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Langhoff, Stephen R.

Publications and source records attributed to Langhoff, Stephen R..

At least 109 records · Page 6
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NASA NTRS1994

Ab Initio Calculations of Singlet and Triplet Excited States of Chlorine Nitrate and Nitric Acid

Ab initio calculations of vertical excitations to singlet and triplet excited states of chlorine nitrate and nitric acid are reported. The nature of the electronic transitions are examined by decomposing the difference density into the sum of detachment and attachment densities. Counterparts for the three lowest singlet excited states of nitric acid survive relatively unperturbed in chlorine nitrate, while other low-lying singlet states of chlorine nitrate appear to be directly dissociative in the ClO chromophore. These results suggest an assignment of the two main peaks in the experimental chlorine nitrate absorption spectrum. In addition, triplet vertical excitations and the lowest optimized triplet geometries of both molecules are studied.

Grana, Ana M.↗
NASA NTRS1994

The Successive H2O Binding Energies for Fe(H2O)n(+)

The binding energy, computed using density functional theory (DFT), are in good agreement with experiment. The bonding is electrostatic (charge-dipole) in origin for all systems. The structures are therefore determined mostly by metal-ligand and ligand-ligand repulsion. The computed structure for FeH2O(+) is C(2v) where sp hybridization is important in reducing the Fe-H2O repulsion. Fe(H2O)2(+) has D2d symmetry where sdo hybridization is the primary factor leading to the linear O-Fe-O geometry. The bonding in Fe(H2O)3(+) and Fe(H2O)4(+) are very complex because ligand-ligand and metal-ligand repulsion, both for the in-plane and out-of-plane water lone-pair orbitals, are important.

Ricca, Alessandra↗
NASA NTRS1994

The Study of the Successive Metal-ligand Binding Energies for Fe(+), Fe(-), V(+) and Co(+)

The successive binding energies of CO and H2O to Fe(+), CO to Fe(-), and H2 to Co(+) and V(+) are presented. Overall the computed results are in good agreement with experiment. The trends in binding energies are analyzed in terms of metal to ligand donation, ligand to metal donation, ligand-ligand repulsion, and changes in the metal atom, such as hybridization, promotion, and spin multiplicity. The geometry and vibrational frequencies are also shown to be directly affected by these effects.

Bauschlicher, Charles W., Jr.↗
NASA NTRS1994

Global Dipole Moment Function for the X1 Sigma+ Ground State of CO

We have studied the dipole moment function (DMF) for the X(sup 1)Sigma(sup +) state of CO as a function of the completeness of the one- and n-particle treatments. Our best DMF is obtained using an augmented correlation-consistent quadruple-zeta basis set with external correlation included using the averaged-coupled-pair functional (ACPF) approach from a complete-active-space self-consistent-field zeroth-order reference. The DMF evaluated using the finite-field approach is in far better agreement with the experimentally deduced DMF than all previous theoretical determinations, but systematic differences still remain in the DMF at larger internuclear distances that give rise to significant discrepancies between the theoretical and experimental Einstein coefficients for transitions involving vibrational quantum numbers above about Upsilon=15.

Langhoff, Stephen R.↗
NASA NTRS1993

Boost-phase discrimination research

The final report describes the combined work of the Computational Chemistry and Aerothermodynamics branches within the Thermosciences Division at NASA Ames Research Center directed at understanding the signatures of shock-heated air. Considerable progress was made in determining accurate transition probabilities for the important band systems of NO that account for much of the emission in the ultraviolet region. Research carried out under this project showed that in order to reproduce the observed radiation from the bow shock region of missiles in their boost phase it is necessary to include the Burnett terms in the constituent equation, account for the non-Boltzmann energy distribution, correctly model the NO formation and rotational excitation process, and use accurate transition probabilities for the NO band systems. This work resulted in significant improvements in the computer code NEQAIR that models both the radiation and fluid dynamics in the shock region.

