Richard L. Jaffe

Review of Chemical-Kinetic Problems of Future NASA Missions, II: Mars Entries

A number of chemical-kinetic problems related to phenomena occurring behind a shock wave surrounding an object flying in the earth atmosphere are discussed, including the nonequilibrium thermochemical relaxation phenomena occurring behind a shock wave surrounding the flying object, problems related to aerobraking maneuver, the radiation phenomena for shock velocities of up to 12 km/sec, and the determination of rate coefficients for ionization reactions and associated electron-impact ionization reactions. Results of experiments are presented in form of graphs and tables, giving data on the reaction rate coefficients for air, the ionization distances, thermodynamic properties behind a shock wave, radiative heat flux calculations, Damkoehler numbers for the ablation-product layer, together with conclusions.

Chemical-Kinetic Parameters of Hyperbolic Earth Entry

Chemical-kinetic parameters governing the e ow in the shock layer over a heat shield of a blunt body entering Earth’ s atmosphere from a hyperbolic orbit are derived. By the use of the assumption that the heat shield is made of carbon phenolic and by allowing for an arbitrary rateof pyrolysis-gasinjection, chemical reactions occurring in the shock layer are postulated, and the collision integrals governing the transport properties, the rate coefe cients of the reactions, and the parameters needed for the bifurcation model and for the e nite-rate kinetic wall boundary conditions are determined using the best available techniques. Sample e owe eld calculations are performed using this set of parameters to show that the heating and surface removal rates are substantially smaller than calculated using theexisting setofsuch parameters and traditionalassumptionsof gas ‐surfaceequilibrium and quasi-steadystate ablation.

A quantum chemistry study of benzene dimer

We have performed a detailed quantum chemistry study of the gas‐phase benzene dimer. Large atomic orbital basis sets with multiple polarization functions were used. The effects of basis set size, electron correlation, and basis set superposition error were investigated for the low‐energy planar sandwich (D6h and C6v), parallel displaced (C2h), and T‐shaped (C2v) dimer structures. Our studies indicate that the C2h‐symmetry parallel displaced geometry is the lowest‐energy structure for the benzene dimer. The T‐shaped structure was found to be a low‐energy saddle point for interconversion between parallel displaced structures, while the planar sandwich structure was found to be a saddle point on a higher‐energy interconversion path between parallel displaced structures. Detailed analysis of the low‐energy (T‐shaped saddle point) path revealed the presence of a shallow minimum corresponding to a tilt angle between phenyl ring planes of about 45°. Much of the behavior of the benzene dimer observed through mole...

Molecular dynamics simulations of carbon nanotube-based gears

We use a molecular dynamics simulation to investigate the properties and design space of molecular gears fashioned from carbon nanotubes with teeth added via a benzyne reaction known to occur with C60. Brenners reactive hydrocarbon potential is used to model interatomic forces within each molecular gear. A Lennard-Jones 6-12 potential or the Buckingham (exp +6) potential plus electrostatic interaction terms are used for intermolecular interactions between gears. A number of gear and gear/shaft configurations are simulated on parallel computers. One gear is powered by forcing the atoms near the end of the nanotube to rotate, and a second gear is allowed to rotate by keeping the atoms near the end of its nanotube constrained to a cylinder. The meshing aromatic gear teeth transfer angular momentum from the powered gear to the driven gear. Results suggest that these gears can operate at up to 50-100 GHz in a vacuum at room temperature. The failure mode involves tooth slip, not bond breaking, so failed gears can be returned to operation by lowering the temperature and/or rotation rate. M This article features multimedia enhancements available from the abstract page in the online journal; see http://www.iop.org.

Rovibrational internal energy transfer and dissociation of N2(1Σg+)−N(4Su) system in hypersonic flows

A rovibrational collisional model is developed to study energy transfer and dissociation of N(2)((1)Σ(g)(+)) molecules interacting with N((4)S(u)) atoms in an ideal isochoric and isothermal chemical reactor. The system examined is a mixture of molecular nitrogen and a small amount of atomic nitrogen. This mixture, initially at room temperature, is heated by several thousands of degrees Kelvin, driving the system toward a strong non-equilibrium condition. The evolution of the population densities of each individual rovibrational level is explicitly determined via the numerical solution of the master equation for temperatures ranging from 5000 to 50,000 K. The reaction rate coefficients are taken from an ab initio database developed at NASA Ames Research Center. The macroscopic relaxation times, energy transfer rates, and dissociation rate coefficients are extracted from the solution of the master equation. The computed rotational-translational (RT) and vibrational-translational (VT) relaxation times are different at low heat bath temperatures (e.g., RT is about two orders of magnitude faster than VT at T = 5000 K), but they converge to a common limiting value at high temperature. This is contrary to the conventional interpretation of thermal relaxation in which translational and rotational relaxation timescales are assumed comparable with vibrational relaxation being considerable slower. Thus, this assumption is questionable under high temperature non-equilibrium conditions. The exchange reaction plays a very significant role in determining the dynamics of the population densities. The macroscopic energy transfer and dissociation rates are found to be slower when exchange processes are neglected. A macroscopic dissociation rate coefficient based on the quasi-stationary distribution, exhibits excellent agreement with experimental data of Appleton et al. [J. Chem. Phys. 48, 599-608 (1968)]. However, at higher temperatures, only about 50% of dissociation is found to take place under quasi-stationary state conditions. This suggest the necessity of explicitly including some rovibrational levels, when solving a global kinetic rate equation.

