect chain and those of the polaronic defects is obtained for K2/K1=0.3. For this ratio the electron polaron and the electron bipolaron give rise to one Raman-active mode while at least two are predicted for the other investigated defects. The shapes of the Raman bands associated with the electron and hole polarons are explained by the chlorine isotopic effect. The vibration associated with the electron bipolaron is predicted to be at about 210 cm-1 where a broad line grows upon photolysis. An elementary calculation based upon a Born-Mayer potential for short-range interactions, adjusted from the interionic distances, leads to force constants of the same magnitude as those adjusted from the experimental frequencies.

Nature of bonding in complexes containing "supershort" metal-metal bonds. raman and theoretical study of M2(dmp)4 [M = Cr (natural abundance Cr, 50Cr, and 54Cr) and Mo; dmp = 2,6-dimethoxyphenyl].

We report an investigation of complexes of the type M(2)(dmp)(4) (M = Mo, Cr; dmp = 2,6-dimethoxyphenyl) using resonance Raman (RR) spectroscopy, Cr isotopic substitution, and density functional theory (DFT) calculations. Assignment of the Mo-Mo stretching vibration in the Mo(2) species is straightforward, as evidenced by a single resonance-enhanced band at 424 cm(-1), consistent with an essentially unmixed metal-metal stretch, and overtones of this vibration. On the other hand, the Cr(2) congener has no obvious metal-metal stretching mode near 650-700 cm(-1), where empirical predictions based on the Cr-Cr distance as well as DFT calculations suggest that this vibration should appear if unmixed. Instead, three bands are observed at 345, 363, and 387 cm(-1) that (a) have relative RR intensities that are sensitive to the Raman excitation frequency, (b) exhibit overtones and combinations in the RR spectra, and (c) shift in frequency upon isotopic substitution ((50)Cr and (54)Cr). DFT calculations are used to model the vibrational data for the Mo(2) and Cr(2) systems. Both the DFT results and empirical predictions are in good agreement with experimental observations in the Mo(2) complex, but both, while mutually consistent, differ radically from experiment in the Cr(2) complex. Our experimental and theoretical results, especially the Cr isotope shifts, clearly demonstrate that the potential energy of the Cr-Cr stretching coordinate is distributed among several normal modes having both Cr-Cr and Cr-ligand character. The general significance of these results in interpreting spectroscopic observations in terms of the nature of metal-metal multiple bonding is discussed.

Highly efficient energy and charge transfer in thin self-assembled multilayered polymer films

We report the synthesis and characterization of multilayer self-assembled polymer films made from a water-soluble conjugated polymer, poly(2,5-methoxy-propyloxy sulfonate phenylene vinylene) (MPS-PPV). We observe a red shift of both the absorption and fluorescence spectra with increasing numbers of active MPS-PPV layers. We attribute this red shift to changing polymer conformation and efficient energy transfer. Upon adding a water-soluble C 60 or C 60 -VBA copolymer top layer, the luminescence spectrum is strongly quenched due to charge transfer. The estimated charge transfer quantum efficiency from PL quenching is ∼95%. We discuss in detail the unidirectional energy transfer followed by charge transfer in the self-assembled multilayered films.

Chemical Speciation of Neptunium(VI) under Strongly Alkaline Conditions. Structure, Composition, and Oxo Ligand Exchange

Hexavalent neptunium can be solubilized in 0.5-3.5 M aqueous MOH (M = Li(+), Na(+), NMe4(+) = TMA(+)) solutions. Single crystals were obtained from cooling of a dilute solution of Co(NH3)6Cl3 and NpO2(2+) in 3.5 M [N(Me)4]OH to 5 °C. A single-crystal X-ray diffraction study revealed the molecular formula of [Co(NH3)6]2[NpO2(OH)4]3·H2O, isostructural with the uranium analogue. The asymmetric unit contains three distinct NpO2(OH)4(2-) ions, each with pseudooctahedral coordination geometry with trans-oxo ligands. The average Np═O and Np-OH distances were determined to be 1.80(1) and 2.24(1) Å, respectively. EXAFS data and fits at the Np L(III)-edge on solid [Co(NH3)6]2[NpO2(OH)4]3·H2O and aqueous solutions of NpO2(2+) in 2.5 and 3.5 M (TMA)OH revealed bond lengths nearly identical with those determined by X-ray diffraction but with an increase in the number of equatorial ligands with increasing (TMA)OH concentration. Raman spectra of single crystals of [Co(NH3)6]2[NpO2(OH)4]3·H2O reveal a ν1(O═Np═O) symmetric stretch at 741 cm(-1). Raman spectra of NpO2(2+) recorded in a 0.6-2.2 M LiOH solution reveal a single ν1 frequency of 769 cm(-1). Facile exchange of the neptunyl oxo ligands with the water solvent was also observed with Raman spectroscopy performed with (16)O- and (18)O-enriched water solvent. The combination of EXAFS and Raman data suggests that NpO2(OH)4(2-) is the dominant solution species under the conditions of study and that a small amount of a second species, NpO2(OH)5(3-), may also be present at higher alkalinity. Crystal data for [Co(NH3)6]2[NpO2(OH)4]3·H2O: monoclinic, space group C2/c, a = 17.344(4) Å, b = 12.177(3) Å, c = 15.273 Å, β = 120.17(2)°, Z = 4, R1 = 0.0359, wR2 = 0.0729.

