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We report the first quantum-based multiscale simulations to study the reactivity of shocked perfect crystals of the insensitive energetic material triaminotrinitrobenzene (TATB). Tracking chemical transformations of TATB experiencing overdriven shock speeds of 9 km/s for up to 0.43 ns and 10 km/s for up to 0.2 ns reveal high concentrations of nitrogen-rich heterocyclic clusters. Further reactivity of TATB toward the final decomposition products of fluid N(2) and solid carbon is inhibited due to the formation of these heterocycles. Our results thus suggest a new mechanism for carbon-rich explosive materials that precedes the slow diffusion-limited process of forming the bulk solid from carbon clusters and provide fundamental insight at the atomistic level into the long reaction zone of shocked TATB.

Raman spectroscopy in a laser heated diamond anvil cell and first principles molecular dynamics simulations have been used to study water in the temperature range 300 to 1500 K and at pressures to 56 GPa. We find a substantial decrease in the intensity of the O-H stretch mode in the liquid phase with pressure, and a change in slope of the melting line at 47 GPa and 1000 K. Consistent with these observations, theoretical calculations show that water beyond 50 GPa is "dynamically ionized" in that it consists of very short-lived (<10 fs) H2O, H3O+, and OH- species, and also that the mobility of the oxygen ions decreases abruptly with pressure, while hydrogen ions remain very mobile. We suggest that this regime corresponds to a superionic state.

Delivery of prebiotic compounds to early Earth from an impacting comet is thought to be an unlikely mechanism for the origins of life because of unfavourable chemical conditions on the planet and the high heat from impact. In contrast, we find that impact-induced shock compression of cometary ices followed by expansion to ambient conditions can produce complexes that resemble the amino acid glycine. Our ab initio molecular dynamics simulations show that shock waves drive the synthesis of transient C-N bonded oligomers at extreme pressures and temperatures. On post impact quenching to lower pressures, the oligomers break apart to form a metastable glycine-containing complex. We show that impact from cometary ice could possibly yield amino acids by a synthetic route independent of the pre-existing atmospheric conditions and materials on the planet.

We present a combined experimental/computational study of the near-edge x-ray absorption fine structure of the liquid water surface which indicates that molecules with acceptor-only hydrogen bonding configurations constitute an important and previously unidentified component of the liquid/vapour interface. A detailed microscopic picture of the liquid water surface underlies many important phenomena, ranging from the terrestrial CO 2 and H 2 O cycles to surface wetting and ice formation. Surface hydrogen bond configurations, which determine important interfacial properties, e.g. surface tension and interfacial mobility, remain incompletely characterized. Molecular dynamics (MD) simulations of water molecules residing in the ∼5 Å [1] liquid-vapour interface have revealed an overall relaxation of bulk properties (e.g. dipole moment, geometry, diffusion constant, density) toward gas-phase values, precipitated by the disintegration of the three-dimensional liquid hydrogen bond network [2-5]. Sum frequency generation (SFG) studies have provided the first important experimental insights into the molecular details of the liquid water surface. Shen and co-workers [6] report a vibrational band slightly red-shifted from the gas-phase symmetric stretch of water vapour consistent with a free O-H oscillator. They further concluded that >20% of the surface molecules are oriented with one free O-H bond extending out of the surface by about 38 • .

Two polarizable six-dimensional water dimer intermolecular potential surfaces have been determined by fitting the distributed multipole ASP ͑anisotropic site potential͒ potential form to microwave, terahertz, and midinfrared cavity ringdown (D 2 O) 2 spectra via a rigorous calculation of the water dimer eigenstates with the PSSH ͑pseudo-spectral split Hamiltonian͒ method. The fitted potentials accurately reproduce most ground-state vibration-rotation-tunneling spectra and yield excellent second virial coefficients for both H 2 O and D 2 O. The calculated dimer structure and dipole moment are close to those determined from microwave spectroscopy and high level ab initio calculations, except that the O-O distance ͑2.952 Å͒ is significantly shorter than the currently accepted experimental value. The dimer binding energy ͑4.85 kcal/mol͒ is considerably smaller than the accepted experimental result, but in excellent agreement with recent theoretical results, as are the acceptor switching and donor-acceptor interchange tunneling barriers and the cyclic water trimer and tetramer structures and binding energies.

Comets are known to harbour simple ices and the organic precursors of the building blocks of proteins-amino acids-that are essential to life. Indeed, glycine, the simplest amino acid, was recently confirmed to be present on comet 81P/Wild-2 from samples returned by NASA's Stardust spacecraft. Impacts of icy bodies (such as comets) onto rocky surfaces, and, equally, impacts of rocky bodies onto icy surfaces (such as the jovian and saturnian satellites), could have been responsible for the manufacture of these complex organic molecules through a process of shock synthesis. Here we present laboratory experiments in which we shocked ice mixtures analogous to those found in a comet with a steel projectile fired at high velocities in a light gas gun to test whether amino acids could be produced. We found that the hypervelocity impact shock of a typical comet ice mixture produced several amino acids after hydrolysis. These include equal amounts of D-and L-alanine, and the non-protein amino acids α-aminoisobutyric acid and isovaline as well as their precursors. Our findings suggest a pathway for the synthetic production of the components of proteins within our Solar System, and thus a potential pathway towards life through icy impacts.

We present new results for the water dimer equilibrium constant K p (T) in the range 190-390 K, using a flexible potential energy surface fitted to spectroscopical data. The increased numerical complexity due to explicit consideration of the monomer vibrations is handled via an adiabatic (6 + 6)d decoupling between intra-and intermolecular modes. The convergence of the canonical partition function of the dimer is ensured by computing all energy levels up to dissociation for total angular momentum values J ) 0-5 and using an extrapolation scheme to higher values. The newly calculated values for K p (T) are in very good agreement with available experimental data at room temperature. At higher temperatures, an analysis of the convergence of the partition function reveals that quasi-bound states are likely to contribute to the equilibrium constant. Additional thermodynamical quantities (∆G, ∆H, ∆S, and C p ) have also been determined and fit to quadratic expressions a + bT + cT 2 .