Crystals of the 2,4,6-tri-tert-butylphenoxyl radical have been isolated and characterized by X-ray diffraction, and calculations have been performed that give the distribution of spin density in the radical.

Reported herein are thermochemical studies of hydrogen atom transfer (HAT) 2+ , is surprisingly well predicted by the trends for electron transfer half-reaction entropies, ΔS o ET , in aprotic solvents. This is because both ΔS o ET and ΔS o HAT have substantial contributions from vibrational entropy, which varies significantly with the metal center involved. The close connection between ΔS o HAT and ΔS o ET provides an important link between these two fields and provides a starting point from which to predict which HAT systems will have important ground-state entropy effects.

We have constructed and studied photoelectrochemical solar cells (PECs) consisting of a photoanode prepared by direct deposition of independently synthesized CdSe nanocrystal quantum dots (NQDs) onto a nanocrystalline TiO(2) film (NQD/TiO(2)), aqueous Na(2)S or Li(2)S electrolyte, and a Pt counter electrode. We show that light harvesting efficiency (LHE) of the NQD/TiO(2) photoanode is significantly enhanced when the NQD surface passivation is changed from tri-n-octylphosphine oxide (TOPO) to 4-butylamine (BA). In the PEC the use of NQDs with a shorter passivating ligand, BA, leads to a significant enhancement in both the electron injection efficiency at the NQD/TiO(2) interface and charge collection efficiency at the NQD/electrolyte interface, with the latter attributed mostly to a more efficient diffusion of the electrolyte through the pores of the photoanode. We show that by utilizing BA-capped NQDs and aqueous Li(2)S as an electrolyte, it is possible to achieve ∼100% internal quantum efficiency of photon-to-electron conversion, matching the performance of dye-sensitized solar cells.

In order to understand how the separation between the electron and proton-accepting sites affects proton-coupled electron transfer (PCET) reactivity, we have prepared ruthenium complexes with 4′-(4-carboxyphenyl)terpyridine ligands, and studied reactivity with hydrogen atom donors (H-X). Ru II (pydic)(tpy-PhCOOH) (Ru II PhCOOH), was synthesized in one pot from [(p-cymene) RuCl 2 ] 2 , sodium 4′-(4-carboxyphenyl)-2,2′:6′,2″-terpyridine ([Na + ]tpy-PhCOO − ), and disodium pyridine-2,6-dicarboxylate (Na 2 pydic). Ru II PhCOOH plus n Bu 4 NOH in DMF yields the deprotonated Ru(II) complex, n Bu 4 N[Ru II (pydic)(tpy-PhCOO)] (Ru II PhCOO − ). The Ru(III) complex (Ru III PhCOO) has been isolated by one-electron oxidation of Ru II PhCOO − with triarylaminium radical cations (NAr 3 •+ ). The bond dissociation free energy (BDFE) of the O-H bond in Ru II PhCOOH is calculated from pK a and E 1/2 measurements as 87 kcal mol -1 , making Ru III PhCOO a strong hydrogen atom acceptor. There are 10 bonds and ca. 11.2 Å separating the metal from the carboxylate basic site in Ru III PhCOO. Even with this separation, Ru III PhCOO oxidizes the hydrogen atom donor TEMPOH (BDFE = 66.5 kcal mol -1 , ΔG°r xn = -21 kcal mol -1 ) by removal of an electron and a proton to form Ru II PhCOOH and TEMPO radical in a concerted proton-electron transfer (CPET) process. The second order rate constant for this reaction is (1.1 ± 0.1) × 10 5 M -1 s -1 with k H /k D = 2.1 ± 0.2, similar to the observed kinetics in an analogous system without the phenyl spacer, Ru III (pydic)(tpy-COO) (Ru III COO − ). In contrast, hydrogen transfer from 2,6-di-tert-butyl-p-methoxyphenol [ t Bu 2 (OMe)ArOH] to Ru III PhCOO is several orders of magnitude slower than the analogous reaction with Ru III COO.Coupling electron transfer to proton transfer is key to a wide range of chemical and biochemical processes, such as converting solar energy to chemical fuels. 1 While many of the fundamentals of electron transfer (ET) are well understood, the principles of proton-coupled electron transfer (PCET) are still being developed. The effects of increasing the distance between reaction centers has been much studied for ET, 2 and we have started to explore PCET systems in which the electron-and proton-accepting sites are increasingly separated. 3 PCET processes with large separations appear to be important in a number of biological systems, such as ribonucleotide reductases and photosystem II. 4 They may also be involved in charge injection into oxide semiconductors from ruthenium polypyridyl-carboxylate complexes. 5 In our previously reported ruthenium terpyridine-4′-carboxylate complex Ru III COO (Scheme 1), the Ru is six bonds and 6.9 Å removed from the basic carboxylate oxygen atoms. 3 Despite this separation, the reported reactions occur with H + and e − transferring in the same kinetic step, by concerted proton-electron transfer (CPET). 1,3,[6][7][8] In this report, the distance between the metal and basic Email: mayer@chem.washington.edu. Supporting Information A...

