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At least 829 records · Page 46
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NASA NTRS1985

Carbon, hydrogen and nitrogen in carbonaceous chondrites Abundances and isotopic compositions in bulk samples

Whole-rock samples of 25 carbonaceous chondrites were analyzed for contents of C, H and N and delta C-13, delta D and delta N-15. Inhomogeneous distribution of these isotopes within individual meteorites is pronounced in several cases. Few systematic intermeteorite trends were observed; N data are suggestive of isotopic inhomogeneity in the early solar system. Several chondrites revealed unusual compositions which would repay further, more detailed study. The data are also useful for classification of carbonaceous chondrites; N abundance and isotopic compositions can differentiate existing taxonomic groups with close to 100 percent reliability; Al Rais and Renazzo clearly constitute a discrete 'grouplet', and there are hints that both CI and CM groups may each be divisible into two subgroups.

Kerridge, J. F.↗
NASA NTRS1985

Chondrules in the Qingzhen type-3 enstatite chondrite Possible precursor components and comparison to ordinary chondrite chondrules

The mineral composition of chondrules from a fragment of Qingzhen (EH3) fall was analyzed by neutron activation method. Unlike the ordinary chondrite (OC) chondrules (Gooding and Keil, 1981), the Qingzhen radial pyroxene (RP) and porphyritic pyroxene (PP) chondrules have similar bulk compositions. Porphyritic olivine-pyroxene (POP) chondrules are richer than PP and RP chondrules in refractory lithophiles and siderophiles. Elements in each of the following sets intercorrelate significantly: (1) Fe-Co-Ni-Ir-Au, probably derived from a metal component; (2) Ca-Eu-Se, which suggests an oldhamite-rich precursor; (3) Al-Sc-Hf, occurring in high concentrations in POP chondrules, this set suggesting the existence of a refractory lithophile-rich and olivine-rich component; (4) Na REE; and (5) Cl-Br. Sets (2) and (4) were not precursors of OC. The interelement ratios of refractory lithophiles such as Ca, Al, Ti, Sc, and REE are similar to CI ratios, suggesting that they originated in the earliest phases as silicates, which were sulfurized before chondrule formation.

Grossman, J. N.↗
NASA NTRS1986

Elemental analysis of a comet nucleus by passive gamma ray spectrometry from a penetrator

The Comet Rendezvous Asteroid Flyby (CRAF) spacecraft, to be launched in 1991, was designed to study the physical and chemical properties of cometary and asteroid bodies. It is proposed that these properties can be determined by utilizing a penetrator experiment delivery system aboard the CRAF, which would deliver a passive gamma ray spectrometer to the comet and determine the composition of subsurface materials. Results of calculations for a model comet (50 percent CI carbonaceous chrondrite, 50 percent ices) provide data on the expected gamma ray fluxes, the minimum detectable limits, and the detection uncertainties for 35 isotopes. These results confirm that this technique can determine the composition of both the rocky and ice components, and the rock-to-ice ratio in the nucleus. In addition, useful information regarding the formation of the solar system is provided.

Evans, L. G.↗
NASA NTRS1986

Theoretical study of the X 2Pi and A 2Sigma(+) states of CuO and CuS

Theoretical spectroscopic constants and dipole moments are determined for the X 2Pi and A 2Sigma(+) states of CuO and CuS, using extended Gaussian basis sets and incorporating correlation using both configuration interaction (CI) and coupled pair (CPF) methods. Relativistic corrections (Darwin plus mass velocity), included using first-order perturbation theory, are found to be relatively small. At the CPF level, significant configuration mixing with the 3d(9)4s(1) occupation occurs for both states, resulting in some bond shortening and a significant increase in the dissociation energy. The best spectroscopic parameters are in excellent agreement with experiment for both CuO and CuS. In analogy with CuO, CuS is predicted to have a yet unobserved 2Sigma(+) state near 10,000 per cm.

Langhoff, S. R.↗
NASA NTRS1986

Calculated potential surfaces for the reactions: O + N2 yields NO + N and N + O2 yields 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/mole and is computed to have an early barrier of 10.2 kcal/mole for the (sup 2)A(sup prime) surface and 18.0 kcal/mole for the (sup 4)A(sup prime) surface. The O + N2 reaction is endoergic by 75 kcal/mole. The (sup 3)A(sup double prime) surface is calculated to have a late barrier of 0.5 kcal/mole, while the (sup 3)A(sup prime) surface is calculated to have a late barrier of 14.4 kcal/mole.

Walch, Stephen P.↗
NASA NTRS1986

The atomic carbon distribution in the coma of Comet Halley

The radial distribution of CO, OI, Ci, and CII emissions in the coma of comet Halley were measured by a long-slit far ultraviolet spectrograph aboard a sounding rocket on 26 Feb. and 13 Mar. 1986. While the CO profiles strongly suggest that CO is vaporized directly from the nucleus, the observed carbon distribution is not consistent with a radial outflow model of CO, suggesting an additional source of atomic carbon in the inner coma. Based on the in situ plasma measurements from the Vega and Giotto spacecraft, it is possible that this additional source of carbon could be the recombination of ionized CO in the inner coma.

