Nir Goldman

Nitrogen-Rich Heterocycles as Reactivity Retardants in Shocked Insensitive Explosives

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.

Spectroscopic determination of the water dimer intermolecular potential-energy surface

Two polarizable six-dimensional water dimer intermolecular potential surfaces have been determined by fitting the distributed multipole ASP (anisotropic site potential) potential form to microwave, terahertz, and midinfrared cavity ringdown (D2O)2 spectra via a rigorous calculation of the water dimer eigenstates with the PSSH (pseudo-spectral split Hamiltonian) method. The fitted potentials accurately reproduce most ground-state vibration-rotation-tunneling spectra and yield excellent second virial coefficients for both H2O and D2O. 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 A) is significantly shorter than the currently accepted experimental value. The dimer binding energy (4.85 kcal/mol) is considerably smaller than the accepted experimental result, but in excellent agreement with recent theoretical results, as are the acceptor switching and donor–acceptor interchang...

Synthesis of glycine-containing complexes in impacts of comets on early Earth

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.

Characterization of hydrogen bond acceptor molecules at the water surface using near-edge x-ray absorption fine-structure spectroscopy and density functional theory

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.

Ab initio simulation of the equation of state and kinetics of shocked water

We report herein first principles simulations of water under shock loading and the chemical reactivity under these hot, compressed conditions. Using a recently developed simulation technique for shock compression, we observe that water achieves chemical equilibrium in less than 2 ps for all shock conditions studied. We make comparison to the experimental results for the Hugoniot pressure and density final states. Our simulations show that decomposition occurs through the reversible reaction H(2)O <--> H(+) + OH(-), in agreement with experiment. Near the approximate intersection of the Hugoniot and the Neptune isentrope, we observe high concentrations of charged species that contribute electronic states near the band gap.

Complete characterization of the water dimer vibrational ground state and testing the VRT(ASP-W)III, SAPT-5st, and VRT(MCY-5f) surfaces

We report the observation of extensive a- and c-type rotation-tunnelling (RT) spectra of (H2O)2 for Ka = 0–3, and (D2O)2 for Ka = 0–4. These data allow a detailed characterization of the vibrational ground state to energies comparable to those of the low-lying (70–80 cm−1) intermolecular vibrations. We present a comparison of the experimentally determined molecular constants and tunnelling splittings with those calculated on the VRT(ASP-W)III, SAPT-5st, and VRT(MCY-5f) intermolecular potential energy surfaces. The SAPT-5st potential reproduces the vibrational ground state properties of the water dimer very well. The VRT(MCY-5f) and especially the VRT(ASP-W)III potentials show larger disagreements, in particular for the bifurcation tunnelling splitting.

Ultrafast transformation of graphite to diamond: An ab initio study of graphite under shock compression

We report herein ab initio molecular dynamics simulations of graphite under shock compression in conjunction with the multiscale shock technique. Our simulations reveal that a novel short-lived layered diamond intermediate is formed within a few hundred of femtoseconds upon shock loading at a shock velocity of 12 kms (longitudinal stress>130 GPa), followed by formation of cubic diamond. The layered diamond state differs from the experimentally observed hexagonal diamond intermediate found at lower pressures and previous hydrostatic calculations in that a rapid buckling of the graphitic planes produces a mixture of hexagonal and cubic diamond (layered diamond). Direct calculation of the x-ray absorption spectra in our simulations reveals that the electronic structure of the final state closely resembles that of compressed cubic diamond.

Quantum mechanical corrections to simulated shock Hugoniot temperatures

We present a straightforward method for the inclusion of quantum nuclear vibrational effects in molecular dynamics calculations of shock Hugoniot temperatures. Using a Gruneisen equation of state and a quasiharmonic approximation to the vibrational energies, we derive a simple, postprocessing method for calculation of the quantum corrected Hugoniot temperatures. We have used our novel technique on ab initio simulations of shock compressed water and methane. Our results indicate significantly closer agreement with all available experimental temperature data for these two systems. Our formalism can be easily applied to a number of different shock compressed molecular liquids or solids, and has the potential to decrease the large uncertainties inherent in many experimental Hugoniot temperature measurements of these systems.

Dissociative melting of ice VII at high pressure.

We have used x-ray diffraction to determine the structure factor of water along its melting line to a static pressure of 57 GPa (570 kbar) and a temperature of more than 1500 K, conditions which correspond to the lower mantle of the Earth, and the interiors of Neptune and Uranus up to a depth of 7000 km. We have also performed corresponding first principles and classical molecular dynamics simulations. Above a pressure of 4 GPa the O-O structure factor is found to be very close to that of a simple soft sphere liquid, thus permitting us to determine the density of liquid water near the melting line. By comparing these results with the density of ice, also determined in this study, we find that the enthalpy of fusion (DeltaH(f)) increases enormously along the melting line, reaching approximately 120 kJ/mole at 40 GPa (compared to 6 kJ/mole at 0 GPa), thus revealing significant molecular dissociation of water upon melting. We speculate that an extended two-phase region could occur in planetary processes involving the adiabatic compression of water.

Prebiotic chemistry within a simple impacting icy mixture.

We present results of prebiotic organic synthesis in shock compressed mixtures of simple ices from quantum molecular dynamics (MD) simulations extended to close to equilibrium time scales. Given the likelihood of an inhospitable prebiotic atmosphere on early Earth, it is possible that impact processes of comets or other icy bodies were a source of prebiotic chemical compounds on the primitive planet. We observe that moderate shock pressures and temperatures within a CO2-rich icy mixture (36 GPa and 2800 K) produce a number of nitrogen containing heterocycles, which dissociate to form functionalized aromatic hydrocarbons upon expansion and cooling to ambient conditions. In contrast, higher shock conditions (48-60 GPa, 3700-4800 K) resulted in the synthesis of long carbon-chain molecules, CH4, and formaldehyde. All shock compression simulations at these conditions have produced significant quantities of simple C-N bonded compounds such as HCN, HNC, and HNCO upon expansion and cooling to ambient conditions. Our results elucidate a mechanism for impact synthesis of prebiotic molecules at realistic impact conditions that is independent of external constraints such as the presence of a catalyst, illuminating UV radiation, or pre-existing conditions on a planet.