Theoretical study of the four lowest doublet electronic states of the hydroperoxyl radical: Application to photodissociation

Potential‐energy curves have been calculated for the four lowest doublet states of the hydroperoxyl radical as a function of the H–O–O bond angle and the oxygen–oxygen and oxygen–hydrogen bond lengths. A polarized double‐zeta basis of Cartesian–Gaussian functions was used. An extensive configuration–interaction treatment and subsequent extrapolation procedure was used to obtain a uniform description of the four electronic states over a wide range of nuclear geometries. The calculated equilibrium oxygen–oxygen bond lenths for the ground state, (1)  2A″, and the low‐lying (1)  2A′ excited state are in excellent agreement with the values determined from absorption spectra in the near infrared. An analysis of the shapes of the potential curves for the (2)  2A″ and (2)  2A′ states, as a function of the oxygen–oxygen and oxygen–hydrogen bond lengths, indicates that OH and atomic oxygen should be the predominant photodissociation products. Calculated dipole moments are reported for each state. These results indi...

Calculated potential surfaces for the reactions: O+N2→NO+N and N+O2→NO+O

Complete active space SCF/contracted CI (CASSCF/CCI) calculations, using large Gaussian basis sets, are presented for selected portions of the potential surfaces for the reactions in the Zeldovich mechanism for the conversion of N2 to NO. The N+O2 reaction is exoergic by 32 kcal/mol and is computed to have an early barrier of 10.2 kcal/mol for the 2A’ surface and 18.0 kcal/mol for the 4A’ surface. The O+N2 reaction is endoergic by 75 kcal/mol. The 3A‘ surface is calculated to have a late barrier of 0.5 kcal/mol, while the 3A’ surface is calculated to have a late barrier of 14.4 kcal/mol relative to NO+N.

Theoretical studies of the potential surface for the F + H2 - HF + H reaction

The F+H2→HF+H potential energy hypersurface has been studied in the saddle‐point and entrance channel regions. Using a large [5s 5p 3d 2f 1g/4s 3p 2d] atomic natural orbital basis set, we obtain a classical barrier height of 1.86 kcal/mol at the CASSCF/multireference CI level (MRCI) after correcting for basis set superposition error and including a Davidson correction (+Q) for higher excitations. Based upon an analysis of the computed results, the true classical barrier is estimated to be about 1.4 kcal/mol. We also compute the location of the bottleneck on the lowest vibrationally adiabatic potential curve, and determine the translational energy threshold from a one‐dimensional tunneling calculation. Using the difference between the calculated and experimental threshold to adjust the classical barrier height on the computed surface yields a classical barrier in the range of 1.0–1.5 kcal/mol. Combining the results of our direct estimates of the classical barrier height with the empirical values obtained f...

An ab initio investigation of the structure, vibrational frequencies, and intensities of HO2 and HOCl

The infrared spectral intensities for HOC1 and HO2 have been calculated using a new ab initio technique. Theoretical results for the geometries, vibrational frequencies, and the dipole moments of these species are also reported. All of the calculations were performed at the SCF level using near Hartree–Fock quality basis sets. Our results for the molecular geometries and the vibrational frequencies are in good agreement with available experimental data. We believe that our computed intensities are accurate to at least 50%. Our results should be helpful in attempts to determine the stratospheric abundance of HOC1 and HO2 by in situ infrared spectroscopic measurements.

Total integral reactive cross sections for F + H2 → HF + H: comparison of converged quantum, quasiclassical trajectory and experimental results

Abstract We report converged quantum total integral reactive cross sections for the reaction F + H 2 → HF + H, for initial rotational states j i = 0 and 1, using a time-dependent method. Our results are compared to classical results and to the experimental results of Neumark . Strong quantum effects are found in the threshold region for both initial states; i.e. in the dependence of the reaction on initial state for low energies. The classical results agree better with experiment than do the quantum results; this appears to be due to errors in the potential used.