Measurement of the rate coefficient for collisional removal of O2(X3Sigma- g, upsilon=1) by O(3P)

We report a laboratory measurement of the rate coefficient for the collisional removal of O(2)(X(3)Sigma(g) (-),upsilon=1) by O((3)P) atoms. In the experiments, 266-nm laser light photodissociates ozone in a mixture of molecular oxygen and ozone. The photolysis step produces vibrationally excited O(2)(a(1)Delta(g)) that is rapidly converted to O(2)(X(3)Sigma(g) (-),upsilon=1-3) in a near-resonant electronic energy-transfer process with ground-state O(2). In parallel, a large amount of O((1)D) atoms is generated that promptly relaxes to O((3)P). Under the conditions of the experiments, only collisions with the photolytically produced O((3)P) atoms control the lifetime of O(2)(X(3)Sigma(g) (-),upsilon=1), because its removal by molecular oxygen at room temperature is extremely slow. Tunable 193-nm laser light monitors the temporal evolution of the O(2)(X(3)Sigma(g) (-),upsilon=1) population by detection of laser-induced fluorescence near 360 nm. The removal rate coefficient for O(2)(X(3)Sigma(g) (-),upsilon=1) by O((3)P) atoms is (3.2+/-1.0)x10(-12) cm(3) s(-1) (2sigma) at a temperature of 315+/-15 K (2sigma). This result is essential for the analysis and correct interpretation of the 6.3-mum H(2)O(nu(2)) band emission in the Earth's mesosphere and indicates that the deactivation of O(2)(X (3)Sigma(g) (-),upsilon=1) by O((3)P) atoms is significantly faster than the nominal values recently used in atmospheric models.     

Essential oil composition of three Australian endemic species of Darwinia (Myrtaceae)

The essential oils from the aerial parts of three rare Australian endemic species of Darwinia have been extracted by hydrodistillation and analyzed by gas chromatography (GC) and gas chromatography coupled to mass spectrometry (GC-MS). In D. procera, myrtenyl acetate (6.1-29.6%), alpha-pinene (6.9-25.1%), gamma-terpinene (6.2-13.6%), bicyclogermacrene (5.5-10.8%) and (E)-nerolidol (3.4-9.7%) were the principal components detected. D. fascicularis ssp. fascicularis produced an oil in which (E)-nerolidol (33.0%), alpha-pinene (15.1%) and gamma-terpinene (10.2%) were the principal components. In D. peduncularis the major constituents were alpha-pinene (33.5%), gamma-terpinene (23.1%) and bicyclogermacrene (6.7%).     

Strontium isotope ratios (87Sr/86Sr) of tooth enamel: a comparison of solution and laser ablation multicollector inductively coupled plasma mass spectrometry methods

Strontium isotope ratios (87Sr/86Sr) in tooth enamel provide a means to investigate migration and landscape use in humans and other animals. Established methods for measuring (87)Sr/(86)Sr in teeth use bulk sampling (5-20 mg) and labor-intensive elemental purification procedures before analysis by either thermal ionization mass spectrometry (TIMS) or multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS). Another method for measuring 87Sr/86Sr is laser ablation MC-ICP-MS, but concerns have been expressed about its accuracy for measuring tooth enamel. In this study we test the precision and accuracy of the technique by analyzing 30 modern rodent teeth from the Sterkfontein Valley, South Africa by laser ablation MC-ICP-MS and solution MC-ICP-MS. The results show a mean difference in 87Sr/86Sr measured by laser ablation and by solution of 0.0003 +/- 0.0002. This degree of precision is well within the margin necessary for investigating the potential geographic origins of humans or animals in many areas of the world. Because laser ablation is faster, less expensive, and less destructive than bulk sampling solution methods, it opens the possibility for conducting 87Sr/86Sr analyses of intra-tooth samples and small and/or rare specimens such as micromammal and fossil teeth.     

Probing phenylalanine/adenine pi-stacking interactions in protein complexes with explicitly correlated and CCSD(T) computations

To examine the effects of pi-stacking interactions between aromatic amino acid side chains and adenine bearing ligands in crystalline protein structures, 26 toluene/(N9-methyl)adenine model configurations have been constructed from protein/ligand crystal structures. Full geometry optimizations with the MP2 method cause the 26 crystal structures to collapse to six unique structures. The complete basis set (CBS) limit of the CCSD(T) interaction energies has been determined for all 32 structures by combining explicitly correlated MP2-R12 computations with a correction for higher-order correlation effects from CCSD(T) calculations. The CCSD(T) CBS limit interaction energies of the 26 crystal structures range from -3.19 to -6.77 kcal mol (-1) and average -5.01 kcal mol (-1). The CCSD(T) CBS limit interaction energies of the optimized complexes increase by roughly 1.5 kcal mol (-1) on average to -6.54 kcal mol (-1) (ranging from -5.93 to -7.05 kcal mol (-1)). Corrections for higher-order correlation effects are extremely important for both sets of structures and are responsible for the modest increase in the interaction energy after optimization. The MP2 method overbinds the crystal structures by 2.31 kcal mol (-1) on average compared to 4.50 kcal mol (-1) for the optimized structures.     

Collisional relaxation of O2(X3Σg(-), υ = 1) and O2(a1Δg, υ = 1) by atmospherically relevant species

Laboratory measurements are reported of the rate coefficient for collisional removal of O(2)(X(3)Σ(g)(-), υ = 1) by O((3)P), and the rate coefficients for removal of O(2)(a(1)Δ(g), υ = 1) by O(2), CO(2), and O((3)P). A two-laser method is employed, in which the pulsed output of the first laser at 285 nm photolyzes ozone to produce oxygen atoms and O(2)(a(1)Δ(g), υ = 1), and the output of the second laser detects O(2)(a(1)Δ(g), υ = 1) via resonance-enhanced multiphoton ionization. The kinetics of O(2)(X(3)Σ(g)(-), υ = 1) + O((3)P) relaxation is inferred from the temporal evolution of O(2)(a(1)Δ(g), υ = 1), an approach enabled by the rapid collision-induced equilibration of the O(2)(X(3)Σ(g)(-), υ = 1) and O(2)(a(1)Δ(g), υ = 1) populations in the system. The measured O(2)(X(3)Σ(g)(-), υ = 1) + O((3)P) rate coefficient is (2.9 ± 0.6) × 10(-12) cm(3) s(-1) at 295 K and (3.4 ± 0.6) × 10(-12) cm(3) s(-1) at 240 K. These values are consistent with the previously reported result of (3.2 ± 1.0) × 10(-12) cm(3) s(-1), which was obtained at 315 K using a different experimental approach [K. S. Kalogerakis, R. A. Copeland, and T. G. Slanger, J. Chem. Phys. 123, 194303 (2005)]. For removal of O(2)(a(1)Δ(g), υ = 1) by O((3)P), the upper limits for the rate coefficient are 4 × 10(-13) cm(3) s(-1) at 295 K and 6 × 10(-13) cm(3) s(-1) at 240 K. The rate coefficient for removal of O(2)(a(1)Δ(g), υ = 1) by O(2) is (5.6 ± 0.6) × 10(-11) cm(3) s(-1) at 295 K and (5.9 ± 0.5) × 10(-11) cm(3) s(-1) at 240 K. The O(2)(a(1)Δ(g), υ = 1) + CO(2) rate coefficient is (1.5 ± 0.2) × 10(-14) cm(3) s(-1) at 295 K and (1.2 ± 0.1) × 10(-14) cm(3) s(-1) at 240 K. The implications of the measured rate coefficients for modeling of atmospheric emissions are discussed.