Cometary Glycolaldehyde as a Source of pre-RNA Molecules.

Over 200 molecules have been detected in multiple extraterrestrial environments, including glycolaldehyde (C2(H2O)2, GLA), a two-carbon sugar precursor that has been detected in regions of the interstellar medium. Its recent in situ detection on the nucleus of comet 67P/Churyumov-Gerasimenko and through remote observations in the comae of others provides tantalizing evidence that it is common on most (if not all) comets. Impact experiments conducted at the Experimental Impact Laboratory at NASA's Johnson Space Center have shown that samples of GLA and GLA mixed with montmorillonite clays can survive impact delivery in the pressure range of 4.5 to 25 GPa. Extrapolated to amounts of GLA observed on individual comets and assuming a monotonic impact rate in the first billion years of Solar System history, these experimental results show that up to 1023 kg of cometary GLA could have survived impact delivery, with substantial amounts of threose, erythrose, glycolic acid, and ethylene glycol also produced or delivered. Importantly, independent of the profile of the impact flux in the early Solar System, comet delivery of GLA would have provided (and may continue to provide) a reservoir of starting material for the formose reaction (to form ribose) and the Strecker reaction (to form amino acids). Thus, comets may have been important delivery vehicles for starting molecules necessary for life as we know it.

Building blocks for molecule-based magnets: radical anions and dianions of substituted 3,6-dimethylenecyclohexane-1,2,4,5-tetrones as paramagnetic bridging ligands.

We have prepared four tetraaryl derivatives of 3,6-dimethylene-1,2,4,5-tetraoxocyclohexane (aryl = Ph; 4-MeOPh; 4-Me(2)NPh; and 3,5-(t-Bu)(2)-4-MeOPh) with guidance from an earlier reported ab initio analysis (Misiolek, A. W.; Jackson, J. E. J. Am. Chem. Soc. 2001, 123, 4774-4780). These electron acceptors may be chemically or electrochemically reduced to the mono- and dianions desired as building blocks for the assembly of molecule-based magnets. Cyclic voltammetry shows that the potential of the first reduction wave depends on the electron donor ability of the aryl ring substituents, ranging from -0.28 V for the tetraphenyl derivative to -0.78 V for the p-dimethylamino substituted analogue (vs ferrocene/ferrocinium(+) at 0.46 V). Spin density distributions in the semiquinone moieties were elucidated by electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) observations of hyperfine couplings to internal (1)H sites and bound alkali metal cations. X-ray diffraction studies of the sodium and potassium salts of the octa-t-butyltetramethoxy derivative reveal the structure of the monoanion and its tendency to self-assemble with metal cations into one-dimensional chains in the solid state. Within the chains the anions display the expected bridging and chelating mode of coordination; SQUID magnetometry revealed weak intermolecular spin-spin couplings of 2J = -0.2 and approximately 0 K for the sodium and potassium salts, respectively. NIR transitions in the electronic spectra of the monoanions in solution are consistent with the expected low energy gap between frontier orbitals and its tunability by substituent variations. EPR studies of the free dianions and monoradical analogues indicate diradical localization into separate triphenylmethyl-like monoradicals via twisting of the diarylmethylene termini.

Chemically induced dynamic electron spin polarization-detected energy transfer. Substrate size effects and solvent dependence.

Time-resolved electron paramagnetic resonance spectroscopy is used to probe energy transfer from aromatic photoexcited triplet states to azo compounds in liquid solution. The observation of chemically induced dynamic electron spin polarization in the spectra gives precise information regarding the spin physics and mechanism of the energy transfer process. The substrate size is varied by altering the chain length of alkyl chains covalently attached to the azo compounds via ester or amide linkages. The solvent dependence of the energy transfer process is also investigated. The results are discussed in terms of Dexter and Förster mechanisms for energy transfer, the properties of the excited states, and the diffusive properties of the molecules in the solvents of interest. Decomposition rate studies and fluorescence measurements are also reported.

Time-resolved EPR studies of main chain radicals from acrylic polymers. Effects of tacticity, solvent, and side group structure on chain stiffness.

Main chain polymeric radicals from several acrylic polymers, produced by laser flash photolysis at 248 nm in liquid solution, have been studied using direct detection time-resolved electron paramagnetic resonance (TREPR) spectroscopy at 9.5 GHz. Highly isotactic poly(methyl methacrylate) (i-PMMA) shows a sharp, well-resolved spectrum at about 95 degrees C. Using synthetic methodology to disrupt the tacticity of i-PMMA, we observed different fast-motion hyperfine coupling constants for the main chain radicals. By raising the temperature of observation, we returned the coupling constants to the same value as those in the highly isotactic sample. This result is related qualitatively to the degree of stiffness of the polymer chains as a function of tacticity. The concept is tested further by comparison to two other acrylic polymers with bulky side chains: poly(fluorooctyl methacrylate) (PFOMA) and poly(adamantyl methacrylate) (PAMA), whose main chain radicals show significant line broadening even at 110 degrees C. Solvent effects on both spectral appearance (the alternating line-width effect) and kinetic decays (attributed to T1 relaxation) are also presented and discussed in terms of main chain conformational motion.