Woods, T. N.↗
NASA NTRS1987

Potential surfaces for O atom-polymer reactions

Ab initio quantum chemistry methods are used to study the energetics of interactions of O atoms with organic compounds. Polyethylene (CH2)n has been chosen as the model system to study the interactions of O(3P) and O(1D) atoms with polymers. In particular, H abstraction is investigated and polyethylene is represented by a C3 (propane) oligomeric model. The gradient method, as implemented in the GRADSCF package of programs, is used to determine the geometries and energies of products and reactants. The saddle point, barrier geometry is determined by minimizing the squares of the gradients of the potential with respect to the internal coordinates. To correctly describe the change in bonding during the reaction at least a two configuration MCSCF (multiconfiguration self consistent field) or GVB (generalized valence bond) wave function has to be used. Basis sets include standard Pople and Dunning sets, however, increased with polarization functions and diffuse p functions on both the C and O atoms. The latter is important due to the O(-) character of the wave function at the saddle point and products. Normal modes and vibrational energy levels are given for the reactants, saddle points and products. Finally, quantitative energetics are obtained by implementing a small CAS (complete active space) approach followed by limited configuration interaction (CI) calculations. Comparisons are made with available experimental data.

Laskowski, B. C.↗
NASA NTRS1986

Theoretical study of the electric dipole moment function of the ClO molecule

The potential energy function and electric dipole moment function (EDMF) are computed for ClO X 2Pi using several different techniques to include electron correlation. The EDMF is used to compute Einstein coefficients, vibrational lifetimes, and dipole moments in higher vibrational levels. The band strength of the 1-0 fundamental transition is computed to be 12 + or - 2 per sq cm atm determined from infrared heterodyne spectroscopy. The theoretical methods used include SCF, CASSCF, multireference singles plus doubles configuration interaction (MRCI) and contracted CI, coupled pair functional (CPF), and a modified version of the CPF method. The results obtained using the different methods are critically compared.

Pettersson, L. G. M.↗
NASA NTRS1986

Theoretical transition probabilities for the OH Meinel system

An electric dipole moment function (EDMF) for the X 2Pi ground state of OH, based on the complete active-space self-consistent field plus a multireference singles-plus-double excitation configuration-interaction procedure (using an extended Slater basis) is reported. Two theoretical EDMFS are considered: the MCSCF (7)-SCEP EDMF of Werner et al., (1983) and a previously unpublished EDMF based on the MCSCF multireference CI(SD) procedure using a large Slater basis. The theoretical treatment follows that of Mies (1974), except that the Hill and Van Vleck (1928) approximation to intermediate coupling is used. This approximation is shown to be accurate to better than 5 percent for the six principal branches of the OH Meinel system.

Langhoff, S. R.↗
NASA NTRS1986

The nature of the bonding in the transition metal trimers

The electronic structure of the transition metal (TM) trimers was studied by comparing the bonding in the Ca3, Sc3, and Cu3 molecules. The complete active space SCF/externally contracted configuration interaction (CI) ratio for the low-lying states of Sc3 and Sc3(+) and the SCF/CI ratio for Ca3 and Cu3 trimers, all for near equilateral triangle geometries, were calculated. In addition, vertical excitation energies for Cu3 were computed, leading to a new assignment of the upper state in the resonant two-photon ionization spectrum. Based on these studies, bonding in other TM trimers was discussed.

Walch, Stephen P.↗
NASA NTRS1987

On the 3d6 4s2 (5D) - 3d7 4s1 (5F) separation in Fe

Full CI calculations are used to calibrate SDSI treatment of the 5D-5F separation in the Fe atom. The (5s4p2d), (5s4p3d), and (5s4p2d1f) basis sets were used for the FCI calculations. The importance of the 3s and 3p correlation and the relativistic effects are studied at SDCI level. The role of polarization functions in computing atomic separations in transition metal atoms is discussed. It is observed that the FCI separation is slightly smaller than the SDCI results, and the SDCI data yield an adequate description of the 5D-5F separation with only 8 electrons correlated. It is concluded that the differential effect of higher excitations becomes more important when the 3s and 3p electrons are correlated.

Bauschlicher, Charles W., Jr.↗
NASA NTRS1987

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

Complete active space SCF/contracted CI calculations using large Gaussian basis sets are presented for selected portions of the potential surfaces for 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-prime surface and 18.0 kcal/mol for the 4A-prime surface. The O + N2 reaction is endoergic by 75 kcal/mol. The 3A-double prime surface is calculated to have a late barrier of 0.5 kcal/mol, while the 3A-prime surface has a late barrier of 14.4 kcal/mol relative to NO + N. These results are significant for determining the physical and chemical conditions which aeroassisted orbital transfer vehicles will encounter while transferring between high and low altitude earth orbits.