Langhoff, Stephen R.↗
NASA NTRS1993

Theoretical study of the lowest 5Pi and 5Sigma(+) states of CO

Large one-particle basis sets and the internally contracted multireference configuration-interaction treatment of electron correlation were used to characterize the lowest 5Pi and 5Sigma(+) states of CO. It was found that the (1)5Sigma(+) state is weakly bound with an r(e) of 4.76 a(0), supporting results of calculations by Rosenkrantz et al. (1992). The (1)5Pi state has a weakly bound outer well and a significantly bound inner well.

Bauschlicher, Charles W., Jr.↗
NASA NTRS1993

Determination of the structure and bond energies of NiO2 and CuO2

On the basis of extensive ab initio calculations, we estimate the metal-O2 binding energies of NiO2 and CuO2 to be 48 +/- 7 and 18 +/- 4 kcal/mol, respectively. We feel that the experimental estimate of 57 +/- 10 kcal/mol for the binding energy of NiO2 is slightly too large, while we are in complete agreement with the experimental estimate of 15 +10/-5 kcal/mol for CuO2. While the 1A1 ground state of NiO2 definitely has a side-on C(2v) structure, matrix isolation studies suggest that CuO2 has an end-on C(s) structure. Calculations at the coupled-cluster singles plus doubles level with a perturbational estimate of triple excitations, CCSD(T), produce a 2A2 state with C(2v) as a global minimum. However, the entire 2A-double prime ground-state surface is exceedingly flat, precluding a reliable determination of the gas-phase equilibrium structure.

Bauschlicher, Charles W., Jr.↗
NASA NTRS1992

Theoretical study of the 2A2-2B2 separation of the alkali superoxides

The computed 2A2-2B2 separations of the alkali superoxides are in good agreement with those deduced from electron-spin resonance spectra. The calculations definitively show that the ground state of CsO2 is 2A2. The larger than expected separation for CsO2, based on the trend from LiO2 to RbO2, is attributed primarily to the differential effects of core relaxation. The CsO2 dissociation energy is computed to be 42.7 kcal/mol, with an uncertainty conservatively estimated as +/- 4 kcal/mol.

Bauschlicher, Charles W., Jr.↗
NASA NTRS1992

Comparative study of the dissociation energies of Ni2 and Ni2(+)

Computations at the internally contracted averaged coupled-pair-functional level of theory yield a dissociation energy (Do) for Ni2(+) that is 0.17 eV larger than that of Ni2. This finding is consistent with the collision-induced dissociation experiments of Lian, Su, and Armentrout, but rules out the results from the resonant two-photon dissociation experiments of Lessen and Brucat, which predict that the Do value of Ni2(+) is about 1 eV larger than that of Ni2.

Bauschlicher, Charles W., Jr.↗
NASA NTRS1992

Theoretical determination of the alkali-metal superoxide bond energies

The bond dissociation energies for the alkali-metal superoxides have been computed using extensive Gaussian basis sets and treating electron correlation at the modified coupled-pair functional level. Our computed D0 values are 61.4, 37.2, 40.6, and 38.4 kcal/mol for LiO2, NaO2, KO2, and RbO2, respectively. These values, which are expected to be lower bounds and accurate to 2 kcal/mol, agree well with some of the older flame data, but rule out several recent experimental measurements.

Partridge, Harry↗
NASA NTRS1992

Theoretical study of the bond dissociation energies of propyne (C3H4)

The C-C and C-H bond dissociation energies (BDEs) of propyne have been computed using the modified coupled-pair functional method. Due to hyperconjugation, the C-C and methyl C-H single bonds are stronger and weaker, respectively than those in ethane. The acetylenic C-H and C triple bond C BDEs are larger and smaller, respectively, than in acetylene, also as a result of the hyperconjugation. Our best estimate of 92.5 +/- 2 kcal/mol for the methyl C-H BDE in propyne is slightly larger than the experimental value. For the acetylenic C-H BDE in propyne we predict 135.9 +/- 2 kcal/mol.

