Spotlights pubs.acs.org/JPCL
Spotlights: Volume 8, Issue 10
■
CATALYTIC DECOMPOSITION OF HYDROXYLAMMONIUM NITRATE IONIC LIQUID: ENHANCEMENT OF NO FORMATION When American astronaut Kathryn Sullivan suited up to be the first woman to perform a spacewalk in 1984, her task was to refuel a satellite with hydrazine, a very effective but highly toxic propellant that had been used by NASA for years, including in the Gemini and Apollo missions of the 1960s and 1970s. More than three decades after Sullivan’s historic walk, the aerospace community is still seeking a greener propellant that will be safer for both astronauts and the environment. One option being considered is the ionic liquid hydroxylammonium nitrate (HAN). To develop an accurate kinetics model of HAN’s ignition properties, a better understanding of its thermal and catalytic decomposition mechanisms is needed. The known thermal decomposition mechanisms are based on millisecond or longer time scale analyses rather than a microsecond time scale, and mass spectrometric detection techniques using electron impact ionization may have misinterpreted the spectra due to ion fragmentation processes that were unaccounted for. The current understanding of the catalytic decomposition of HAN is limited to the concept that the hydroxylamine is reacting catalytically on hot iridium, whereas the HNO3 product is simply vaporized from hot iridium. Chambreau et al. address some of these knowledge gaps in their Letter (10.1021/acs.jpclett.7b00672) and report that HAN is a promising candidate as a green alternative to hydrazine in monopropellant thruster space applications. They studied the reactivity of HAN aerosols on heated copper and iridium targets using tunable vacuum ultraviolet photoionization timeof-flight aerosol mass spectrometry, and they identified the reaction products by their mass-to-charge ratios and their ionization energies. Products include NH3, H2O, NO, hydroxylamine (HA), HNO3, and a small amount of NO2 at high temperature; however, the authors detected no N2O under these experimental conditions, despite the fact that it is one of the expected products according to the generally accepted thermal decomposition mechanism of HAN. The authors also observed a significant enhancement of the NO/HA ratio when the iridium catalyst was introduced, indicating that the formation of NO via decomposition of hydroxylamine is an important pathway in the catalytic decomposition of HAN.
forms, the safety of sunscreen itself has been questioned in recent years. Because of its makeup, chemical sunscreen is more likely to cause allergic reactions and hormone disruption. Research suggests that these risks may be lowered by the use of larger chemical filters that cannot penetrate the skin. One such filter is ethylhexyl triazone (EHT), which has been approved for use in sunscreen products. The EHT molecule is significantly larger than the molecules of many popular chemical filters used commercially, including octocrylene and avobenzone. Despite this size advantage, there have been few studies of the ultrafast excited-state dynamics of EHT, meaning that the photodeactivation mechanisms immediately after radiation absorption remain unknown. Baker et al. (10.1021/ acs.jpclett.7b00633) used femtosecond transient absorption spectroscopy with electronic structure calculations to unravel the complete photodeactivation mechanism that EHT undergoes after UV−B irradiation. Applying a global analysis procedure, they determined two dominant ultrafast, nonradiative processes occurring with time constants of ∼400 fs and ∼20 ps. They also observed metastable transient states (∼400 ps) that are characterized through state-of-the-art electronic structure calculations as well as steady-state measurements. The authors propose the complete photodeactiveation mechanism of EHT, a significant finding given the fact that efficient sunscreens such as EHT undergo numerous excitation/recovery cycles during their lifespans.
■
DISCRIMINATION OF DIVERSE COHERENCES ALLOWS IDENTIFICATION OF ELECTRONIC TRANSITIONS OF A MOLECULAR NANORING For the past decade, the field of physical chemistry has been engaged in a debate over the ubiquitous coherences observed in ultrafast spectroscopy experiments. Based on these observations, some researchers claim that coherent energy transfer is at the core of photosynthetic light harvesting; however, the subject remains controversial because it is extremely difficult to determine the nature of the observed coherences. Butkus et al. (10.1021/acs.jpclett.7b00612) studied synthetic porphyrin hexamer nanorings, which present a model system for investigating coherent dynamics. The authors used twodimensional electronic spectrum dynamics to disentangle and characterize three types of coherences, finding electronic, vibrational, and mixed coherences with a wide variety of dephasing times. They then used the coherences to identify six electronic transitions in the porphyrin nanoring, which was unexpected given the ring-like symmetry of the molecule.
■
ULTRAFAST TRANSIENT ABSORPTION SPECTROSCOPY OF THE SUNSCREEN CONSTITUENT ETHYLHEXYL TRIAZONE Sunscreen is widely used to prevent skin damage caused by the sun’s ultraviolet rays, and it has been around in various forms for centuries. Modern-day consumers have a plethora of choices, which can generally be divided into two types of protection. Organic sunscreens, often called “chemical sunscreens,” are transparent after application and thus are more popular among consumers than inorganic, or “physical,” sunscreens, which can leave a white residue on the skin. Although it has been shown to have health benefits in both © 2017 American Chemical Society
Published: May 18, 2017 2322
DOI: 10.1021/acs.jpclett.7b01139 J. Phys. Chem. Lett. 2017, 8, 2322−2322