Walch, Stephen P.↗
NASA NTRS1987

Theoretical study of the dipole moment function of OH(X 2Pi)

A complete theoretical electric dipole moment function mu(r) is computed for OH(X 2Pi) at the full configuration-interaction (FCI) level for DZP + diffuse Gaussian basis set. The energy and dipole moment are also determined with 2d functions on oxygen at seven r values. The following methods are compared to the FCI for both the oxygen 1d and 2d basis sets: single-reference singles-plus-doubles configuration interaction (SDCI), SDCI with Davidson's correction, the coupled pair functional approach (CPF) and a modified form of CPF, CASSCF, and CASSCF-MRSDCI, and the external contracted CI(CCI) method. The dipole moments are evaluated both as an expectation value and by finite-field methods. The results support the superiority of evaluating the dipole moment as an energy derivative rather than as an expectation value.

Langhoff, Stephen R.↗
NASA NTRS1987

Accurate ab initio calculations which demonstrate a 3Pi(u) ground state for Al2

The computational requirements for accurately describing the spectroscopic constants for all three candidates for the ground state of Al2 are determined. Full CI(FCI) calculations are used to calibrate approximate methods of including the electron correlation. CASSCF/MRCI calculations which accurately reproduce the FCI results in the valence DZ + 2d Gaussian basis are carried to chemical accuracy by using extensive one-particle basis sets. The effect of 2s and 2p correlation and relativistic effects are considered as well as the valence 3s and 3p correlation. Several excited states are also considered. It is shown that the computed vibrational frequency of the (2) 3Pi(g) state and the Franck-Condon factors for the (1) 3Pi(u) - (2) 3Pi(g) transition are consistent with the spectrum obtained by Douglas et al. (1983) and Abe and Kolb (1983). It is therefore concluded that the ground state is 3Pi(u).

Bauschlicher, Charles W., Jr.↗
NASA NTRS1987

Evolutionary models of interstellar chemistry

The goal of evolutionary models of interstellar chemistry is to understand how interstellar clouds came to be the way they are, how they will change with time, and to place them in an evolutionary sequence with other celestial objects such as stars. An improved Mark II version of an earlier model of chemistry in dynamically evolving clouds is presented. The Mark II model suggests that the conventional elemental C/O ratio less than one can explain the observed abundances of CI and the nondetection of O2 in dense clouds. Coupled chemical-dynamical models seem to have the potential to generate many observable discriminators of the evolutionary tracks. This is exciting, because, in general, purely dynamical models do not yield enough verifiable discriminators of the predicted tracks.

Prasad, Sheo S.↗
NASA NTRS1987

Effective core potential parameters for first- and second-row atoms

An improved effective core potential (ECP) technique is described and used to give ECP parameters for the atoms of the first two rows of the periodic table. A given basis set is parametrized which allows for a direct comparison with all-electron calculations. Extensive test calculations on first- and second-row molecules using the ECP have been performed, giving excellent agreement with the all-electron results at the SCF, CASSCF, and CI levels of treatment. Correlating and diffuse functions may be added without modifying the ECP parameters. The present ECP descriptions result in CPU time reductions of the order of 50 percent in addition to reduced disk storage.

Pettersson, Lars G. M.↗
NASA NTRS1988

Core-core and core-valence correlation

The effect of (1s) core correlation on properties and energy separations was analyzed using full configuration-interaction (FCI) calculations. The Be 1 S - 1 P, the C 3 P - 5 S and CH+ 1 Sigma + or - 1 Pi separations, and CH+ spectroscopic constants, dipole moment and 1 Sigma + - 1 Pi transition dipole moment were studied. The results of the FCI calculations are compared to those obtained using approximate methods. In addition, the generation of atomic natural orbital (ANO) basis sets, as a method for contracting a primitive basis set for both valence and core correlation, is discussed. When both core-core and core-valence correlation are included in the calculation, no suitable truncated CI approach consistently reproduces the FCI, and contraction of the basis set is very difficult. If the (nearly constant) core-core correlation is eliminated, and only the core-valence correlation is included, CASSCF/MRCI approached reproduce the FCI results and basis set contraction is significantly easier.

Bauschlicher, Charles W., Jr.↗
NASA NTRS1987

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

The F + H2 yields HF + H potential energy hypersurface was 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, a classical barrier height of 1.86 kcal/mole was obtained 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/mole. The location of the bottleneck on the lowest vibrationally adiabatic potential curve was also computed and the translational energy threshold determined 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 to 1.5 kcal/mole. Combining the results of the direct estimates of the classical barrier height with the empirical values obtained from the approximation calculations of the dynamical threshold, it is predicted that the true classical barrier height is 1.4 + or - 0.4 kcal/mole. Arguments are presented in favor of including the relatively large +Q correction obtained when nine electrons are correlated at the CASSCF/MRCI level.

Bauschlicher, Charles W., Jr.↗