Bauschlicher, Charles W., Jr.↗
NASA NTRS1992

Theoretical study of transition-metal ions bound to benzene

Theoretical binding energies are reported for all first-row and selected second-row transition metal ions (M+) bound to benzene. The calculations employ basis sets of at least double-zeta plus polarization quality and account for electron correlation using the modified coupled-pair functional method. While the bending is predominantly electrostatic, the binding energies are significantly increased by electron correlation, because the donation from the metal d orbitals to the benzene pi* orbitals is not well described at the self-consistent-field level. The uncertainties in the computed binding energies are estimated to be about 5 kcal/mol. Although the calculated and experimental binding energies generally agree to within their combined uncertainties, it is likely that the true binding energies lie in the lower portion of the experimental range. This is supported by the very good agreement between the theoretical and recent experimental binding energies for AgC6H6(+).

Bauschlicher, Charles W., Jr.↗
NASA NTRS1992

Theoretical study of Cr(+) and Co(+) bound to H2 and N2

Binding energies of Co(H2)n(+) (n = 1-3) and CrN2(+) computed using the modified coupled-pair functional approach agree well with measured values. The calculations show that both H2 and N2 bond much more strongly to Co(+) than to Cr(+). Theoretical studies also show that despite the similar polarizabilities of N2 and Ar, the CrN2(+) binding energy is twice as large as that of CrAr(+) due to the charge-quadrupole contribution. Finally, the bonding of H2 and N2 with Na(+) and Mg(+) is contrasted with that for Cr(+) and Co(+).

Bauschlicher, Charles W., Jr.↗
NASA NTRS1992

Theoretical study of the BeLi, BeNa, MgLi, MgNa, and AlBe molecules and their negative ions

The alkaline earth-alkali diatomics are found to have weak bonds, because the diffuse alkali valence s orbitals cannot form a bond of sufficient strength to pay the promotion energy of the alkaline-earth atoms. This leads to van der Waals bonding in the neutrals as well as the negative ions. In fact, the negative ions have larger binding energies than the neutrals as a result of the much larger polarizability of the negative ion. The binding energy of AlBe is significantly larger than the Be-alkali molecules, due to a covalent contribution to the bonding. The binding energy in AlBe(-) is considerably larger than AlBe; the binding energy of the X 3Sigma(-) state of AlBe(-) is computed to be 1.36 eV, as compared with 0.57 eV for the X 2Pi state of AlBe.

Bauschlicher, Charles W., Jr.↗
NASA NTRS1992

Theoretical study of the bond dissociation energies of methanol

A theoretical study of the bond dissociation energies for H2O and CH3OH is presented. The C-H and O-H bond energies are computed accurately with the modified coupled-pair functional method using a large basis set. For these bonds, an accuracy of +/- 2 kcal/mol is achieved, which is consistent with the C-H and C-C single bond energies of other molecules. The C-O bond is much more difficult to compute accurately because it requires higher levels of correlation treatment and more extensive one-particle basis sets.

Bauschlicher, Charles W., Jr.↗
NASA NTRS1992

Theoretical study of the bonding of the first-row transition-metal positive ions to ethylene

Ab initio calculations were performed to study the bonding of the first-row transition-metal ions with ethylene. While Sc(+) and Ti(+) insert into the pi bond of ethylene to form a three-membered ring, the ions V(+) through Cu(+) form an electrostatic complex with ethylene. The binding energies are compared with those from experiment and with those of comparable calculations performed previously for the metal-acetylene ion systems.

Sodupe, M.↗
NASA NTRS1992

Theoretical study of the bonding of the first- and second-row transition-metal positive ions to methylene

The geometries of the molecules formed by the interaction of the first- and second-row transition-metal cations with methylene are optimized at the modified coupled-pair functional (MCPF) level of theory using large Gaussian basis sets, and their dissociation energies are computed employing both the MCPF and internally contracted averaged coupled-pair functional (ICACPF) methods. The computed binding energies are generally in good agreement with the available experimental results, although the calculations indicate that the experimental values for ScCH2(+), TiCH2(+), and NbCH2(+) are probably too large. The nature of the bonding in each case and trends in the bonding patterns across the transition-metal rows are discussed.

Bauschlicher, Charles W., Jr.↗