We have designed and prepared ruthenium complexes with terpyridine-4′-carboxylate (tpyCOO) ligands, in which there are six bonds between the redox-active Ru and the basic carboxylate. The protonated Ru(II) complex, Ru II (dipic)(tpyCOOH) (Ru II COOH), is prepared in one-pot from [(pcymene)RuCl 2 ] 2 , tpyCOONa, and then sodium pyridine-2,6-dicarboxylate [Na(dipic)]. A crystal structure of the deprotonated Ru(II) complex, Ru II COO − , shows a distance of 6.9Å between the metal and basic sites. The Ru(III) complex (Ru III COO) has been isolated by one-electron oxidation of Ru II COO − with triarylaminium radical cations (NAr 3 ·+ ). Ru III COO has a bond dissociation free energy (BDFE) of 81 ± 1 kcal mol −1 , from pK a and E 1/2 measurements. It oxidizes 2,4,6-tri-tertbutylphenol (BDFE = 77 ± 1 kcal mol −1 ) by removal of e − and H + (H · ) to form 2,4,6-tri-tertbutylphenoxyl radical and Ru II COOH, with a second order rate constant of (2.3 ± 0.2) × 10 4 M −1 s −1 and a k H /k D of 7.7 ± 1.2. Thermochemical analysis suggests a concerted proton-electron transfer (CPET) mechanism for this reaction, despite the 6.9 Å distance between the redox-active Ru and the H + -accepting oxygen. Ru III COO also oxidizes the hydroxylamine TEMPOH to the stable free radical TEMPO, and xanthene to bixanthyl. These reactions appear to be similar to processes that have been previously termed hydrogen atom transfer.Reactions that involve transfer of both a proton and an electron are important in a wide range of chemical and biochemical processes. 1 When the two particles transfer in a single step from a donor to an acceptor, XH + Y → X + YH, such reactions are termed concerted proton-electron transfer (CPET) or in certain cases, hydrogen atom transfer (HAT); the exact definition of these terms is a matter of continuing discussion. 2 This reactivity has been observed even when the proton and electron-accepting (or donating) sites are separated, as in reactions of phenols, ascorbate, and many transition metal species. 1 Within this framework, a CPET process has four relevant distances: the distances traveled by the electron and by the proton, and the separations of the H + and e − in the donor and in the acceptor. CPET reactions in which at least one of these distances is long have been implicated in photosystem II and class 1 ribonucleotide reductases, 3 and the distance dependence of pure electron transfer has long been studied. 4 We have reported HAT and CPET reactions of iron and ruthenium complexes with imidazole or related ligands, in which H + transfers to or from a nitrogen that s 3 bonds and ~4 Å distant from the metal center where the redox change primarily occurs. 5-7 Even with this separation, the rate constants typically correlate well with those for related organic processes. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript site is ~6.9 Å removed from the metal center, to probe how this distance affects CPET reactivity. 8The protonated ruthenium(n) complex, Ru II (pydic)(tpyCOOH) (Ru II C...

The sensitivity of explosives is controlled by factors that span from intrinsic chemical reactivity to mesoscale structure, and has been a topic of extensive study for over 50 years.

The explosive erythritol tetranitrate (ETN) has been known since 1849 and has applications as a vasodilator; however, little is known about its structure and bonding. Here we present the X-ray crystal structure of erythritol tetranitrate (ETN), along with characterization by nuclear magnetic resonance (NMR), infrared spectroscopy (IR), elemental analysis, and X-ray diffraction (XRD). Crystal packing and morphology are discussed in relation to explosive handling sensitivity (impact, spark, and friction testing). We compare the structure and property relationship to a closely related common nitrate ester, pentaerythritol tetranitrate (PETN).

Understanding the mechanisms for photodegradation of nanocrystal quantum dots is an important step toward their application in real-world technologies. A usual assumption is that photochemical modifications in nanocrystals, such as their photooxidation, are triggered by absorption of a photon in the dot itself. Here, we demonstrate that, contrary to this commonly accepted picture, nanocrystal oxidation can be initiated by photoexcitation of solvent-oxygen ion pairs that relax to produce singlet oxygen, which then reacts with the nanocrystals. We make this conclusion on the basis of photolysis studies of solutions of CdSe nanocrystals. Our measurements indicate a sharp spectral onset for photooxidation, which depends on solvent identity and is 4.8 eV for hexane and 3.4 eV for toluene. Importantly, the photooxidation onset correlates with the position of a new optical absorption feature, which develops in a neat solvent upon its exposure to oxygen. This provides direct evidence that nanocrystal photooxidation is mediated by excitation of solvent-oxygen pairs and suggests that the stability of the nanocrystals is defined by not only the properties of their surfaces (as has been commonly believed) but also the properties of their environment, that is, of the surrounding solvent or matrix.