Main chain free radicals produced from 248 nm photolysis of poly(alkyl acrylate)s and poly(alkyl methacrylate)s have been unambiguously characterized for the first time by time-resolved electron paramagnetic resonance (TREPR) spectroscopy. Side chain cleavage via the Norrish I process dominates, leading to an oxo-acyl radical from the ester side chain and a main chain polymeric radical, the existence of which has been previously postulated but never confirmed by direct spectroscopic observation. There is a strong stereochemical influence on the methacrylate spectra, which manifests itself through changes in the TREPR spectra as a function of polymer tacticity and temperature. There is also a strong solvent dependence. Computer simulation provides unambiguous assignment of the signal carriers for the acrylate polymers at room temperature and above, and for PMMA main chain radicals at high temperature, where the fast motion limit for the β-methylene hyperfine coupling constants is achieved. The methacrylate spectra give remarkably similar coupling constants for all tacticities at high temperature, a phenomenon explained by an unexpected (and fortuitous) pseudosymmetry relationship between the diastereotopic protons in the radicals.

Sugars of extraterrestrial origin have been observed in the interstellar medium (ISM), in at least one comet spectrum, and in several carbonaceous chondritic meteorites that have been recovered from the surface of the Earth. The origins of these sugars within the meteorites have been debated. To explore the possibility that sugars could be generated during shock events, this paper reports on the results of the first laboratory impact experiments wherein glycolaldehyde, found in the ISM, as well as glycolaldehyde mixed with montmorillonite clay, have been subjected to reverberated shocks from ~5 to >25 GPa. New biologically relevant molecules, including threose, erythrose and ethylene glycol, were identified in the resulting samples. These results show that sugar molecules can not only survive but also become more complex during impact delivery to planetary bodies.

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.

Main chain radicals from several acrylic polymers are characterized in liquid solution at high temperatures (∼100 °C) using time-resolved electron paramagnetic resonance (TREPR) spectroscopy. The radicals are produced by laser flash photolysis (248 nm) and subsequent loss of the side chain ester functionality by Norrish I α-cleavage. At these temperatures fast-motion spectra with conformationally averaged hyperfine interactions are observed. The spectra are strongly spin polarized from the triplet mechanism (TM) of chemically induced dynamic electron spin polarization (CIDEP). Computer simulation of the TREPR spectra leads to unambiguous structural characterization of both the main chain radical and the side chain oxo-acyl radical. Hyperfine coupling constants, g-factors, and line widths are reported and discussed for radicals produced from five different acrylic polymers. Variation of the side chain on the polymer backbone leads to significant changes in the observed hyperfine coupling constants. For the main chain radical from poly(fluorooctyl methacrylate) (PFOMA), fast motion of the polymer chain is not accessible at these temperatures. The side chain oxo-acyl radical from PFOMA exhibits long-range 19F hyperfine interactions. Additional TREPR experiments on small molecule model compounds and gel permeation chromotography results of the photolyzed polymers support the conclusion that the primary photodegradation mechanism proposed in this paper is general for acrylic polymers.

The temperature dependencies of the time-resolved electron paramagnetic resonance (TREPR) spectra of five main chain acrylic free radicals are presented and discussed in terms of conformational dynamics. The radicals are produced in liquid solution at temperatures ranging from 25 °C to over 100 °C by direct excitation (248 nm) of the ester group in the polymers leading to Norrish I α-cleavage of the side chain ester moiety. Restricted rotational motion near the radical center leads to modulation of the β-hyperfine coupling constants, which manifests itself in some of the spectra as an alternating line width effect near room temperature. A standard hyperfine modulation model (two-site exchange) is proposed and has been added to the simulation routine. The model works especially well for two of the polymers (poly(ethyl acrylate) and poly(methyl d 3-methacrylate), for which activation barriers for rotation are extracted from Arrhenius plots. The model is somewhat successful for poly(ethyl cyanoacrylate) but fails for poly(ethyl methacrylate) and poly(methyl methacrylate). This failure is discussed in terms of radical structure and dynamics and the possibility that jumping between more than two low-energy conformational sites is involved. Disruption of the symmetry of the hyperfine couplings at intermediate temperatures supports this explanation.

A combination of time-resolved electron paramagnetic resonance (TREPR) and laser flash photolysis (LFP) studies of flexible acyl-containing biradicals over a wide temperature range is reported. In contrast to previous reports, it is shown that the main channel of intersystem crossing in these biradicals is the electron spin relaxation of the acyl moiety rather than spin-orbit interaction in the biradical. This relaxation determines the decay rate of the electron spin polarization at low temperatures and the biradical lifetime at high temperatures. The relaxation mechanism is attributed to the spin-rotation interaction, associated with the rotation of the carbonyl group about the neighboring C-C bond axis. From a model simulation of the time profile of the spin-polarized TREPR signal based on the numerical solution of the stochastic Liouville equation of the spin density matrix in frame of realistic model of biradical, the Arrhenius parameters for correlation times of spin rotation interaction and activation energies for molecular and spin dynamics were determined in two solvents, 2-propanol and hexane.

Over 200 molecules have been detected in multiple extraterrestrial environments, including glycolaldehyde (C 2 (H 2 O) 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 10 23 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.

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