A Model for Trivalent Actinides in Media Containing High Carbonate Concentrations − Structural Characterization of the Lanthanide Tetracarbonate [Co(NH3)6][Na(μ-H2O)(H2O)4]2[Ho(CO3)4]·4H2O

The identity of the limiting HoIII species in aqueous solutions with high carbonate concentrations has been determined to be Ho(CO3)45−. Single crystals of [Co(NH3)6][Na(μ−H2O)(H2O)4]2[Ho(CO3)4]·4H2O were obtained by the addition of [Co(NH3)6]3+ to an aqueous 0.04 M solution of HoIII in 2.1 M Na2CO3. The asymmetric unit contains the anion, [Ho(CO3)4]5−, a [Co(NH3)6]3+ cation and two Na+ cations, which are bound to H2O molecules in an edge-sharing bioctahedral geometry. The [Ho(CO3)4]5− anion is eight coordinate with four bidentate carbonate ligands bound to the Ho atom. The molecule has essentially C2v symmetry with two coplanar carbonates making a vane, which is perpendicular to a similar vane produced by the other two carbonate ligands. An alternative way to this view molecule is through the geometry of the C atoms, which are found in a distorted tetrahedron. The average Ho−O distance was determined to be 2.361(5) A, while the average Ho−C distance was 2.784(6) A. The IR and Raman spectra were determined in both the solid state and solution in order to confirm the solution speciation. The Raman data show a single CO32− stretch for the solid at 1062 cm−1. The solution data show multiple peaks with the most prominent being at 1048 cm−1, which is consistent with the literature reports of an equilibrium mixture. The IR data for the solids confirm the X-ray results showing bidentate carbonate ligands by the splitting of the ν3 band of the CO32−. The crystal data for [Co(NH3)6][Na(μ-H2O)(H2O)4]2[Ho(CO3)4]·4H2O are as follows: monoclinic space group P2/n, a = 8.7091(5) A, b = 10.8744(6) A, c = 15.7971(9) A, β = 93.117(1)°, Z = 2, R1 = 0.0307, wR2 = 0.0756.

Raman Spectroscopic Study of the Aging and Nitration of Actinide Processing Anion-Exchange Resins in Concentrated Nitric Acid

Degradation of two types of anion exchange resins, Dowex 11 and Reillex HPQ, from the action of concentrated nitric acid (4 to 12 M) and radiolysis [from depleted uranium as UO22+ nitrate species and 239Pu as Pu(IV) nitrate species] was followed as a function of time with Raman vibrational spectroscopy. Elevated temperatures (∼ 50 °C) were used in the absence of actinide metal loading to simulate longer exposures of the resin to a HNO3 process stream and waste storage conditions. In the absence of actinide loading, only minor changes in the Dowex resin at acid concentrations ⩽10 M were observed, while at 12 M acid concentration, the emergence of a Raman peak at 1345 cm−1 indicates the addition of nitro functional groups to the resin. Similar studies with the Reillex resin show it to be more resistant to nitric acid attack at all acid concentrations. Incorporation of weakly radioactive depleted uranium as the UO22+ nitrate species to the ion-exchange sites of Dowex 11 under differing nitric acid concentrations (6 to 12 M) at room temperature showed no Raman evidence of resin degradation or nitration, even after several hundred days of contact. In contrast, Raman spectra for Dowex 11 in the presence of 239Pu as Pu(IV) nitrate species reveal numerous changes indicating resin alterations, including a new mode at 1345 cm−1 consistent with a Pu(IV)-nitrate catalyzed addition of nitro groups to the resin backbone.

FTIR characterization of heterocycles lumazine and violapterin in solution: effects of solvent on anionic forms.

Fourier transform infrared (FTIR) spectra have been obtained from solution samples of the heterocycles uracil, lumazine, and violapterin and reveal interpretable carbonyl stretching frequencies. Spectra of conjugate bases of lumazine and violapterin demonstrate decreases in these carbonyl stretching frequencies upon ionization. Based on isotopic shifts from amide deuterated analogs, semiempirical QCFF/PI calculations were used to assign the vibrational frequencies in the region 1100-1800 cm-1 observed from samples in dimethylsulfoxide (DMSO) and aqueous solutions to specific normal modes. The observed deuterium shifts and the calculations suggest that, in some cases, N-H bending motions are coupled to the C=O stretching motions of the pyrimidine ring. These data suggest that for lumazine anions a change in solvent can significantly change the mixing of the N-H bending and C=O stretching vibrational motions. This implies that vibrational analysis for lumazine species in relatively noninteracting media like nonpolar solvents, mulls or pellets cannot necessarily be transferred to the system when it is dissolved in a polar, hydrogen-bonding solvent such as water. Although other explanations can be offered, our vibrational analysis suggests that the changes in normal mode composition of the predominantly C=O stretching vibrations of lumazine anion on going from dimethylsulfoxide to water solution are consistent with a change in the predominant tautomer of the heterocycle. This change appears to correspond to a shifting of the location of the remaining acidic proton to a different ring nitrogen atom. This interpretation is of interest in view of recent ab initio calculations which suggest that proton shifts may occur during the hydroxylation of lumazine as mediated by the enzyme xanthine oxidase.